Selective intrapartum chemoprophylaxis of neonatal group B streptococcal early-onset disease. II. Predictive value of prenatal cultures.

To determine the value of prenatal cultures in defining maternal colonization status at delivery, 5,586 pregnant women were screened at prenatal visits for vaginal and rectal carriage of group B streptococci (GBS). GBS were isolated from 1,272 (22.8%). At delivery, semiquantitative cultures were obtained from 393 prenatal carriers, of whom 264 (67.2%) retained carriage at delivery. Seventeen (8.5%) of 200 women with negative prenatal cultures acquired carriage. The predictive value of a positive prenatal culture was highest (72.5%) in women with prenatal vaginal and rectal colonization and lowest (59.7%) in women with only rectal colonization. The predictive value varied inversely with the interval between prenatal sampling and delivery. In mothers with prenatal carriage, density of colonization at parturition was not predicted by the sites of prenatal colonization. Density of colonization, however, strongly influenced rates of vertical transmission to neonates and rates of heavy infant colonization. Ten infants born to prenatally cultured mothers developed group B streptococcal early-onset disease; the mothers of eight (80%) of the 10 had prenatal colonization with the homologous GBS serotype.