When is a vaginal‑rectal Group B Streptococcus (GBS) swab indicated in pregnant women?

When is GBS Swab Recommended in Pregnancy

All pregnant women should undergo universal vaginal-rectal GBS screening at 35-37 weeks' gestation (specifically 36 0/7 to 37 6/7 weeks) using a single swab collected first from the lower vagina and then inserted through the anal sphincter into the rectum. 1, 2

Universal Screening Protocol

Routine prenatal screening includes:

• Timing: Screen between 35-37 weeks' gestation for all pregnant women 1, 2
• Collection technique: Use one swab to collect from the vaginal introitus first, then insert the same swab through the anal sphincter into the rectum 1, 2
• Specimen handling: Place swabs in non-nutritive transport medium (Stuart's or Amies) and refrigerate if processing is delayed 1
• Laboratory processing: Specimens should undergo 1-4 hour incubation at 35-37°C in enrichment broth medium to enhance GBS recovery 1, 2

This screening applies even to women planning cesarean delivery because they remain at risk for labor or membrane rupture before the scheduled procedure. 3

Additional Screening Situations Beyond Routine 35-37 Week Testing

Preterm Labor or Preterm Premature Rupture of Membranes (< 37 weeks)

Women admitted with signs and symptoms of preterm labor or membrane rupture before 37 weeks' gestation should be screened immediately at hospital admission unless a vaginal-rectal GBS culture was performed within the preceding 5 weeks. 1, 2

• If a valid negative screen exists from within the past 5 weeks, no repeat screening is needed 1
• If the patient reaches 35-37 weeks' gestation and has not yet delivered, repeat the vaginal-rectal culture 1
• GBS prophylaxis should be started immediately upon admission while awaiting culture results 1, 2

Intrapartum Screening (Optional in Select Settings)

In facilities with rapid intrapartum nucleic acid amplification testing (NAAT) available, obstetric providers may choose to perform intrapartum testing of vaginal-rectal samples from women with unknown GBS status who are delivering at term. 1

Situations Where Screening is NOT Needed

Do not perform vaginal-rectal screening at 35-37 weeks if:

• GBS bacteriuria at any concentration was detected during the current pregnancy – these women are presumed to be heavily colonized and automatically qualify for intrapartum prophylaxis regardless of screening results 1, 4, 2
• Previous infant had invasive GBS disease – these women automatically receive intrapartum prophylaxis without need for screening 2, 3

Critical Validity Window

A negative GBS screen is valid for only 5 weeks. 1, 2 If a woman with a history of preterm labor is readmitted with signs and symptoms of preterm labor and had a negative GBS screen more than 5 weeks prior, she should be rescreened and managed according to the preterm labor algorithm. 1

Special Considerations for Penicillin-Allergic Women

For women with documented penicillin allergy at high risk for anaphylaxis (history of anaphylaxis, angioedema, respiratory distress, or urticaria following penicillin or cephalosporin), antimicrobial susceptibility testing for clindamycin and erythromycin should be performed on antenatal GBS isolates. 1, 2, 3 Testing for inducible clindamycin resistance (D-zone testing) should be performed on isolates that are erythromycin-resistant but clindamycin-susceptible. 1, 4

Common Pitfalls to Avoid

• Never use cervical, perianal, perirectal, or perineal specimens – only vaginal introitus and rectal (through anal sphincter) specimens are acceptable 1
• Do not use a speculum for culture collection – this reduces sensitivity 1
• Do not treat asymptomatic GBS vaginal colonization with oral or IV antibiotics before labor – this is completely ineffective at eliminating colonization and promotes antibiotic resistance 1, 4, 2
• Ensure laboratories are informed when urine specimens are from pregnant women so they report GBS at concentrations ≥10,000 CFU/mL 1, 4