Polymerase chain reaction for Group B Streptococci (GBS) at labor highly correlates with vaginal GBS load.

Objective: To explore factors associated with a high vaginal GBS load during labor considering (1) the recto-vaginal GBS load at 35-37 weeks' gestation determined by culture and (2) the vaginal GBS colonization determined by a polymerase chain reaction (PCR) assay during labor.

Methods: From an unselected cohort of 902 pregnant women, we obtained (1) recto-vaginal swabs for culture of GBS at 35-37 weeks' gestation (GBS), (2) vaginal swabs for GBS PCR detection at labor (PCR), and (3) vaginal swabs for culture of GBS at labor (GBS). The GBS load was classified semi quantitatively according to a culture protocol without prior broth enrichment of the swab samples: none (0), few (+), some (++), or many (+++) GBS colonies.

Results: Among 902 unselected pregnant women, 859 (95%) had a vaginal swab culture taken at labor, which was classified semi quantitatively. High load GBS (+++) were found in 31 participants. GBS showed a sensitivity of 90% (28/31) and a PPV of 23% (28/121), whereas PCR had a sensitivity of 98% (30/31, non-significant difference) and a PPV of 42% (30/71,  < .01). PCR at labor had a lower sensitivity (78%) for detection of vaginal colonization with GBS at labor (any load) compared to recto/vaginal colonization with GBS at 36 weeks (92%). Vaginal colonization with GBS at 36 weeks seemed to have a lower sensitivity for detecting GBS in vagina at labor for high load (48%) and for any load (39%).

Conclusion: PCR at labor has higher detection rate (non-significant) and PPV in identification of laboring women with a high load of vaginal GBS compared with recto-vaginal culture at 36 weeks' gestation.