Evaluation of a Zone Size Chart for Antibiotic Susceptibility Tests by Disk Diffusion

Abstract

A rapid method to determine bacterial susceptibility to antibiotics using the disk diffusion method is described. A series of zone size charts were prepared to determine susceptibility of Gram-negative and Grampositive, rapidly growing pathogens to 17 antibiotics. The chart permitted the susceptibility determinations, based on zone size, to be made in lieu of actual measurement of each zone size with a ruler. Comparison of results obtained with the chart with those obtained by zone measurements showed a 96% correlation against 13,132 Gram-negative organisms; a 98% agreement was found against 8,034 Gram-positive cocci. Discrepancies between the chart and zone measurements are discussed. The chart is reliable and is well adapted for use in most clinical laboratories.