Tables
A guide to the use of the tables
There are 4 sets of tables containing information essential for the performance of the CDS Test. These tables are intended for both Clinical and Veterinary laboratories. Antibiotics used in veterinary medicine only are flagged.
Calibration tables 1a, 1b, & 1c collectively summarise much of the basic information used in the CDS Test. They list all the organisms and antimicrobials tested, the media used, conditions of incubation, disc potencies, cut off sizes for unusual annular radii and the MICs for susceptible strains (breakpoints). The tables are updated regularly and operators should ensure that they are using the latest versions of these tables. It is most important to pay particular attention to the footnotes included with each table as these highlight exceptions, restrictions and some specific directions. For organisms not included in the Table of calibrations, extrapolate the testing from that established for similar organisms.
Surrogate disc testing tables 2a and 2b list those antimicrobials where the susceptibility can be inferred from the results obtained with a closely related agent, the “surrogate disc”. The table is arranged according to bacterial species and the relationship between the antimicrobials is valid only for the species indicated. This table is also updated regularly as data are accumulated which either invalidate the relationship or enable us to add agents to the list. Laboratories may find it useful to include a comment on the susceptibility report that the result reported with a particular antimicrobial indicates the susceptibility to another.
Quality assurance tables 3a, 3b, & 3c list the 12 reference strains, the media, and the incubation conditions of testing and the expected range of zone sizes observed with each disc of a stated potency. The footnotes explain how the acceptable ranges of zone sizes were derived and recommend the indications for and the frequency of testing the reference strains.
Table 4 is a guide through the maze of testing and reporting the susceptibility of Gram negative aerobic species to β-lactam antibiotics. The production of one or more β-lactamases is an important and common mechanism of resistance in these species but, for several reasons, resistance may be difficult or impossible to demonstrate by the usual methods of antibiotic susceptibility testing. The table sets out, in some detail, those species where resistance should be assumed on the basis of previous documentation of the presence of a stable mechanism of resistance and those where susceptibility can be reliably demonstrated by the disc test. This table also relies heavily on the footnotes to draw attention to exceptions and special circumstances.
11.1. Calibrations
Table 11.1.a. Calibrations: Gram Positive Organisms
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) |
| Corynebacterium species (Blood Sensitest, CO2, 35‑37°C)b |
|||
| Ampicillinc | 5 | 4 mm | ≤ 2 |
| Benzylpenicillin | 0.5 u | ≤ 0.125 | |
| Chloramphenicol | ≤ 8 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | |
| Clindamycind | 2 | ≤ 0.5 | |
| Erythromycin | 5 | ≤ 0.5 | |
| Florfenicole | 30 | ≤ 8 | |
| Fusidic acid | 2.5 | ≤ 0.5 | |
| Marbofloxacine | 5 | ≤ 2 | |
| Moxifloxacin | 2.5 | ≤ 1 | |
| Pradofloxacine | 5 | ≤ 2 | |
| Quinupristin/Dalfopristin | 15 | ≤ 2 | |
| Rifampicin | 1 | ≤ 0.5 | |
| Teicoplanin | 15 | 2 mm | ≤ 8 |
| Tetracycline | 10 | ≤ 4 | |
| Tigecycline | 15 | ≤ 1 | |
| Vancomycin | 5 | 2 mm | ≤ 4 |
| Enterococci (Blood Sensitest, CO2, 35-37°C) |
|||
| Ampicillinf | 5 | 4 mmf | ≤ 4 |
| Chloramphenicol | 30 | 4 mm | ≤ 8 |
| Doxycyclineg | 30 | 4 mm | ≤ 16 |
| Florfenicole | 30 | ≤ 8 | |
| Fosfomycing | 200 | ≤ 64 | |
| Gentamicin | 200 | ≤ 256 | |
| Linezolid | 10 | ≤ 4 | |
| Marbofloxacine | 5 | ≤ 2 | |
| Nitrofurantoing | 200 | 4 mm | ≤ 64 |
| Pradofloxacine | 5 | ≤ 2 | |
| Quinupristin/Dalfopristinh | 15 | ≤ 2 | |
| Streptomycin | 300 | 4 mm | ≤ 512 |
| Teicoplanin | 15 | 2 mm | ≤ 8 |
| Tetracycline | 10 | ≤ 4 | |
| Tigecycline | 15 | ≤ 1 | |
| Vancomycini,j | 5 | (See foot note)i | ≤ 4 |
- The standard 6 mm cut‑off applies where no exception has been specified.
- Slow growers are incubated for 48h.
- Corynebacterium species: benzylpenicillin 0.5 u/ampicillin 5 μg: If the inhibitory zone is < 6 mm with a benzylpenicillin 0.5 u disc and ≥ 4 mm with an ampicillin 5 μg disc, report the susceptibility as follows: “There is decreased susceptibility to penicillin, ampicillin and amoxycillin with the MIC between 0.25 mg/L and 2.0 mg/L”
- Isolates with inducible clindamycin resistance (iMLSB phenotype) will have a large zone to clindamycin, but should be reported as resistant (see section 4.7).
- Antibiotic calibrated for veterinary medicine.
- Perform a nitrocefin based test to detect β–lactamase activity if the zone of inhibition has a sharp edge and an annular radius > 4 mm. β–lactamase positive isolates are reported as resistant.
- For testing urine isolates only.
- Enterococcus faecalis is intrinsically resistant to pristinamycin and quinupristin/dalfopristin.
- A zone of inhibition with a hazy edge and a light growth towards the disc indicates a low level resistance to vancomycin (VanB type), irrespective of the size of the inhibitory zone. The light growth will be more evident if the plate is reincubated for 48 h.
- Enterococcus gallinarum and Enterococcus casseliflavus possess the natural vanC type resistance and should be reported as resistant (see section 4.2.7).
Table 11.1.a. Calibrations: Gram Positive Organisms cont.
