Clinical and laboratory efficiency of rapid screening urine cultures by nephelometry

Urine cultures represent 40-50% of specimens processed by hospital clinical microbiology laboratories. An accurate rapid screening method to detect positive urine cultures could greatly improve laboratory efficiency. Three hundred eighty-two unselected urines were screened by nephelometry at 2, 4, and 5 hours of shaking and incubation in Autobac cuvettes. Results were compared in parallel with standard quantitative-loop plate cultures of the same specimens. Fifty-eight of the 382 specimens showed significant positive (greater than or equal to 10(5) CFU/ml) standard plate cultures. Fifty-four of the 58 were positive at 4 hours by the nephelometric method (sensitivity 93%, 7% false negatives). Plate cultures of the four false negative specimens grew greater than 10(5) CFU/ml Candida (two specimens), Staphylococcus epidermidis or diphtheroids (one specimen each) and represented colonization or contamination without evidence of infection. For 324 specimens negative by plate culture, nephelometry was negative in 287 (89% specificity or 11% false positives). Overall predictive value of a negative test was 99%. Results were comparable for clean-voided or catheterized specimens. The rapid nephelometric urine culture method gave significant savings of material and time.