Table 1.
Key differences between the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST) disk diffusion methodologies
| Methodological discrepancy | CLSI | EUCAST | |
|---|---|---|---|
| Incubation temperature | 35 ± 2°C | 35 ± 1°C | |
| Duration of incubation | 16–18 h for Enterobacteriaceae, Staphylococcus spp.a and Pseudomonas aeruginosa; 20–24 h for most other organisms | 16–20 h | |
| Media | MHA supplemented with 5% sheep blood for Streptococcus spp., Campylobacter jejuni/coli, Pasteurella spp. and Neisseria meningitidis; Haemophilus Test Medium for Haemophilus spp.; GC agar base with 1% defined growth supplement for Neisseria gonorrhoeae | MH-F agar for Streptococcus spp., Campylobacter jejuni/coli, Pasteurella spp., H. influenzae, Moraxella catarrhalis, Listeria monocytogenes, Kingella kingae, Aerococcus spp. and Corynebacterium spp. | |
| Antimicrobial disk contents (μg unless otherwise stated) | Amoxicillin–clavulanic acid | 20–10 | 20–10 and 2–1b |
| Ampicillin | 10 | 10 and 2c | |
| Cefotaxime | 30 | 5 | |
| Ceftaroline | 30 | 5 | |
| Ceftazidime | 30 | 10 | |
| Ceftazidime–avibactam | 30–20 | 10–4 | |
| Gentamicind | 120 | 30 | |
| Linezolid | 30 | 10 | |
| Nitrofurantoin | 300 | 100 | |
| Penicillin G | 10 units | 1 unit | |
| Piperacillin | 100 | 30 | |
| Piperacillin–tazobactam | 100–10 | 30–6 | |
| Vancomycin | 30 | 5 | |
| Zone diameter breakpoints | None for Aerococcus spp., Kingella kingae, Listeria monocytogenes, Corynebacterium spp. Rarely aligned with EUCAST |
None for N. gonorrhoeae, N. meningitidis, Burkholderia cepacia, Vibrio spp. Fewer intermediate categories |
|
| ATCC strains for routine disk diffusion quality control |
Staphylococcus aureus ATCC 25923, H. influenzae ATCC 49247 |
S. aureus ATCC 29213, H. influenzae ATCC 49766 |
|
MHA, Mueller–Hinton agar; MH-F agar, MHA supplemented with 5% horse blood and 20 mg/L β-NAD.
a
Except when testing cefoxitin against coagulase-negative staphylococci which requires 24 h.
b
For testing H. influenzae, Pasteurella multocida, Moraxella catarrhalis, and Listeria monocytogenes.
c
For testing H. influenzae, viridans group streptococci, Staphylococcus saprophyticus and Enterococcus spp.
d
For testing high-level resistance in Enterococcus spp.