Group B Streptococcus PCR screening in pregnancy

Group B Streptococcus (GBS) screening in pregnancy by real-time PCR

Group B Streptococcus (Streptococcus agalactiae) is carried without symptoms by many healthy women, but it can pass to a baby during birth and cause serious early-onset infection. Screening late in pregnancy identifies carriers so that preventive antibiotics can be offered during labour.

When screening is done

United States guidance from the American College of Obstetricians and Gynecologists recommends universal screening between 36 0/7 and 37 6/7 weeks of pregnancy, using a swab of the lower vagina and rectum. The Inogen I-GBS Real-time PCR Kit instructions follow this timing, although screening policy differs between countries.

How the sample is prepared

Vaginal-rectal swabs are collected with sterile swabs and sent to the laboratory promptly. The American Society for Microbiology (2020) recommends incubating the swab in a selective enrichment broth for 18 to 24 hours at 35 to 37 °C before testing, and the kit's instructions adopt this step. DNA is then extracted, for example with a magnetic bead kit.

What the PCR detects

The test amplifies a specific sequence of the GBS cfb gene. In silico analysis covers 11 GBS variants (Ia, Ib, Ic, II to VIII, ND). A beta-actin internal control confirms that sampling, extraction and amplification worked. The limit of detection is 100 CFU/mL, and no cross-reactivity was found with 12 other species tested.

Run time

After extraction, the PCR run takes about two hours.