What is the GeneXpert (Molecular Diagnostic Test) Express GBS (Group B Streptococcus) test for pregnant women?

GeneXpert Express GBS Test

What It Is

The GeneXpert GBS test is a rapid molecular diagnostic assay (real-time PCR) that detects Group B Streptococcus DNA from vaginal-rectal swab specimens, providing results in approximately 50-70 minutes compared to 24-48 hours for traditional culture methods. 1, 2

Test Performance Characteristics

The GeneXpert system offers two versions for GBS detection, each with distinct clinical applications:

Direct Testing (Xpert GBS Assay)

• Sensitivity: 85.7% when performed directly on unprocessed vaginal-rectal swabs without enrichment 1
• Specificity: 96.2% for direct specimen testing 1
• This lower sensitivity makes direct testing less reliable for routine prenatal screening 1

Enriched Broth Testing (Xpert GBS LB Assay)

• Sensitivity: 99.0% after 18-24 hours of broth enrichment 1
• Specificity: 92.4% compared to gold standard culture 1
• Superior performance to direct testing, approaching culture sensitivity 1
• Another study reported sensitivity of 91% and specificity of 100% 2

Clinical Applications

Intrapartum Point-of-Care Testing

• The CDC guidelines indicate that intrapartum nucleic acid amplification tests (NAAT) like GeneXpert can be used when GBS status is unknown at labor onset 3
• Results available within 1 hour allow real-time decision-making about intrapartum antibiotic prophylaxis 2
• If intrapartum NAAT is negative but other risk factors are present (delivery <37 weeks, membrane rupture ≥18 hours, or temperature ≥100.4°F), prophylaxis is still indicated 3

Prenatal Screening Alternative

• Can be used for routine 35-37 week screening when validated by the laboratory with appropriate quality controls 3
• The CDC allows NAAT methods including PCR assays after enrichment as acceptable alternatives to traditional culture 3
• Requires laboratory validation and quality control implementation before clinical use 3

Advantages Over Traditional Culture

• Rapid turnaround time: Results in 50-70 minutes versus 24-48 hours for culture 1, 2
• High sensitivity: Particularly with enriched specimens (99.0%) 1
• Automated processing: Minimal hands-on time and technical expertise required 2
• Point-of-care capability: Can be performed at labor and delivery units 2

Important Limitations and Caveats

Test Performance Issues

• Direct testing without enrichment has inadequate sensitivity (85.7%) for reliable prenatal screening 1
• Some studies report lower sensitivity (71.4%) compared to culture, with 12 false-negative results in one cohort 4
• High cycle threshold (Ct) values may indicate low bacterial loads that could be clinically significant but reported as negative 4
• Requires 3-4 fold fewer test repetitions compared to some competing molecular assays 4

Clinical Context Requirements

• The CDC emphasizes that accurate results are more important than rapid turnaround time for antenatal screening 3
• Negative intrapartum NAAT does not eliminate need for prophylaxis if other risk factors are present 3
• Culture at 35-37 weeks remains the gold standard for detecting vaginal colonization 4

Comparison to Standard Culture

Traditional enriched culture remains the reference standard because:

• Specimens undergo 1-4 hour incubation at 35-37°C in enrichment broth to maximize GBS recovery 3
• Culture allows antimicrobial susceptibility testing for penicillin-allergic patients at high risk for anaphylaxis 3
• The CDC prioritizes accuracy over speed for routine prenatal screening 3

When Susceptibility Testing Is Critical

• Antimicrobial susceptibility testing must be performed on GBS isolates from penicillin-allergic women at high risk for anaphylaxis 3
• Testing for inducible clindamycin resistance is required for isolates susceptible to clindamycin but resistant to erythromycin 3
• Molecular tests like GeneXpert cannot provide susceptibility data, requiring culture in these cases 3

Optimal Testing Strategy

For routine prenatal screening at 35-37 weeks, traditional enriched culture remains preferred because it provides both detection and susceptibility testing when needed 3, 4. However, GeneXpert has valuable applications for intrapartum testing when GBS status is unknown at labor onset, allowing rapid decision-making about antibiotic prophylaxis 3, 2.