TABLE 1.

Stability of imipenem disks with 1,000 μg of EDTA added during storage as determined by change of inhibition zone diameter

Storage (wk)	Diam (mm) of:
	IMPb-EDTA disk zone										IMP disk zone, P. aeruginosa ATCC 27853a
	IMP resistant, MBL positive				IMP resistant, MBL negative				P. aeruginosa ATCC 27853
	P. aeruginosa		A. baumannii		P. aeruginosa		A. baumannii
	4°C	−20°C	4°C	−20°C	4°C	−20°C	4°C	−20°C	4°C	−20°C
Immediate	24	24	23	23	15	15	17	17	24	24	27
1	23	23	22	23	12	13	15	15	22	23	24
4	21	22	22	22	13	13	14	16	21	22	25
8	20	21	22	23	13	12	15	15	20	21	25
12	21	24	23	23	11	13	14	16	20	22	22
16	20	23	23	25	11	13	9	15	19	22	25
24	18	23	22	23	11	12	12	13	19	22	25
Mean	21.0	22.9	22.4	23.1	12.3	13.0	13.7	15.3	20.7	22.3	24.7

^aRoutine quality control results with disks stored as recommended by the National Committee for Clinical Laboratory Standards. The acceptable imipenem disk zone diameter is 20 to 28 mm.

^bIMP, imipenem.