Latex agglutination detection of group-B streptococcal inoculum in urine

Abstract

It is not uncommon for a newborn to be blood culture negative, but urine latex particle agglutination (LPA) positive for group-B streptococcus (GBS). We hypothesized that whole-cell GBS contamination of urine, which is possible in bag-collected specimens, could cause LPA positivity. Serial tenfold dilutions through 10 −8 of an 18-hr broth suspension of GBS serotype III were prepared in sterile urine. Directigen (Becton-Dickenson Microbiologic Systems, Cockeyville, MD) LPA testing was performed on each sample at inoculation and, if negative, every 2 hr until becoming LPA positive. All tubes were quantitatively cultured when becoming LPA positive. The assay was performed four times to evaluate independently the effects of urine sample centrifugation, volume, glucose content, and incubation temperature. An 18-hr GBS suspension dilution of 1:10,000 in urine and as few as 5.7 × 10 3 organisms/ml caused LPA positivity at time 0. Urine supported sufficient replication to enable log 0 concentrations (10 −8 dilution) to become LPA positive within 8 hr at 35°C, a routine infant incubator setting. Alteration by centrifugation, volume, or glucose content had no effect on LPA positivity or time needed to become positive. We conclude that Directigen LPA can detect a relatively small concentration of GBS organisms in urine. Due to potential contamination from skin bacteria, positive LPA results from bag urine should be interpreted with caution in the absence of positive cultures.