The Urine Nitrite Dipstick Test: Potential Usefulness for Guiding Empiric Antibiotic Therapy for Patients (pts) Admitted with Urinary Tract Infections (UTI)

Background: Escherichia coli and other enteric gram-negatives are the most common causative agents of UTI; these organisms are able to produce nitrite from nitrate in contrast to Enterococcus, a less frequently encountered but also important cause of UTI, which does not reduce nitrate. Enterococci are resistant to third-generation cephalosporins and other agents that are commonly used to treat UTI. Results of the urine nitrite dipstick test may help clinicians with the choice of empiric antibiotic therapy for pts with suspected UTI, but few data are available from the more recent literature.

Objective: To determine if the urine nitrite dipstick test is useful to differentiate between UTI caused by Enterococcus and UTI caused by E. coli among newly hospitalized pts.

Methods: A cross-sectional study was performed on all pts who were hospitalized with an enterococcal UTI at an 800-bed tertiary care center between 1/08 and 7/10. Pts with enterococcal UTI were compared to a randomly selected cohort of pts admitted with E. coli UTI with regard to a variety of demographic, laboratory, and clinical characteristics. Inclusion criteria for all patients comprised 1. Presence of pyuria defined as >10 leukocytes per high-power field on urine microscopy; 2. A urine culture was obtained within 24 hours after admission and revealed more than 10⁴ colony-forming units per ml; 3. Presence of clinical symptoms of UTI; and 4. Adequate antimicrobial therapy was provided. Pts whose urine culture revealed more than one organism or who had a UTI within the previous month were excluded.

Results: A total of 24,164 urine cultures were obtained during the study period. 35 pts with enterococcal UTI were identified according to our inclusion criteria and compared to 67 randomly selected pts with E. coli UTI. The median age of the total cohort (N=102) was 78 years, 37 (36%) were male, 32 (31%) had an upper UTI; the median Charlson comorbidity score was two. On bivariate analyses (P<0.05), pts with enterococcal UTI were older (median age, 81 vs. 76 years; P=0.02) and more likely to have an anatomic abnormality of the genitourinary tract (63% vs. 27%; P<0.001) compared to pts with E. coli UTI. The urine dipstick nitrite test was positive in 1 (3%) of 35 pts with enterococcal UTI and 44 (66%) of 67 pts with E. coli UTI (P<0.0001). Thus, a positive nitrite test correctly predicted an E. coli UTI in 44 (98%) of 45 pts; however, a negative test result was predictive of an enterococcal UTI in only 34 (68%) of 57 episodes.

Conclusions: A positive urine nitrite dipstick test was rare in pts with enterococcal UTI. Based on these results, clinicians may consider omitting empiric anti-enterococcal coverage in non-critically ill pts who are hospitalized with a suspected UTI and have a positive urine nitrite test.