COMPARISON OF DIRECT AND STANDARDIZED TESTING OF INFECTED URINE FOR ANTIMICROBIAL SUSCEPTIBILITIES BY DISK DIFFUSION

A total of 14,272 urine specimens were examined over one year to deter mine the validity of direct antimicrobial agent susceptibility testing against ampicillin, amoxicillin-clavulanic acid, cephalothin, gentami cin, norfloxacin, and trimethoprim. A comparison between direct and st andardized disk diffusion tests was made for a total of 1, 106 urine s pecimens containing greater-than-or-equal-to 10(5) organisms per ml in pure culture. There were 5,821 individual organism-antimicrobial agen t challenges compared for the two testing methods, and there was compl ete agreement of susceptibility category in 5,492 comparisons (94.3%). Initially, discordant results were reduced from 5.7 to 2.1% when the intermediate category was considered susceptible. Intralaboratory vari ation was assessed by testing another 453 organisms by the standard Na tional Committee for Clinical Laboratory Standards (NCCLS) method on t wo consecutive days; there was complete agreement in 96.1% of comparis ons. When results of direct and standardized testing were simply class ified as susceptible or resistant, there was 1.1% discordance. When si mple same-day tests were used together with predictable patterns of su sceptibility and resistance, 536 (48.5%) of 1,106 isolates could be id entified satisfactorily to the genus or species level. For laboratory reporting purposes, the direct method is equivalent to the standard me thod when the urine being tested is infected with greater-than-or-equa l-to 10(5) organisms of a single type per ml. The presence or absence of preexisting antimicrobial agents in urine did not appreciably influ ence the results. This procedure allows the earlier reporting of susce ptibility results and facilitates less expensive identification of man y organisms. Costs and benefits need to be determined in each institut ion.