Is a single positive blood culture for Group B strep (Streptococcus agalactiae) out of two cultures clinically significant and requires treatment?

Clinical Significance of a Single Positive Blood Culture for Group B Streptococcus

A single positive blood culture for Group B Streptococcus (Streptococcus agalactiae) out of two cultures should be considered clinically significant and requires treatment, especially when accompanied by clinical signs of infection.

Understanding Blood Culture Results for Group B Streptococcus

• According to the CDC guidelines for laboratory-confirmed bloodstream infections (BSI), a recognized pathogen (such as Group B Streptococcus) cultured from one or more blood cultures meets the criteria for a true infection when not related to infection at another site 1
• Group B Streptococcus is considered a recognized pathogen, not a common skin contaminant, making even a single positive culture potentially significant 1
• Unlike coagulase-negative staphylococci and other common contaminants that require two positive cultures from separate occasions, pathogens like Group B Streptococcus require only one positive culture to be considered significant 1

Clinical Significance of Group B Streptococcus Bacteremia

• Group B Streptococcus is an emerging cause of disease in non-pregnant adults, particularly those with underlying conditions such as liver disease, heart disease, and malignancy 2
• The overall mortality rate for GBS bacteremia can be as high as 18.9%, with an attributable mortality rate of 7.8%, highlighting the importance of prompt recognition and treatment 2
• GBS remains a significant source of morbidity and mortality in high-risk populations, including the elderly and those with comorbidities 3

Evaluating the Single Positive Culture

When evaluating a single positive blood culture for GBS, consider:

• Patient symptoms: Presence of fever (>100.4°F), chills, or hypotension strongly suggests true bacteremia rather than contamination 1
• Source of infection: Skin and soft tissue (24.4%), urinary tract, and bone/joint are common sources of GBS bacteremia 2
• Underlying conditions: Assess for diabetes mellitus, malignancy, and hepatic failure, which are significant risk factors for GBS sepsis 4

Treatment Approach

• Penicillin G remains the mainstay of therapy for GBS infections 3
• For penicillin-allergic patients, clindamycin (300 mg four times daily for 10 days) is recommended, though increasing resistance has been observed 5, 3
• In cases of reduced susceptibility to penicillin or resistance to non-beta-lactam antibiotics, antimicrobial susceptibility testing should guide therapy 3

Common Pitfalls to Avoid

• Dismissing a single positive culture: Unlike common skin contaminants that require two positive cultures, a single positive culture for a recognized pathogen like GBS meets CDC criteria for true bacteremia 1
• Delaying treatment: Given the significant mortality associated with GBS bacteremia, prompt initiation of appropriate antimicrobial therapy is essential 2, 4
• Inadequate source control: Identifying and addressing the source of infection (skin/soft tissue, urinary tract, etc.) is crucial for successful treatment 2

Special Considerations

• Blood cultures should be obtained before initiating antibiotic therapy to maximize yield 6
• At least two blood culture sets should be obtained from separate peripheral sites with ≥7 mL of blood per bottle 6
• For recurrent or persistent GBS infections, consider screening household contacts and pets as potential sources of reinfection 5

In conclusion, a single positive blood culture for Group B Streptococcus should not be dismissed as a contaminant but evaluated in the context of the patient's clinical presentation and risk factors. Prompt initiation of appropriate antimicrobial therapy is warranted to reduce the significant morbidity and mortality associated with GBS bacteremia.