Evaluation of the Sirscan automated zone reader in a clinical microbiologylaboratory

We compared readings of Kirby-Bauer plates by the Sirscan, an automated ima ge analyzer that measures zone diameters, to those of experienced clinical microbiologists measuring zones with a hand-held caliper interfaced to a co mputer and with a ruler. To read plates of Escherichia coli, Morganella mor ganii, and Pseudomonas aeruginosa containing 12 antibiotic disks the Sirsca n took 11 s; technologists took 28 s by caliper and 39 s by ruler. Reading times of four different technologists ranged from 22 to 44 s with the calip er and 10 to 12 s with Sirscan. Upon repeated testing zone size variation r arely exceeded 3 mm by caliper and 1 mm by Sirscan, over a 4-month period, 368 clinical isolates were tested prospectively by both methods in the Clin ical Microbiology Laboratory of the Miriam Hospital, There was good correla tion of zone sizes for most antibiotics, but Sirscan zone diameter measurem ents tended to be 3 to 5 mm larger than caliper readings for ciprofloxacin, norfloxacin, aztreonam, erythromycin, clindamycin, and trimethoprim-sulfam ethoxazole. Very major errors (resistant by caliper and susceptible by Sirs can) occurred with 10 of 3,770 readings (0.3%), mainly where breakpoint cri teria lacked an intermediate zone. They occurred in testing staphylococci w ith amoxicillin-clavulanate (5 of 127 isolates, 3.9%), pseudomonas with pip eracillin (1 of 28, 3.6%), coagulase-negative staphylococci with oxacillin (2 of 74, 2.75), gram-negative bacilli with cefuroxime (1 of 209, 0.5%), an d mixed species with trimethoprim-sulfamethoxazole (1 of 366, 0.3%). The Si rscan zone reader facilitates accurate, fully quantitative susceptibility t esting in clinical microbiology laboratories.