Correlation of cefpodoxime susceptibility with cephalothin, cefuroxime, and ceftriaxone for urinary tract isolates

Purpose: Current guidelines recommend that cefpodoxime susceptibility should be extrapolated from the cephalothin result for urinary tract isolates.  The purpose of the study was to determine whether cefuroxime or ceftriaxone was superior to cephalothin as a surrogate marker for cefpodoxime susceptibility.

Methods: Automated susceptibility testing for cephalothin, cefuroxime and ceftriaxone was conducted on consecutive urine cultures with a colony count of at least 50,000 organisms via the Microscan^© system.  Pseudomonas, Enterococcus, Acinetobacter and Methicillin-resistant Staphylococcus isolates were excluded.  Simultaneously, a manual E-test for cefpodoxime was placed on the culture plate, the minimum inhibitory concentration was determined, and susceptibility was based on the FDA-approved breakpoints.  The interpretation for cefpodoxime was compared to that of the other 3 agents, and a correlation rate was calculated defined as the percentage of identical susceptibility interpretations.  Chi-square was used for comparisons.

Results: A total of 292 isolates were assessed.  The correlation rate was 93% for ceftriaxone, 91% for cefuroxime, and 63% for cephalothin.  The concordance of both ceftriaxone and cefuroxime were superior to cephalothin (p < 0.01).  Of 107 discordant isolates, 43 tested resistant and 59 were intermediate to cephalothin while testing susceptible to cefpodoxime.   Thirteen isolates susceptible to cefuroxime were either resistant (9) or intermediate (4) to cefpodoxime.  Conversely, 11 isolates susceptible to cefpodoxime were either resistant (9) or intermediate (2) to cefuroxime.   For ceftriaxone, 17 susceptible isolates were either resistant (13) or intermediate (4) to cefpodoxime.    Three isolates were susceptible to cefpodoxime but not ceftriaxone.  The likelihood of inaccurately extrapolating susceptibility was higher for ceftriaxone than cefuroxime (p = 0.02).

Conclusion: Both cefuroxime and ceftriaxone were better predictors of cefpodoxime susceptibility than cephalothin.  Cefuroxime appears to be the preferred surrogate agent for interpreting cefpodoxime urinary susceptibilities.