INEXPENSIVE 4-HOUR MICRO-AGAR DILUTION SUSCEPTIBILITY DETERMINATION METHOD

Using a micro-agar dilution (MAD) method in which microscope slides: a re covered with a thin film of agar, and MICs are read microscopically after a 4-h incubation, 18 antibiotics were tested against 29 to 32 m icroorganisms each, Identical MICs were obtained for microscopic MAD M ICs performed in duplicate in 87.1% of the antibiotic-microorganism co mbinations, and 97.9% were identical within one dilution. When read ma croscopically after an 18-h incubation, identical duplicate MICs were obtained in 86.8% of the cases, and 98.4% were identical within one di lution. Using agar dilution as the ''gold standard,'' the correlation obtained with MAD slides read microscopically at 4 h was 94.3%, and ma croscopic correlation at 18 h was 97.6%., The correlation of MAD slide s with agar dilution for the groups of microorganisms most frequently used was as follows (microscopic/macroscopic): Staphylococcus aureus 9 6%/98%; Streptococcaceae 97%/98%; Enterobacteriaceae 98%/99%; and Pseu domonadaceae 95%/98%, At the present rate of exchange (A 1.60 = $1.00) , the cost of a MAD slide, including labor, is $1.28 (20 microorganism s tested) or $0.06 per microorganism-antibiotic combination tested. Th is method is easy to perform, rapid, and inexpensive, It is suitable f or use in routine and research laboratories.