ABSTRACT. Objective. To assess the usefulness of laboratory parameters, including peripheral white blood cell (WBC) count, C-reactive protein (CRP) concentration, erythrocyte sedimentation rate (ESR), and microscopic urinalysis (UA), for identifying febrile infants younger than 8 weeks of age at risk for urinary tract infection (UTI), and comparison of standard UA and hemocytometer WBC counts for predicting the presence of UTI.Methods. A total of 162 febrile children <8 weeks of age were enrolled in this prospective study. All underwent clinical evaluation and laboratory investigation, including WBC count and differential; ESR; CRP; blood culture; a lumbar puncture for cell count and differential, glucose level, protein level, Gram stain, and culture; and a UA and urine culture. All urine specimens were obtained by suprapubic aspiration and microscopically analyzed with standard UA as well as with hemocytometer WBC counts. Quantitative urine cultures were performed. Sensitivity, specificity, accuracy, likelihood ratios, and receiver operating characteristic (ROC) curves were determined for each of the screening tests.Results. There were 22 positive urine culture results of at least 100 colony-forming unit/mL. Eighteen of these 22 patients were males, and all were uncircumcised. There were significant differences for pyuria м5 WBCs/ hpf, pyuria м10 WBC/L, CRP >20 mg/L, and ESR >30 mm/hour between culture-positive and culture-negative groups (P < .05). The ROC area for hemocytometer WBC count, standard UA, peripheral WBC count, ESR, and CRP concentration were .909 ؎ .045, .791 ؎ .065, .544 ؎ .074, .787 ؎ .060, and .822 ؎ .036, respectively. The ROC curve analysis indicates that the CRP, ESR, and standard UA were powerful but imperfect tools with which to discriminate for UTI in potentially infected neonates. Hemocytometer WBC counts had the highest sensitivity, specificity, accuracy, and likelihood ratios for identifying very young infants with positive urine culture results. For all assessments, hemocytometer WBC counts were significantly different, compared with the standard urinalysis. ESR, CRP, and peripheral WBC counts were not helpful in identifying UTI in febrile infants.Conclusion. UTI had a prevalence of 13.6% in febrile infants <8 weeks of age. The CRP, ESR, and standard UA were imperfect tools in discriminating for UTI, and the sensitivity of these laboratory parameters was relatively low. Hemocytometer WBC count was a significantly better predictor of UTI in febrile infants. Pediatrics 2000; 105(2). URL: http://www.pediatrics.org/cgi/content/full/ 105/2/e20; urinary tract infection, standard urinalysis, hemocytometer white blood cell counts, receiver operator characteristic curves.ABBREVIATIONS. UTI, urinary tract infection; UA, urinalysis; hpf, high-power microscopic field; WBC, white blood cell; CFU, colonyforming unit; CRP, C-reactive protein; ESR, erythrocyte sedimentation rate; MMH, Mackay Memorial Hospital; ROC, receiver operator characteristic curves; AUC, area under the curve; LR, likelihood ra...