GBS Swab Collection in Pregnancy: Timing and Procedure
Group B Streptococcus (GBS) swabs should be collected between 35-37 weeks of gestation from both the lower vagina and rectum to identify women who require intrapartum antibiotic prophylaxis. 1
Optimal Timing for GBS Screening
The timing of GBS screening is critical for accurate identification of colonization status at delivery:
- Collection at 35-37 weeks' gestation provides the best sensitivity and specificity for detecting women who will remain colonized at the time of delivery 1
- Screening too early in pregnancy is not recommended as GBS colonization can be transient, chronic, or intermittent 1
- Colonization early in pregnancy is not predictive of neonatal sepsis, making the 35-37 week window crucial 1
Proper Collection Technique
The CDC guidelines specify the following collection procedure:
- Swab the lower vagina (vaginal introitus) first, followed by the rectum (insert swab through the anal sphincter) using the same swab or two different swabs 1
- Do not use a speculum for collection 1
- Cervical, perianal, perirectal, or perineal specimens are not acceptable alternatives 1
- Cultures should be collected in the outpatient setting by the healthcare provider or, with appropriate instruction, by the patient herself 1
Specimen Handling
After collection, proper handling of specimens is essential:
- Place swabs into a nonnutritive transport medium (e.g., Stuart's or Amies with or without charcoal) 1
- Clearly indicate on specimen requisitions that they are for GBS testing 1
- When feasible, specimens should be refrigerated before processing 1
- Specimens should undergo 1-4 hour incubation at 35°-37°C in an appropriate enrichment broth medium to enhance GBS recovery 1
Special Considerations
- Women with GBS bacteriuria during the current pregnancy do not need screening as they should automatically receive intrapartum prophylaxis 1, 2
- Women who previously delivered an infant with invasive GBS disease do not need screening as they should automatically receive intrapartum prophylaxis 1, 2
- For women with penicillin allergy, susceptibility testing for clindamycin and erythromycin should be ordered on positive GBS cultures 1
Clinical Impact
The importance of proper timing and technique for GBS screening cannot be overstated:
- Maternal GBS colonization is a major risk factor for early-onset disease in infants 1
- Women with prenatal GBS colonization are >25 times more likely to deliver infants with early-onset GBS disease compared to women with negative cultures 1
- Intrapartum antibiotic prophylaxis based on accurate screening has reduced early-onset GBS disease by 70-95% 2
- Inadequate screening and incorrect intrapartum antibiotic prophylaxis lead to increased incidence of early-onset disease in newborns 3
Common Pitfalls to Avoid
- Collecting specimens too early in pregnancy (before 35 weeks)
- Using a speculum for collection
- Collecting only vaginal or only rectal specimens (both sites must be sampled)
- Failing to place specimens in appropriate transport medium
- Treating GBS-positive women with oral antibiotics during pregnancy (not effective in eliminating carriage or preventing neonatal disease) 1
- Failing to communicate penicillin allergy status when ordering GBS cultures
By following these guidelines for proper timing and collection of GBS swabs, healthcare providers can significantly reduce the risk of early-onset GBS disease in newborns through appropriate identification of colonized women and administration of intrapartum antibiotic prophylaxis.