A comparison of antibiotic susceptibility profiles using single and multiple isolates per patient

We compared the antibiogram statistics generated by including all clinical isolates to those obtained by tabulating no more than one isolate of a particular organism from a given patient. We found that 48.3% of the isolates presented multiple occurrences in individual patients but found no practical differences in the profiles obtained by the two methods. We also tabulated the occurrence of minimum inhibitory concentration (MIC)-distinct organisms among the sets of multiple isolates of E. coli and of S. aureus and found that only 20.9% of those isolates represented duplicate organisms by MIC profile. This heterogeneity of MIC sensitivity that occurs in multiple isolates from individual patients was not expected but provides an explanation for the lack of difference between the two methods of tabulating antibiotic susceptibility statistics.