Limits of random variability in disk diffusion test results for assessing bacterial antibiotic susceptibility

Matveev V.A., Skipor T.E.

Vitebsk State Medical University, Vitebsk, Belarus
Mogilev Regional Children's Hospital, Mogilev, Belarus

Objective. To determine the control limits of random variation in the results of disk diffusion method (DDM) when assessing bacterial susceptibility to antibiotics.

Material and Methods. A total of 280 DDM tests were performed using Gram-negative and Gram-positive control strains: E. coli ATCC 25922, P. aeruginosa ATCC 27853, and Staphylococcus aureus ATCC 25923. Nine and four antibiotics were tested for Gram-negative and Gram-positive bacteria respectively.

Results. It was established that for both Gram (-) and Gram (+) control cultures, the mean square deviation of the diameters of bacterial growth inhibition zones when using all antibiotics was less than 2: 1.64±0.05 and 1.57±0.1 on average, respectively. In all cases the distribution of results was close to normal. The ±2σ limits, within which 95% of the results are expected to fall, amounted to slightly more than ±3 mm in absolute terms.

Conclusion. Considering the limitations of the measurement methodology, a change in the diameter of the bacterial growth inhibition zone of ≥±4 mm should be regarded as significant when using the DDM.