Diagnosing symptomatic urinary tract infections in infants by catheter urine culture

BACKGROUND: Diagnosing infantile urinary tract infection (UTI) is difficult due to contamination during urine collection. Catheterization is convenient but diagnostic criteria (colony-forming units per millilitre (CFU/mL)) is controversial, especially in uncircumcised males. OBJECTIVES: To study the value of catheter urine cultures in terms of likelihood ratios (LRs), sensitivity, specificity, positive and negative predictive values of different CFU/mL in uncircumcised boys and girls. METHODS: Infants aged 1-18 months who had catheter urine cultures from July 1999 to June 2002 were reviewed to decide if they had symptomatic UTI (group A) or not (group B). Urinary tract infection was confirmed if patients had a positive urine culture plus acute fever, pyuria, positive leucocyte esterase and nitrite tests and good response to antibiotics, with pyelonephritic evidence on early dimercaptosuccinic acid (DMSA) scans in doubtful cases. Group B included infants with negative urine culture results, and those with positive results but were asymptomatic and admitted for micturating cystourethrogram. RESULTS: Nine hundred and fifty-two patients were studied (492 boys, 460 girls; 212 in group A, 740 in group B). No single cut-off CFU/mL has high sensitivity and specificity to simultaneously diagnose and exclude UTI. The CFU counts of 100-10(3), 10(3)-10(4), 10(4)-10(5) and >10(5) were associated with LRs of 0.11, 0.45, 1.52 and 20.5, respectively in uncircumcised boys, and with LRs of 1.39, 2.49, 8.95 and 18.8, respectively in girls. The LR for mixed growths was 0.21. CONCLUSION: Unlike suprapubic tap urine, catheter urine culture has to be interpreted against the clinical context or pretest probability and in terms of probability. In the scenario of a febrile infant where the pretest probability of UTI was about 5%, UTI was highly likely if counts exceeded 10(5)/mL, and unlikely if counts were below 10(4)/mL in uncircumcised boys. In female infants, UTI was highly likely if counts were >10(4) CFU/mL, but lower counts could not exclude UTI.