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) |
| Listeria species (Blood Sensitest, CO2, 35‑37°C) |
|||
| Ampicillin | 5 | ≤ 1 | |
| Gentamicin | 10 | ≤ 1 | |
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | ≤ 0.5/9.5 | |
| Staphylococci (Sensitest, Air 35‑37°C) |
|||
| Ampicillink | 5 | ≤ 0.5 | |
| Benzylpenicillinl | 0.5 u | ≤ 0.06 | |
| Cefoxitinm | 10 | ≤ 4 | |
| Ceftarolinen | 5 | ≤ 1 | |
| Cephalexink | 100 | ≤ 16 | |
| Chloramphenicol | 30 | ≤ 8 | |
| Ciprofloxacin | 2.5 | ≤ 1 | |
| Clindamycino | 2 | ≤ 0.5 | |
| Erythromycin | 5 | ≤ 0.5 | |
| Florfenicole | 30 | ≤ 8 | |
| Fusidic acid | 2.5 | ≤ 0.5 | |
| Gentamicin | 10 | ≤ 1 | |
| Linezolid | 10 | ≤ 4 | |
| Marbofloxacine | 5 | ≤ 2 | |
| Moxifloxacin | 2.5 | ≤ 1 | |
| Mupirocin | 5 | ≤ 2 | |
| Neomycine | 30 | 4 mm | ≤ 8 |
| Nitrofurantoing | 200 | ≤ 32 | |
| Novobiocine | 5 | ≤ 1 | |
| Oxacillinp | 1 | ≤ 0.25 | |
| Pradofloxacine | 5 | ≤ 2 | |
| Quinupristin/Dalfopristin | 15 | ≤ 2 | |
| Rifampicin | 1 | ≤ 0.5 | |
| Teicoplanin | 15 | 2 mm | ≤ 8 |
| Tetracycline | 10 | ≤ 4 | |
| Tigecycline | 15 | ≤ 1 | |
| Trimethoprim | 5 | ≤ 4 | |
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | ≤ 1/19 | |
| Vancomycin | 5 | 2 mm | ≤ 4 |
a The standard 6 mm cut‑off applies where no exception has been specified.
e Antibiotic calibrated for veterinary medicine.
g For testing urine isolates only.
k For testing Staphylococcus saprophyticus only.
l Not for testing Staphylococcus saprophyticus.
m For testing Staphylococcus aureus and Staphylococcus lugdunensis only.
n For testing MRSA only.
o Isolates with inducible clindamycin resistance (iMLSB phenotype) will have a large zone to clindamycin, but should be reported as resistant. Inducible clindamycin resistance can be detected as described in section 4.7.
p For testing coagulase‑negative staphylococci (except Staphylococcus saprophyticus and Staphylococcus lugdunensis).
Table 11.1.a. Calibrations: Gram Positive Organisms cont.
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) | |
| Streptococci & Erysipelothrix species (Blood Sensitest, CO2, 35‑37°C) |
||||
| Ampicillinq | 5 | 4 mm | ≤ 2 | |
| Benzylpenicillin | 0.5 u | ≤ 0.125 | ||
| Cefotaxime | 0.5 | 4 mm | ≤ 0.5 | |
| Cefotaximeq | 5 | ≤ 2 | ||
| Ceftriaxone | 0.5 | 4 mm | ≤ 0.5 | |
| Ceftriaxoneq | 5 | ≤ 2 | ||
| Chloramphenicol | 30 | ≤ 8 | ||
| Clindamycino | 2 | ≤ 0.5 | ||
| Erythromycin | 5 | ≤ 0.5 | ||
| Florfenicole | 30 | ≤ 8 | ||
| Marbofloxacine | 5 | ≤ 2 | ||
| Moxifloxacin | 2.5 | 4 mm | ≤ 1 | |
| Nitrofurantoing | 200 | 4 mm | ≤ 32 | |
| Pradofloxacine | 5 | ≤ 2 | ||
| Quinupristin/Dalfopristin | 15 | ≤ 2 | ||
| Rifampicin | 1 | ≤ 0.5 | ||
| Teicoplanin | 15 | 2 mm | ≤ 8 | |
| Tetracycline | 10 | ≤ 4 | ||
| Tigecycline | 15 | ≤ 1 | ||
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | ≤ 0.5/9.5 | ||
| Vancomycin | 5 | 2 mm | ≤ 4 | |
a The standard 6 mm cut‑off applies where no exception has been specified.
e Antibiotic calibrated for veterinary medicine
g For testing urine isolates only
o Isolates with inducible clindamycin resistance (iMLSB phenotype) will have a large zone to clindamycin, but should be reported as resistant. Inducible clindamycin resistance can be detected as described in section 4.7.
q NOT for testing Streptococcus pneumoniae from CSF. If Streptococcus pneumoniae or any other Streptococcus species from a site other than CSF is resistant to benzylpenicillin 0.5 u, cefotaxime 0.5 μg or ceftriaxone 0.5 μg then test ampicillin 5 μg, cefotaxime 5 μg and ceftriaxone 5 μg.
Table 11.1.b. Calibrations: Gram Negative Organisms
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) | |
| Acinetobacter species, Enterobacteriaceae & Vibrionaceae (Sensitest, air, 35‑37°C)b |
||||
| Amikacin | 30 | 4 mm | ≤ 16 | |
| Amoxycillin-clavulanic acidc | 40/20 | ≤ 8/4 | ||
| Ampicillind | 25 | ≤ 8 | ||
| Azithromycine | 15 | 4 mm | ≤ 16 | |
| Aztreonam | 30 | ≤ 8 | ||
| Cefazolin | 30 | ≤ 4 | ||
| Cefepime | 10 | ≤ 2 | ||
| Cefotaxime | 5 | ≤ 1 | ||
| Cefotetan | 30 | ≤ 8 | ||
| Cefoxitin | 30 | ≤ 8 | ||
| Cefpodoxime | 10 | ≤ 2 | ||
| Ceftazidime | 10 | ≤ 4 | ||
| Ceftolozane-tazobactam | 30/10 | ≤ 2 | ||
| Ceftriaxone | 5 | ≤ 1 | ||
| Cefuroxime | 30 | ≤ 8 | ||
| Cephalexind | 100 | ≤ 16 | ||
| Chloramphenicol | 30 | ≤ 8 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Doripenem | 10 | ≤ 4 | ||
| Ertapenemf | 10 | ≤ 4 | ||
| Florfenicolh | 30 | ≤ 8 | ||
| Fosfomycing | 200 | ≤ 32 | ||
| Gentamicin | 10 | 4 mm | ≤ 2 | |
| Imipenemi | 10 | ≤ 4 | ||
| Marbofloxacinh | 5 | ≤ 2 | ||
| Meropenem | 5 | ≤ 2 | ||
| Moxifloxacin | 2.5 | ≤ 1 | ||
| Nalidixic acide | 30 | ≤ 4 | ||
| Neomycinh | 30 | 4 mm | ≤ 8 | |
| Nitrofurantoing | 200 | ≤ 32 | ||
| Norfloxacing | 10 | ≤ 4 | ||
| Piperacillin-tazobactamc | 50/5 | ≤ 16/2 | ||
| Polymyxin Bj | 300 u | 4 mm | ≤ 1 | |
| Pradofloxacinh | 5 | ≤ 2 | ||
| Spectinomycinh | 25 | ≤ 32 | ||
| Streptomycinh | 25 | ≤ 16 | ||
| Tetracycline | 10 | 4 mm | ≤ 4 | |
| Ticarcillin-clavulanic acidc | 75/10 | ≤ 16/2 | ||
| Tigecyclinei | 15 | ≤ 1 | ||
| Tobramycin | 10 | 4 mm | ≤ 2 | |
| Trimethoprim | 5 | ≤ 4 | ||
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | ≤ 1/19 | ||
a The standard 6 mm cut-off applies where no exception has been specified.
