Frequency and risk factors of colonization of pregnant women with group B streptococci

Background. Group B streptococci (GBS) are opportunistic bacteria capable of causing severe infections in pregnant women and newborn infants. For effective prevention of obstetrical and perinatal GBS-associated pathology, investigation into epidemiology of GBS carriage and GBS-associated diseases, as well as elaboration of accurate and rapid methods for GBS detection are necessary.

Objectives. Validation of a molecular test for the detection of GBS and evaluation of frequency and risk factors of colonization of pregnant women with this microorganism.

Methods. For validation of PCR test for the detection of GBS DNA (AmpliSens Streptococcus agalactiae-screen-titer-FL, Central Research Institute of Epidemiology, Moscow), 1496 clinical samples from 650 women and 112 newborn infants submitted for routine GBS culture were used. For evaluation of frequency and risk factors of GBS-colonization during pregnancy, clinical samples (urine, vaginal and rectal swab samples) from 496 women at their first prenatal visit before 12 weeks of gestation were used. GBS testing was performed using culture and the validated PCR test. For evaluation of risk factors, binomial logistic regression was used.

Results. PCR method for GBS DNA detection showed high analytical sensitivity (3 ∙ 102 copies/ml) and specificity (no cross-reactions with other microorganisms). Diagnostic sensitivity and specificity of the test (81 and 97.6%, respectively) were comparable with those of culture (77.6 and 100%, respectively). GBS was detected in 16.3% of pregnant women. Testing of rectal samples in addition to vaginal samples enabled to detect 66.7% more cases of GBS carriage. Women aged 18 to 22 years had a higher risk of GBS colonization during pregnancy than older women (23 to 32 years). At the same time, women who had their sexual debut at an older age (after 18) were significantly more often colonized during pregnancy than women started their sex life at a younger age (before 18).

Conclusion. PCR method for GBS detection has high sensitivity and specificity and can be used as an alternative to culture. Simultaneous testing of vaginal and rectal samples increases the sensitivity of revealing colonized women by two thirds in comparison with testing vaginal samples only. Younger age and later sexual debut are independent risk factors for GBS colonization during pregnancy.