TABLE 3.

Interlaboratory variations among three participating laboratories by two methods with readings of MICs at 24 and 48 h^a

Antimicrobial agent	Agar dilutionb		Broth microdilution		% Acceptable variationc
	24 h	48 h	24 h	48 h	Agar dilution	Broth microdilution
Amikacin	9	9	6	8	96.9	97.6
Gentamicin	10	13	14	12	96.1	95.6
Tobramycin	14	10	13	13	95.9	95.6
Ciprofloxacin	14	15	18	13	95.1	94.7
Piperacillin	22	25	20	22	90.8	92.9
Ticarcillin	17	17	18	15	94.2	94.4
Piperacillin-tazobactam	27	27	30	20	90.8	91.5
Ticarcillin-clavulanate	16	13	17	13	95.1	95.0
Ceftazidime	20	17	24	16	93.7	93.2
Aztreonam	21	16	26	18	93.7	92.5
Imipenem	20	20	18	14	93.2	94.6
Meropenem	15	19	18	21	94.2	93.4
Total no. of variations	205	201	222	185	94.1 (mean)	94.2 (mean)

^aVariations are results more than 1 log₂ dilution from the geometric mean MIC generated by the three participating laboratories. 

^bThe numbers of variations are provided for agar dilution and broth microdilution readings at 24 and 48 h which reflect 294 observations (98 isolates studied by three laboratories) for each agent for each method at each time point. 

^cResults in the three laboratories within 1 log₂ dilution from the geometric mean MIC.