b Yersinia enterocolitica is incubated in air at 30° C.
c If an ESBL is present, report amoxycillin-clavulanic acid, ticarcillin-clavulanic acid and piperacillin-tazobactam for isolates from uncomplicated UTI only.
d Test ampicillin 25 and cephalexin 100 in parallel. Strains resistant to cephalexin but appear susceptible to ampicillin (presence of chromosomal inducible AmpC β-lactamase) are reported as resistant to ampicillin/amoxicillin.
e For the testing of Salmonella Typhi and other Salmonella species isolated from blood culture.
f Acinetobacter species are considered resistant to ertapenem.
g For testing urinary isolates only.
h Antibiotic calibrated for veterinary medicine.
i Proteus species, Providencia species and Morganella morganii are considered resistant to tigecycline, refer to section 5.5 regarding imipenem testing.
j Polymyxin B tested for screening purposes only.
Table 11.1.b. Calibrations: Gram Negative Organisms cont.
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) | |
| Moraxella catarrhalis (Blood Sensitest, 5% CO2, 35‑37°C) |
||||
| Benzylpenicillin | 0.5 u | ≤ 0.25 | ||
| Cefaclor | 30 | ≤ 4 | ||
| Cefpodoxime | 10 | ≤ 2 | ||
| Cefuroxime | 30 | ≤ 4 | ||
| Chloramphenicol | 10 | ≤ 4 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Erythromycin | 5 | ≤ 0.5 | ||
| Florfenicolh | 30 | ≤ 8 | ||
| Marbofloxacinh | 5 | ≤ 2 | ||
| Moxifloxacin | 2.5 | ≤ 1 | ||
| Tetracycline | 10 | ≤ 4 | ||
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | ≤ 1/19 | ||
| Campylobacter species (Blood Sensitest, microaerophilic, 42°C) |
||||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Erythromycin | 5 | 4 mm | ≤ 0.5 | |
| Gentamicin | 10 | ≤ 1 | ||
| Tetracycline | 10 | ≤ 4 | ||
| Haemophilus species (HTM agarj, 5% CO2, 35‑37°C) |
||||
| Amoxycillin-clavulanic acid | 10/5 | ≤ 2/1 | ||
| Ampicillin | 5 | ≤ 1 | ||
| Cefaclor | 30 | ≤ 4 | ||
| Cefotaxime | 0.5 | ≤ 0.25 | ||
| Cefotaxime | 5 | ≤ 1 | ||
| Cefpodoxime | 10 | ≤ 2 | ||
| Ceftriaxone | 0.5 | ≤ 0.25 | ||
| Ceftriaxone | 5 | ≤ 1 | ||
| Cefuroxime | 30 | ≤ 4 | ||
| Chloramphenicol | 10 | ≤ 2 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Florfenicolh | 30 | ≤ 8 | ||
| Marbofloxacinh | 5 | ≤ 2 | ||
| Moxifloxacin | 2.5 | ≤ 1 | ||
| Tetracycline | 10 | ≤ 4 | ||
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | ≤ 1/19 | ||
a The standard 6 mm cut‑off applies where no exception has been specified.
h Antibiotic calibrated for veterinary medicine.
j Haemophilus Test Medium Base containing 15 mg/L of freshly prepared haematin and NAD.
Table 11.1.b. Calibrations: Gram Negative Organisms cont.
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) | |
| Helicobacter pylori (Chocolate Columbia Blood Agar, microaerophilic, 35‑37°C) |
||||
| Amoxycillin | 2 | ≤ 1 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Erythromycink | 5 | ≤ 0.5 | ||
| Metronidazole | 5 | ≤ 4 | ||
| Rifampicinl | 5 | ≤ 2 | ||
| Tetracycline | 10 | ≤ 4 | ||
| Neisseria meningitidis
(Blood Sensitest, 5% CO2, 35‑37°C) |
||||
| Benzylpenicillin | 0.5 u | 4 mm | ≤ 0.25 | |
| Cefotaxime | 0.5 | ≤ 0.25 | ||
| Ceftriaxone | 0.5 | ≤ 0.25 | ||
| Chloramphenicol | 10 | ≤ 2 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Rifampicin | 1 | ≤ 0.5 | ||
a The standard 6 mm cut‑off applies where no exception has been specified.
k Erythromycin 5 μg is the surrogate disc for reporting the susceptibility to clarithromycin. The MIC of clarithromycin for susceptible strains is ≤ 0.5 mg/L.
l Rifampicin 5 μg is the surrogate disc for reporting the susceptibility to rifabutin.
Table 11.1.b. Calibrations: Gram Negative Organisms cont.
