Potential unreliability of nitrofurantoin disks in susceptibility testing.

The Kirby-Bauer disk diffusion antimicrobial susceptibility test was studied with three lots of commercial nitrofurantoin disks by using 50 strains of Proteus mirabilis. One lot of disks consistently gave zone diameters 3 to 6 mm larger than the other two and resulted in a substantial number of Proteus mirabilis strains being erroneously categorized as sensitive. Antimicrobial content in disks of the three lots did not account for these differences. The pH of eluate from the unreliable disks was higher than from disks of the other two and resulted in more rapid diffusion of the antimicrobial and consequently larger zones of inhibition.