Analytical Variability in the Single Disk Antimicrobial Susceptibility Test

Although one method of performing the single disk test for antimicrobial susceptibility has been identified as producing the most reliable results, insufficient criteria have been established for evaluation and control of precision and accuracy. The authors compared barium sulfate and powdered glass turbidity standards. The latter appeared more stable. Both produced inoculum density of about 1 × 108± 5 × 107 colony-forming units. Plate size and disk location did not produce significant differences in precision and accuracy. Variability observed over an extended period was greater than that observed from day to day. The authors suggest that this may be a clue to interlaboratory disagreement and may be caused by variation in lots of disks, batches of commercial dehydrated culture media, and criteria for measurement of zone diameters.