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) | |
| Pasteurella species (Blood Sensitest, 5% CO2, 35‑37°C) |
||||
| Ampicillinm | 5 | ≤ 2 | ||
| Benzylpenicillin | 0.5 u | 4 mm | ≤ 0.25 | |
| Cefotaxime | 0.5 | ≤ 0.06 | ||
| Ceftriaxone | 0.5 | ≤ 0.06 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Erythromycinh | 5 | 4 mm | ≤ 2 | |
| Florfenicolh | 30 | ≤ 8 | ||
| Marbofloxacinh | 5 | ≤ 2 | ||
| Moxifloxacin | 2.5 | ≤ 1 | ||
| Pradofloxacinh | 5 | ≤ 2 | ||
| Tetracycline | 10 | ≤ 4 | ||
| Pseudomonas species, Burkholderia species & Chryseobacterium species (Sensitest, air, 35‑37°C) |
||||
| Amoxycillin-clavulanic acidn | 40/20 | ≤ 8/4 | ||
| Amikacin | 30 | 4 mm | ≤ 16 | |
| Aztreonam | 30 | ≤ 8 | ||
| Cefepime | 10 | ≤ 2 | ||
| Cefpirome | 10 | ≤ 2 | ||
| Ceftazidime | 10 | ≤ 4 | ||
| Ceftolozane-tazobactam | 30/10 | ≤ 2 | ||
| Ciprofloxacin | 2.5 | ≤ 1 | ||
| Doripenem | 10 | ≤ 4 | ||
| Ertapenem | 10 | ≤ 4 | ||
| Florfenicolh | 30 | ≤ 8 | ||
| Fosfomycing | 200 | ≤ 32 | ||
| Gentamicin | 10 | 4 mm | ≤ 4 | |
| Imipenem | 10 | ≤ 4 | ||
| Marbofloxacinh | 5 | ≤ 2 | ||
| Meropenem | 5 | ≤2 | ||
| Moxifloxacin | 2.5 | ≤ 1 | ||
| Norfloxacing | 10 | ≤ 4 | ||
| Piperacillin-tazobactam | 50/5 | ≤ 16/2 | ||
| Polymyxin B | 300 u | 4 mm | ≤ 2 | |
| Pradofloxacinh | 5 | ≤ 2 | ||
| Ticarcillinh | 75 | ≤ 32 | ||
| Ticarcillin-clavulanic acid | 75/10 | ≤ 16/2 | ||
| Tobramycin | 10 | 4 mm | ≤ 4 | |
| Trimethoprim | 5 | ≤ 4 | ||
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | ≤ 2/38 | ||
| Stenotrophomonas maltophiliao (Sensitest, air, 35‑37°C) |
||||
| Trimethoprim-sulfamethoxazoleo | 1.25/23.75 | ≤ 2/38 | ||
a The standard 6 mm cut‑off applies where no exception has been specified.
g For testing urine isolates only.
h Antibiotic calibrated for veterinary medicine.
m Pasteurella multocida is tested against ampicillin 5 μg and NOT benzylpenicillin 0.5 u.
n Burkholderia pseudomallei is usually susceptible to amoxycillin-clavulanic acid and can be tested against this antibiotic.
o See Section 5.10 for notes on testing antibiotic susceptibilities of Sulphonamide resistant Stenotrophomonas maltophilia.
Table 11.1.c. Calibrations: Anaerobic Organisms.
Antibiotics, disc potencies, annular radii and MIC for susceptible strains, media and incubation conditions.
| Antibiotic | Disc potency (μg) | Exception to the standard interpretationa | MIC (mg/L) | |
| Anaerobic organismsb (Supplemented Brucella Medium Base, anaerobic, 35‑37°C)c |
||||
| Amoxycillin-clavulanic acid | 2/1 | 4 mm | ≤ 2/1 | |
| Benzylpenicillin | 0.5 u | ≤ 0.25 | ||
| Cefoxitin | 10 | ≤ 4 | ||
| Clindamycind | 2 | 4 mm | ≤ 0.5 | |
| Meropenem | 5 | ≤ 2 | ||
| Metronidazole | 5 | 4 mm | ≤ 2 | |
| Moxifloxacin | 2.5 | 4 mm | ≤ 2 | |
| Piperacillin-tazobactam | 50/5 | ≤ 8/2 | ||
a The standard 6 mm cut off applies where no exception has been specified
b Slow growers should be incubated for 48 h.
c Brucella Medium Base containing 5% defibrinated horse blood, haemin 5 mg/L and vitamin K 1 mg/L.
d An erythromycin 5µg disc must be placed 13mm edge to edge to detect inducible clindamycin resistance as described in section 4.7.
11.2. Surrogate discs
Table 11.2.a. Surrogate disc testing: Gram Positive Organisms
Antibiotics that can be reported based on susceptibility results obtained with a surrogate disc.
| Antibiotic reported | Surrogate disc used | Disc potency (ug) |
| Corynebacterium species | ||
| Amoxycillin/Ampicillin/Penicillin V | Benzylpenicillin | 0.5 u |
| Azithromycin/Clarithromycin/Roxithromycin | Erythromycin | 5 |
| Carbapenems | Benzylpenicillin | 0.5 u |
| Cefovecina /Ceftiofura /other Cephalosporinsb | Benzylpenicillin | 0.5 u |
| Enrofloxacina /Orbifloxacina | Moxifloxacin | 2.5 |
| Lincomycin | Clindamycin | 2 |
| Norfloxacinc | Ciprofloxacin | 2.5 |
| Pristinamycin | Quinupristin/Dalfopristin | 15 |
| Tetracyclines | Tetracycline | 10 |
| Tylosina | Erythromycin | 5 |
| Enterococci | ||
| Amoxycillin/Benzylpenicillin | Ampicillin | 5 |
| Enrofloxacina /Orbifloxacina | Moxifloxacin | 2.5 |
| Pristinamycind | Quinupristin/Dalfopristin | 15 |
| Listeria species | ||
| Amoxycillin/Benzylpenicillin | Ampicillin | 5 |
| Staphylococci (except S. saprophyticus from urine) | ||
| Amoxycillin/Ampicillin/Penicillin V | Benzylpenicillin | 0.5 u |
| Amoxycillin-clavulanic acid | Oxacilline/Cefoxitinf | 1/10 |
| Azithromycin/Clarithromycin/Roxithromycin | Erythromycin | 5 |
| Carbapenems | Oxacilline/Cefoxitinf | 1/10 |
| Cefovecina /Ceftiofura /other Cephalosporinsb | Oxacilline/Cefoxitinf | 1/10 |
| Cloxacillin/ Dicloxacillin/ Flucloxacillin | Oxacilline/Cefoxitinf | 1/10 |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Framycetina | Neomycin | 30 |
| Lincomycin | Clindamycin | 2 |
| Norfloxacinc/ Ofloxacin | Ciprofloxacin | 2.5 |
| Pristinamycin | Quinupristin/Dalfopristin | 15 |
| Tetracyclines | Tetracycline | 10 |
| Tylosina | Erythromycin | 5 |
| Staphylococcus saprophyticus from urine | ||
| Amoxycillin/Benzylpenicillin/Penicillin V | Ampicillin | 5 |
| Amoxycillin-clavulanic acid | Cephalexin | 100 |
| Cefovecina/Ceftiofura/other Cephalosporinsb | Cephalexin | 100 |
| Cloxacillin/Dicloxacillin/Flucloxacillin | Cephalexin | 100 |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Norfloxacinc/Ofloxacin | Ciprofloxacin | 2.5 |
| Tetracyclines | Tetracycline | 10 |
a Antibiotic used in veterinary medicine only.
