Diagnosis of Group B Streptococcal (GBS) Infection in Males
The gold standard for diagnosing Group B Streptococcal (GBS) infection in males is culture of the suspected site of infection on a sheep blood agar plate, which remains the most reliable diagnostic method with 90-95% sensitivity.
Diagnostic Approach for GBS in Males
Sample Collection
- Appropriate sampling is critical:
- For suspected urinary tract infections: Clean-catch urine culture with ≥10^5 CFU/ml considered significant 1
- For skin/soft tissue infections: Swab or aspirate from the affected area 2, 3
- For respiratory infections: Sputum culture or bronchial aspirate 4
- For suspected bacteremia: Blood cultures (multiple sets recommended) 5
Culture Methods
- Blood agar plate culture:
Identification of GBS
Bacitracin disk test:
- Most widely used for differentiating streptococcal groups
- GBS typically does not show inhibition zones (unlike Group A strep) 6
Group-specific antigen detection:
- Commercial kits with group-specific antisera provide highly specific identification
- Appropriate for microbiology laboratory use 6
Rapid Diagnostic Methods
- While rapid antigen detection tests (RADTs) are available for Group A strep, similar rapid tests for GBS have not been as widely validated in male populations 6
Clinical Considerations
Common Sites of GBS Infection in Males
- Respiratory tract (most common source of bacteremia) 5, 4
- Skin and soft tissue infections 5, 2, 3
- Urinary tract 5, 1
- Intravenous access sites (hospital-acquired) 5
Risk Factors for GBS Infection
- Advanced age (highest incidence in adults over 60) 2
- Underlying conditions:
- Diabetes mellitus
- Malignancies
- Liver disease 2
- Hospitalization (70% of bacteremia cases are hospital-acquired) 5
Diagnostic Pitfalls to Avoid
False negatives may occur due to:
- Improper sample collection technique
- Recent antibiotic use prior to sampling
- Low bacterial counts 6
Urinary samples interpretation:
- Even with ≥10^5 CFU/ml in voided urine, up to one-third may represent contamination rather than true bacteriuria 1
- Suprapubic aspiration may be necessary in ambiguous cases
Polymicrobial infections:
- GBS often occurs with other pathogens (43% of bacteremia cases), particularly Staphylococcus aureus 5
- Always consider and test for co-infections
Antibiotic Susceptibility Testing
- Penicillin remains the most effective antibiotic against GBS
- Tetracycline and gentamicin show the least activity 4
- Susceptibility testing should be performed in serious infections or treatment failures
By following this systematic diagnostic approach, clinicians can accurately identify GBS infections in males, leading to appropriate treatment and improved outcomes in terms of morbidity and mortality.