When is group B strep (Streptococcus agalactiae) screening performed during pregnancy?

Group B Streptococcus Screening During Pregnancy

All pregnant women should be screened for Group B Streptococcus (GBS) colonization at 36 0/7 to 37 6/7 weeks of gestation through vaginal-rectal swab culture. 1, 2

Standard GBS Screening Protocol

Timing of Screening

• The optimal timing for GBS screening is between 36 0/7 and 37 6/7 weeks of gestation 2
  • This represents an updated recommendation from the previous 35-37 weeks window 3
  • This timing provides the best sensitivity and specificity for detecting women who will remain colonized at delivery 1

Collection Technique

• Proper specimen collection is critical for accurate results:
  • Swab the lower vagina (vaginal introitus) followed by the rectum (through anal sphincter)
  • Use the same swab or two different swabs for both sites
  • Insert the swab approximately 2 cm into the vagina and 1 cm into the anus 4
  • Patients can perform their own swabs with appropriate instruction 1, 4

Processing

• Collected specimens should be placed into a nonnutritive transport medium 1
• Some centers may use nucleic acid amplification tests (NAATs) for detection of GBS in addition to standard cultures 5

Exceptions to Standard Screening

Women Who Do NOT Need Screening

1. Women with GBS bacteriuria during current pregnancy

   • These women should receive intrapartum antibiotic prophylaxis without additional screening 3, 1
   • This applies regardless of the colony count (though a threshold of at least 10⁴ colony-forming units per mL is considered positive) 5
   • Prior treatment with oral antibiotics does not eliminate the need for intrapartum prophylaxis 1

2. Women who previously delivered an infant with invasive GBS disease

   • These women should receive intrapartum antibiotic prophylaxis without screening 3, 1

Management Based on Screening Results

Positive GBS Screening

• Intrapartum antibiotic prophylaxis should be given to all pregnant women who test positive for GBS colonization 3
• Exception: Cesarean delivery performed before onset of labor on a woman with intact amniotic membranes 3, 1

Unknown GBS Status at Labor

Intrapartum antibiotic prophylaxis should be given when screening results are unavailable if any of these risk factors are present:

• Delivery at <37 weeks' gestation
• Amniotic membrane rupture ≥18 hours
• Intrapartum temperature ≥100.4°F (≥38.0°C) 3

Common Pitfalls to Avoid

1. Inappropriate timing of screening

   • Screening too early (before 36 weeks) or too late may reduce accuracy

2. Improper collection technique

   • Failure to sample both vaginal and rectal sites reduces sensitivity

3. Relying on previous pregnancy results

   • GBS status in a previous pregnancy does not predict current status unless there was a previous GBS-affected infant 1

4. Using oral antibiotics before labor

   • Antimicrobial agents should not be used before the intrapartum period to eradicate GBS colonization, as this is ineffective and may cause adverse consequences 3

5. Overlooking GBS bacteriuria

   • Women with GBS bacteriuria during any trimester require intrapartum prophylaxis regardless of subsequent screening results 1

Despite universal screening and prophylaxis protocols, GBS remains a significant cause of neonatal sepsis and mortality, highlighting the importance of adherence to screening guidelines 6.