b Ceftazidime is inactive against Gram positive organisms.
c Reporting of norfloxacin is for urine isolates only.
d Enterococcus faecalis is intrinsically resistant to pristinamycin and quinupristin/dalfopristin.
e Oxacillin is used for testing coagulase‑negative staphylococci (except Staphylococcus saprophyticus and Staphylococcus lugdunensis).
f Cefoxitin is used for testing Staphylococcus aureus and Staphylococcus lugdunensis.
Table 11.2.a. Surrogate disc testing: Gram Positive Organisms cont.
Antibiotics that can be reported based on susceptibility results obtained with a surrogate disc.
| Antibiotic reported | Surrogate disc used | Disc potency (ug) | ||
| Streptococcig | ||||
| Amoxycillin/ amoxycillin-clavulanic acid/ Ampicillin/Penicillin V | Benzylpenicillin | 0.5 u | ||
| Amoxycillin/ amoxycillin-clavulanic acid / Benzylpenicillin | Ampicillinh | 5 | ||
| Amoxycillin-clavulanic acidi | Benzylpenicillin | 0.5 u | ||
| Amoxycillin-clavulanic acidi | Ampicillin | 5 | ||
| Azithromycin/Clarithromycin/Roxithromycin | Erythromycin | 5 | ||
| Carbapenems | Benzylpenicillin | 0.5 u | ||
| Cefovecina/Ceftiofura | Benzylpenicillin | 0.5 u | ||
| Cefovecina/Ceftiofura | Cefotaximej | 5 | ||
| Cephalosporins (except Ceftiofur)a | Cefotaxime/Ceftriaxonek | 0.5/5 | ||
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 | ||
| Flucloxacillin | Benzylpenicillin | 0.5 u | ||
| Lincomycin | Clindamycin | 2 | ||
| Pristinamycin | Quinupristin/Dalfopristin | 15 | ||
| Tetracyclines | Tetracycline | 10 | ||
| Tylosina | Erythromycin | 5 | ||
a Antibiotic used in veterinary medicine only.
g For streptococci groups A, B, C, G and Streptococcus anginosus group, the susceptibility to benzylpenicillin, ampicillin, amoxycillin and cephalosporin antibiotics (except ceftazidime) is extrapolated from the testing of benzylpenicillin 0.5 u.
h NOT for testing Streptococcus pneumoniae from CSF. Test if isolate is resistant to benzylpenicillin 0.5 u, cefotaxime 0.5 mg or ceftriaxone 0.5 mg and reported as having a decreased susceptibility.
i For the reporting of all Streptococcus species including S. pneumoniae from sites other than CSF. If the organism is resistant to benzylpenicillin 0.5 u and susceptible to Ampicillin 5 µg, report as having a decreased susceptibility.
j If the organism is resistant to benzylpenicillin 0.5 u and susceptible to cefotaxime 5 µg, report as having a decreased susceptibility to Cefovecina/Ceftiofura.
k If the organism is resistant to cefotaxime/ceftriaxone 0.5 and susceptible to cefotaxime/ceftriaxone 5µg, report as having a decreased susceptibility.
Table 11.2.b. Surrogate disc testing: Gram Negative Organisms
Antibiotics that can be reported based on susceptibility results obtained with a surrogate disc.
| Antibiotic reported | Surrogate disc used | Disc potency (ug) |
| Acinetobacter, Enterobacteriaceae & Vibrionaceae | ||
| Amoxycillin | Ampicillin | 25 |
| Cefovecin /Ceftiofura | Cefotaxime | 5 |
| Cephalothinb | Ampicillin | 25 |
| Ceftriaxone | Cefotaxime | 5 |
| Cefotaxime | Ceftriaxone | 5 |
| Colistin | Polymyxin B | 300 u |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Framycetina | Neomycin | 30 |
| Ofloxacin | Ciprofloxacin | 2.5 |
| Piperacillin | Ampicillin | 25 |
| Tetracycliness | Tetracycline | 10 |
| Ticarcillina | Ampicillin | 25 |
| Moraxella catarrhalis | ||
| Azithromycin/Clarithromycin/Roxithromycin | Erythromycin | 5 |
| Amoxycillin/Ampicillin/Penicillin V | Benzylpenicillin | 0.5 u |
| Amoxycillin-clavulanic acid | Cefuroxime/Cefaclor | 30 |
| Cefovecina/Ceftiofura | Cefuroxime/Cefaclor | 30 |
| Cephalosporins | Cefuroxime/Cefaclor | 30 |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Ofloxacin | Ciprofloxacin | 2.5 |
| Tetracyclines | Tetracycline | 10 |
| Campylobacter species | ||
| Azithromycin /Clarithromycina/Roxithromycina | Erythromycin | 5 |
| Enrofloxacina/Orbifloxacina/Ofloxacin | Ciprofloxacin | 2.5 |
| Tetracyclines | Tetracycline | 10 |
| Haemophilus influenzae/Haemophilus species | ||
| Amoxycillin | Ampicillin | 5 |
| Cefepime | Cefotaxime/Ceftriaxone | 0.5 |
| Cefotaxime | Ceftriaxone | 0.5 |
| Cefpirome | Cefotaxime/Ceftriaxone | 0.5 |
| Ceftazidime | Cefotaxime/Ceftriaxone | 0.5 |
| Cefovecina/Ceftiofura | Cefuroxime/Cefaclor | 30 |
| Ceftriaxone | Cefotaxime | 0.5 |
| Cephalexin | Cefuroxime/Cefaclor | 30 |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Ofloxacin | Ciprofloxacin | 2.5 |
| Tetracyclines | Tetracycline | 10 |
| Helicobacter pylori | ||
| Clarithromycin | Erythromycin | 5 |
| Rifabutin | Rifampicin | 5 |
a Antibiotic used in veterinary medicine only
b Not for Acinetobacter species.
