Sensitivity of intrapartum group B streptococcal screening and in vitro comparison of four rapid antigen tests.

Objectives: To evaluate the sensitivity of intrapartum screening for group B streptococcal (GBS) colonization and to compare 4 rapid GBS antigen tests in vitro.

Design: Two swabs of the lower vagina of 769 parturients were taken; one swab was cultured, the other was frozen at -70 degrees C until antigen testing with the Group B Strep Test (Quidel) of the culture positive samples was performed. The Quidel test was then compared with 3 other rapid GBS antigen tests in vitro: Wellcogen Strep B (Wellcome Diagnostics), Slidex méningite Strepto B (bioMérieux) and ICON Strep B (Hybritech). The supernatant of 29 GBS cultures in Todd-Hewitt broth was tested in bacterial concentrations of 10(6), 10(7), and 10(8) Colony-forming Units (CFU)/ml, respectively.

Results: Lower vagina GBS carrier rate was 13.4% (103/769) and heavy colonization (growth density 3 and 4 on blood agar plates) was found in 5.2% (40/769). The Group B Strep Test detected 11% (11/103) of GBS carriers, with a sensitivity for heavy colonization of 25% (10/40). In vitro none of the tests scored positively with a concentration of 10(6) CFU/ml, while with 10(7) CFU/ml the enzyme immunoassay tests (Quidel, Hybritech) were more sensitive (McNemar test, P < 0.05) than the latex agglutination tests (Wellcome Diagnostics, bioMérieux).

Conclusions: Although in vitro the enzyme immunoassay tests are more sensitive than the latex agglutination tests, sensitivity in vivo is too low to recommend the use of rapid antigen tests for general screening.