SENSITIVITY AND SPECIFICITY OF RAPID DIAGNOSTIC-TESTS FOR DETECTION OF GROUP-B STREPTOCOCCAL ANTIGEN IN BACTEREMIC NEONATES

Latex particle agglutination (LPA) testing for antigen to group B stre ptococcus (GBS) has been useful in the diagnosis of GBS sepsis in newb orns. However, recent reports have demonstrated that the sensitivity o f LPA assays may be as low as 27 to 54%. The purposes of the present s tudy were to directly compare the abilities of four urine antigen assa ys to detect GBS antigen with clinical urine samples from neonates wit h GBS bacteremia and to evaluate the effect of the urine concentration on the sensitivities and specificities of these assays. Urine samples were collected serially from neonates with blood cultures positive fo r GBS or on admission from healthy full-term infants. One milliliter o f urine was removed, and the remainder was concentrated to a volume of 1 ml. Unconcentrated samples were serially diluted with normal saline and were assayed to determine the highest dilution which would produc e a positive test result. The Wellcogen, Bactigen, and Directigen LPA tests and ICON immunoassay were directly compared by using concentrate d and unconcentrated urine specimens and urine specimens with known ti ters. A total of 94 urine specimens, including 61 concentrated and 75 unconcentrated specimens, from bacteremic infants were available for s ensitivity testing, and 220 urine specimens from uninfected infants we re available for specificity testing. There were significant differenc es in sensitivity among the four assays when they were performed on co ncentrated urine specimens, as follows: Directigen, 98%; Bactigen, 92% ; ICON, 89%; Wellcogen, 68%. When the assays were performed on unconce ntrated urine specimens, the Directigen (84%) and Bactigen (76%) assay s were each significantly more sensitive than the ICON (59%) or Wellco gen (43%) assay, All four assays were significantly more sensitive in detecting GBS antigen in concentrated than in unconcentrated urine. Th e Directigen assay detected antigen in higher dilutions (geometric mea n titer, 1:5) than the ICON (1:3), Bactigen (1:2), or Wellcogen (1:1) assay. The specificity was 99.5% for all four assays when concentrated urine was used and for the Bactigen, Directigen, and ICON assays when unconcentrated urine was used; the Wellcogen assay was 100% specific when unconcentrated urine was used. We conclude that there are signifi cant differences in sensitivity but not specificity among the commerci ally available assays for the detection of GBS antigenuria when concen trated and unconcentrated urine specimens are tested. These difference s in sensitivity may affect the abilities of clinicians to accurately diagnose GBS sepsis before culture results are available.