Table 11.2.b. Surrogate disc testing: Gram Negative Organisms cont.
Antibiotics that can be reported based on susceptibility results obtained with a surrogate disc.
| Antibiotic reported | Surrogate disc used | Disc potency (ug) |
| Neisseria meningitidis | ||
| Ampicillin/Amoxycillin | Benzylpenicillin | 0.5 u |
| Cefotaxime | Ceftriaxone | 0.5 |
| Cefovecina/Ceftiofura | Benzylpenicillin | 0.5 u |
| Ceftriaxone | Cefotaxime | 0.5 |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Pasteurella species | ||
| Amoxycillin/Benzylpenicillin | Ampicillin | 5 |
| Amoxycillin-clavulanic acid | Ampicillin | 5 |
| Ampicillin/Amoxycillinc | Benzylpenicillin | 0.5 u |
| Cephalexin | Ampicillin | 5 |
| Cefovecina/Ceftiofura | Ampicillin | 5 |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Norfloxacin /Ofloxacin | Ciprofloxacin | 2.5 |
| Tetracyclines | Tetracycline | 10 |
| Pseudomonas species, Burkholderia species & Chryseobacterium species | ||
| Colistin | Polymyxin B | 300 u |
| Enrofloxacina/Orbifloxacina | Moxifloxacin | 2.5 |
| Norfloxacin/Ofloxacin | Ciprofloxacin | 2.5 |
a Antibiotic used in veterinary medicine only.
c Pasteurella multocida is tested against ampicillin 5 μg and NOT benzylpenicillin 0.5 u.
11.3. Quality assurance
Table 11.3.a. Reference strains: Gram Positive Organisms
Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa.
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | |
| Enterococcus faecalis ATCC 29212
(Blood Sensitest, CO2, 35°C) |
|||
| Ampicillin | 5 | 6.2 – 9.8 | |
| Chloramphenicol | 30 | 6.9 – 9.8 | |
| Doxycycline | 30 | 5.7 – 8.2 | |
| Fosfomycin | 200 | 9.6 – 12.3 | |
| Gentamicin | 200 | 8.0 – 11.0 | |
| Linezolid | 10 | 6.9 – 9.7 | |
| Nitrofurantoin | 200 | 6.1 – 8.7 | |
| Streptomycin | 300 | 7.1 – 8.6 | |
| Teicoplanin | 15 | 3.7 – 6.1 | |
| Tetracycline | 10 | – b | |
| Tigecycline | 15 | 6.8 – 10.6 | |
| Vancomycin | 5 | 2.5 – 4.0 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar and discs. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
b Tetracycline should be tested against Staphylococcus aureus NCTC 6571 or Escherichia coli ATCC 25922.
Table 11.3.a. Reference strains: Gram Positive Organisms cont.
Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa.
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | ||
| Staphylococcus aureus ATCC 9144 (NCTC 6571)
(Sensitest & Blood Sensitest, air 35°C) |
||||
| Sensitest | Blood Sensitest | |||
| Amoxycillinb | 2 | 10.6 – 17.4 | 9.0 – 11.6 | |
| Ampicillin | 5 | 11.9 – 18.7 | 10.8 – 14.5 | |
| Benzylpenicillin | 0.5 u | 10.7 – 16.7 | 9.5 – 12.7 | |
| Cefoxitin | 10 | 7.1 – 10.5 | 7.9 – 9.7 | |
| Ceftaroline | 5 | 11.0 – 16.6 | 10.2 – 13.2 | |
| Cephalexin | 100 | 12.0 – 15.7 | 11.9 – 14.7 | |
| Chloramphenicol | 30 | 8.6 – 11.7 | 8.6 – 10.7 | |
| Ciprofloxacin | 2.5 | 9.8 – 11.9 | 9.2 – 11.8 | |
| Clindamycin | 2 | 9.4 – 11.5 | 8.1 – 11.2 | |
| Erythromycin | 5 | 8.5 – 11.8 | 9.0 – 10.6 | |
| Florfenicolc | 30 | 9.4 – 12.4 | – | |
| Fusidic acid | 2.5 | 9.4 – 12.2 | 6.3 – 8.0 | |
| Gentamicin | 10 | 7.7 – 9.9 | 7.5 – 8.9 | |
| Linezolid | 10 | 8.4 – 11.4 | 8.5 – 11.5 | |
| Marbofloxacinc | 5 | 9.5 – 11.5 | 8.9 – 10.5 | |
| Moxifloxacin | 2.5 | 11.2 – 14.0 | 10.8 – 13.0 | |
| Mupirocin | 5 | 7.8 – 11.0 | 8.1 – 11.6 | |
| Neomycinc | 30 | 7.2 – 9.6 | 7.6 – 9.3 | |
| Nitrofurantoin | 200 | 6.2 – 8.3 | 6.3 – 8.1 | |
| Novobiocinc | 5 | 8.8 – 11.3 | 6.4 – 9.2 | |
| Oxacillin | 1 | 9.0 – 11.3 | 7.9 – 10.6 | |
| Pradofloxacinc | 5 | 11.7 – 15.1 | – | |
| Quinupristin/Dalfopristin | 15 | 8.5 – 11.5 | 7.0 – 10.0 | |
| Rifampicin | 1 | 9.1 – 12.6 | 9.4 – 11.6 | |
| Rifampicinb | 5 | 11.0 – 14.6 | 10.5 – 14.2 | |
| Teicoplanin | 15 | 3.9 – 6.1 | 4.1 – 6.0 | |
| Tetracycline | 10 | 10.1 – 13.7 | 10.1 – 13.3 | |
| Tigecycline | 15 | 9.4 – 11.7 | 9.2 – 11.6 | |
| Trimethoprim | 5 | 9.2 – 12.0 | 9.2 – 11.0 | |
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | 10.9 – 13.6 | 10.9 – 13.1 | |
| Vancomycin | 5 | 3.4 – 4.9 | 3.0 – 4.8 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of agar, discs and measuring devices. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
b The amoxycillin 2 µg and rifampicin 5 µg discs are used for the susceptibility testing of H. pylori only.
c Antibiotic used in veterinary medicine only.
