Limited diagnostic value of routine screening of neonates with the urinary group B streptococcal antigen tests.

Background: A urinary latex test for detection of antigens from group B Streptococcus (GBS) has been used for the diagnosis of invasive GBS disease. However, the value of routine screening of infants with this test has not been determined.

Methods: All infants admitted to Linkou Chang-Gung Memorial Hospital (Taoyuan, Taiwan) from January 2005 to May 2013 were screened with a urinary GBS antigen test (Wellcogen Strep B). Medical records were retrospectively reviewed to determine the diagnostic value of this test.

Results: A total of 14,277 infants were tested and 38 cases had confirmed diagnoses of invasive GBS disease (34 bacteremia, 18 meningitis, 14 both), corresponding to a prevalence of 0.27% among our admitted infants. A total of 106 infants had positive results, but only 26 had confirmed disease. Among infants with confirmed disease, 12 had negative antigen results. These data allowed calculation of the sensitivity (68.4%), specificity (99.4%), positive predictive value (24.5%), and negative predictive value (99.9%). Adjusting for prevalence, the disease probability of a positive test result was 23.6%, and the probability of a negative post-test result was 0.09%. The absolute risk reduction of a negative result was very small (0.18%). Analysis of demographic, clinical, and laboratory parameters indicated that late age of onset (≥7 days-old), presence of seizure, fever, respiratory distress, leukopenia, bandemia, thrombocytopenia, coagulopathy, metabolic acidosis, and elevated levels of C-reactive protein (CRP) were significantly related to the presence of a true positive test result.

Conclusion: In our study population, the positive predictive value of the GBS antigen test was poor and the risk reduction of a negative result was weak. These results indicate that routine screening with this test has a limited diagnostic value. However, GBS antigen testing appears to be useful for early detection of disease in infants with certain demographic, clinical, and laboratory risk factors.