Table 11.3.a. Reference strains: Gram Positive Organisms cont.
Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa.
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | |
| Streptococcus pneumoniae ATCC 49619
(Blood Sensitest, 5% CO2, 35°C) |
|||
| Ampicillin | 5 | 10.7 – 15.4 | |
| Benzylpenicillin | 0.5 u | 2.8 – 6.9 | |
| Cefotaxime | 0.5 | 7.1 – 10.3 | |
| Cefotaxime | 5 | 10.7 – 14.7 | |
| Ceftriaxone | 0.5 | 7.3 – 10.8 | |
| Ceftriaxone | 5 | 10.4 – 14.8 | |
| Chloramphenicol | 30 | 9.3 – 14.1 | |
| Clindamycin | 2 | 7.6 – 11.5 | |
| Erythromycin | 5 | 8.7 – 12.6 | |
| Florfenicolc | 30 | 11.4 – 13.9 | |
| Moxifloxacin | 2.5 | 8.3 – 12.1 | |
| Quinupristin/Dalfopristin | 15 | 7.1 – 9.4 | |
| Rifampicin | 1 | 8.2 – 12.5 | |
| Teicoplanin | 15 | 5.4 – 9.9 | |
| Tetracycline | 10 | 9.5 – 14.8 | |
| Tigecycline | 15 | 8.2 – 13.7 | |
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | 7.2 – 10.7 | |
| Vancomycin | 5 | 5.4 – 8.9 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar, discs and measuring devices. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
c Antibiotic used in veterinary medicine only.
Table 11.3.b. Reference strains: Gram Negative Organisms
Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa.
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | |
| Bacteroides fragilis ATCC 25285 (Blood Sensitest, anaerobic, 35‑37°C) |
|||
| Metronidazoleb | 5 | 10.2 – 13.8 | |
| Campylobacter jejuni ATCC 33560 (Blood Sensitest, microaerophilic, 42°C) |
|||
| Ciprofloxacin | 2.5 | 9.3 – 12.8 | |
| Erythromycin | 5 | 7.2 – 9.4 | |
| Gentamicin | 10 | 6.8 – 8.9 | |
| Tetracycline | 10 | 8.5 – 10.8 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar, discs and measuring devices. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
b Bacteroides fragilis ATCC 25285 may be used as the reference strain when testing H. pylori against metronidazole. When testing anaerobic organisms, Clostridium perfringens ATCC 13124 should be used as the reference organism.
Table 11.3.b. Reference strains: Gram Negative Organisms cont.
| Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa. |
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | |
| Escherichia coli ATCC 25922c (Sensitest, air, 35–37°C) |
|||
| Amikacin | 30 | 6.7 – 8.9 | |
| Ampicillin | 25 | 7.1 – 9.1 | |
| Azithromycin | 15 | 4.7 – 7.2 | |
| Apramycind | 15 | 4.6 – 5.9 | |
| Aztreonam | 30 | 11.2 – 14.9 | |
| Cefazolin | 30 | 7.7 – 10.6 | |
| Cefepime | 10 | 10.7 – 14.0 | |
| Cefotaxime | 5 | 9.5 – 13.2 | |
| Cefotetan | 30 | 10.3 – 14.4 | |
| Cefoxitin | 30 | 8.2 – 11.1 | |
| Cefpodoxime | 10 | 8.1 – 11.2 | |
| Ceftazidime | 10 | 8.9 – 12.3 | |
| Ceftolozane-tazobactam | 30/10 | 8.9 – 11.7 | |
| Ceftriaxone | 5 | 8.9 – 13.1 | |
| Cefuroxime | 30 | 7.0 – 9.5 | |
| Cephalexin | 100 | 7.1 – 9.6 | |
| Chloramphenicol | 30 | 8.4 – 10.8 | |
| Ciprofloxacin | 2.5 | 11.9 – 15.4 | |
| Doripenem | 10 | 8.8 – 13.4 | |
| Ertapenem | 10 | 8.8 – 14.2 | |
| Fosfomycin | 200 | 8.4 – 11.7 | |
| Florfenicold | 30 | 8.0 – 10.5 | |
| Gentamicin | 10 | 7.0 – 9.7 | |
| Imipenem | 10 | 9.0 – 13.1 | |
| Marbofloxacind | 5 | 11.5 – 15.6 | |
| Meropenem | 5 | 10.2 – 14.0 | |
| Moxifloxacin | 2.5 | 10.3 – 13.5 | |
| Nalidixic acid | 30 | 9.0 – 11.6 | |
| Neomycind | 30 | 5.9 – 8.5 | |
| Nitrofurantoin | 200 | 5.9 – 8.9 | |
| Norfloxacin | 10 | 10.9 – 14.4 | |
| Polymyxin B | 300 u | 4.7 – 6.7 | |
| Pradofloxacind | 5 | 12.3 – 15.6 | |
| Spectinomycind | 25 | 4.7 – 6.4 | |
| Streptomycind | 25 | 5.6 – 7.1 | |
| Tetracycline | 10 | 3.9 – 7.8 | |
| Tigecycline | 15 | 7.9 – 10.8 | |
| Tobramycin | 10 | 6.1 – 8.6 | |
| Trimethoprim | 5 | 7.7 – 10.9 | |
| Trimethoprim-sulfamethoxazole | 1.25/23.75 | 8.7 – 12.3 | |
| Escherichia coli ATCC 35218 (Sensitest, air, 35‑37°C) |
|||
| Amoxycillin-clavulanic acid | 40/20 | 6.9 – 9.3 | |
| Piperacillin-tazobactam | 50/5 | 7.3 – 11.0 | |
| Ticarcillin-clavulanic acid | 75/10 | 7.2 – 9.5 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar, discs and measuring devices. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
c If antibiotic discs are tested with Escherichia coli ATCC 25922, there is no need to test these against Pseudomonas aeruginosa ATCC 27853 as well and vice versa.
d Antibiotic used in veterinary medicine only
Table 11.3.b. Reference strains: Gram Negative Organisms cont.
Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa.
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | |
| Haemophilus influenzae ATCC 49766 (HTMe agar, 5% CO2, 35‑37°C) |
|||
| Ampicillin | 5 | 5.9 – 10.7 | |
| Cefaclor | 30 | 8.9 – 13.0 | |
| Cefotaxime | 0.5 | 9.9 – 12.6 | |
| Cefotaxime | 5 | 13.1 – 16.2 | |
| Cefpodoxime | 10 | 11.1 – 15.1 | |
| Ceftriaxone | 0.5 | 10.6 – 13.8 | |
| Ceftriaxone | 5 | 13.5 – 16.6 | |
| Cefuroxime | 30 | 9.9 – 13.5 | |
| Chloramphenicol | 10 | 11.4 – 13.8 | |
| Ciprofloxacin | 2.5 | 12.6 – 16.7 | |
| Moxifloxacin | 2.5 | 11.6 – 15.0 | |
| Tetracycline | 10 | 8.9 – 12.2 | |
| Trimethoprime-sulfamethoxazole | 1.25/23.75 | 9.8 – 15.0 | |
| Haemophilus influenzae ATCC 35056 (HTMe agar, 5% CO2, 35‑37°C) |
|||
| Amoxycillin-clavulanic acid | 10/5 | 7.6 – 9.9 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar, discs and measuring devices. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
e Haemophilus Test Medium Base containing 15 mg/L freshly prepared Haematin and NAD.
Table 11.3.b. Reference strains: Gram Negative Organisms cont.
Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa.
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | |
| Pseudomonas aeruginosa ATCC 27853f (Sensitest, air, 35‑37°C) |
|||
| Amikacin | 30 | 7.4 – 8.9 | |
| Aztreonam | 30 | 8.1 – 10.8 | |
| Cefepime | 10 | 7.1 – 10.5 | |
| Ceftazidime | 10 | 7.6 – 10.4 | |
| Ceftolozane-tazobactam | 30/10 | 8.4 – 11.7 | |
| Ciprofloxacin | 2.5 | 9.9 – 12.6 | |
| Doripenem | 10 | 9.8 – 15.4 | |
| Ertapenem | 10 | – g | |
| Fosfomycin | 200 | 6.8 – 10.0 | |
| Gentamicin | 10 | 6.2 – 7.8 | |
| Imipenem | 10 | 8.0 – 10.0 | |
| Meropenem | 5 | 9.9 – 12.7 | |
| Moxifloxacin | 2.5 | – g | |
| Norfloxacin | 10 | 10.1 – 12.1 | |
| Polymyxin B | 300 u | 4.4 – 6.3 | |
| Ticarcillin | 75 | 7.9 – 10.3 | |
| Tobramycin | 10 | 7.5 – 9.2 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar, discs and measuring devices. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
f If antibiotic discs are tested with Escherichia coli ATCC 25922 there is no need to test these against Pseudomonas aeruginosa ATCC 27853 as well and vice versa.
g Ertapenem and moxifloxacin should be tested against Escherichia coli ATCC 25922.
Table 11.3.c. Reference strains: Anaerobic Organisms
Antibiotics, disc potencies and acceptable zones of inhibition for reference strainsa.
| Antibiotic reported | Disc potency (μg) | Annular radii (mm) | |
| Clostridium perfringens ATCC 13124 (Supplemented Brucella Medium Base, anaerobic, 35‑37°C)b |
|||
| Benzylpenicillin | 0. 5 u | 6.1 – 8.2 | |
| Cefoxitin | 10 | 6.7 – 10.4 | |
| Clindamycin | 2 | 6.2 – 8.7 | |
| Meropenem | 5 | 10.5 – 14.3 | |
| Metronidazole | 5 | 4.8 – 8.6 | |
| Moxifloxacin | 2.5 | 4.8 – 6.9 | |
| Bacteroides fragilis ATCC 25285 (Supplemented Brucella medium Base, anaerobic, 35‑37°C)b |
|||
| Amoxycillin-clavulanic acid | 2/1 | 7.3 – 10.1 | |
| Piperacillin-tazobactam | 50/5 | 10.3 – 14.2 | |
| Ticarcillin-clavulanic acid | 75/10 | 13.9 – 19.0 | |
Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar, discs and measuring devices. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.
a Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.
b Brucella Medium Base containing 5% defibrinated horse blood, haemin 5 mg/L and vitamin K 1 mg/L.
11.4 Testing and reporting β‑lactam antibiotics
Table 11.4.a. A guide to the testing and reporting of β‑lactam antibiotics for Gram negative organisms.
a R = The isolate is resistant to the antibiotic because it possesses a mechanism of resistance that may not be demonstrated by disc testing.
T = Can be tested and reported.
T*= See section 5.1 on Aeromonas & section 5.5.7 on Morganella morganii.
U = Test isolates from uncomplicated UTI only. Isolates from other sites are considered RESISTANT.
b
| AMP | ampicillin | CTX | cefotaxime | DOR | doripenem | KZ | cefazolin |
| AMC | amoxycillin-clavulanic acid | CL | cephalexin | ETP | ertapenem | MEM | meropenem |
| ATM | aztreonam | CPD | cefpodoxime | FEP | cefepime | TIM | ticarcillin-clavulanic acid |
| CAZ | ceftazidime | CRO | ceftriaxone | FOX | cefoxitin | TZP | piperacillin-tazobactam |
| CXM | cefuroxime | CTT | cefotetan | IPM | imipenem |
c Enterobacter aerogenes, Enterobacter cloacae complex, Citrobacter freundii complex.
d A. caviae does not possess a carbapenemase (A2) and can be tested against imipenem, meropenem, ertapenem and doripenem.
e Isolates with high β-lactamase activity may give no zone around CTX 5 µg but show a “key-hole” effect that may be mistaken as an indication of the presence of an ESBL. However, unlike ESBL producers, they may be susceptible to ceftazidime.
f This group varies in susceptibility to β-lactams – see sections 5.5.3 & 5.5.7.
g B. pseudomallei is usually susceptible to amoxycillin-clavulanic acid and can be tested against this antibiotic.
h See section 5.10 for notes on testing S. maltophilia.