Strep B Test: Purpose, Test Details & Results

Group B Streptococcus (GBS), or Strep B, is a bacterium that can be harmless in adults but poses serious health risks to newborns if transmitted during childbirth. Screening for Strep B is a routine but crucial part of prenatal care, designed to prevent life-threatening infections in newborns. In this article, we'll break down the purpose of the Strep B test, explore how it's performed, and clarify what the results mean for you and your baby.

Purpose of Strep B Test

Understanding why the Strep B test is performed is the first step to appreciating its significance during pregnancy and neonatal care. The primary goal is to identify GBS carriers—especially pregnant women—to prevent severe infections in newborns.

What is Strep B and Why is it Important?

Strep B (Group B Streptococcus, or GBS) is a type of bacteria commonly found in the lower genital tract and rectum of healthy adults. For most, it causes no harm. However, GBS can be dangerous for newborns if they become infected during labor or shortly after birth. GBS is the leading bacterial cause of meningitis and sepsis in newborns, which can result in severe illness or even death if not promptly treated 3, 5.

Who Should Be Tested?

• Pregnant Women: Most guidelines recommend routine screening of all pregnant women between 35 and 37 weeks' gestation. This timing ensures the results are relevant for the delivery period 5.
• Neonates: Tests may be performed on newborns who display symptoms of infection soon after birth, especially if the mother’s GBS status is unknown or positive 1.

The Impact of Testing

The identification of GBS carriers enables healthcare providers to give appropriate antibiotics during labor, drastically reducing the risk of transmitting the infection to the newborn 5. Early detection and treatment have significantly lowered the incidence of severe neonatal GBS disease in countries with routine testing.

Preventing GBS-Related Complications

Without screening, GBS infections in newborns can lead to:

• Sepsis (blood infection)
• Pneumonia
• Meningitis (brain and spinal cord infection)

Testing is therefore a key preventive measure—simple for the mother, but potentially lifesaving for the baby 1, 3, 5.

Strep B Test Details

Knowing why you need the test is one thing, but understanding how it's done, what methods are available, and their pros and cons can help demystify the process.

How is the Test Performed?

Sample Collection

For pregnant women, a swab is taken from the lower vagina and rectum, usually between 35 and 37 weeks of pregnancy. The process is quick and generally painless.

For newborns or symptomatic individuals, urine, blood, or cerebrospinal fluid samples may be tested if infection is suspected 1.

Main Testing Methods

1. Culture-Based Testing

• Considered the gold standard for GBS detection.
• Swab samples are placed in a selective enrichment broth (often Lim broth) and then subcultured onto agar plates.
• Results are typically available in 24–48 hours.
• Highly sensitive and specific for identifying GBS 2, 5, 6.

2. PCR (Polymerase Chain Reaction) Testing

• Newer PCR-based assays detect GBS DNA directly from swab samples.
• Results can be available in as little as 30 minutes to 2 hours, making it useful for rapid decision-making during labor.
• PCR tests have very high sensitivity and specificity (up to 100% sensitivity in some studies) 3, 5.
• Can detect GBS even when bacteria are present in low numbers or are nonviable (dead but still present) 3.

3. Immunoassays (Antigen Detection)

• Immunoassays, including latex agglutination and optical immunoassays (OIA), detect GBS antigens in patient samples.
• Results are rapid (about 30 minutes), which is helpful during labor.
• However, sensitivity is generally lower than culture or PCR, especially in cases with low bacterial load. Their specificity is high, but they may miss mild or early colonization 2, 4.

4. CAMP Test

• A laboratory method that helps presumptively identify GBS based on a unique reaction on blood agar plates.
• Mainly used in microbiology labs for confirmation rather than direct patient testing 6.

Rapid vs. Standard Tests

• Rapid tests (PCR, immunoassay): Offer quick results and can be useful during labor if GBS status is unknown, but may sacrifice sensitivity, especially immunoassays 2, 3, 4.
• Standard culture: Remains the most reliable method, particularly for routine screening in pregnancy, though slower 2, 5.

Test Limitations

• Immunoassays may miss cases with low bacterial counts and are not recommended as the sole method for intrapartum detection 4.
• PCR and culture methods are more reliable, but PCR may be more expensive and less widely available 3, 5.
• False positives can rarely occur due to contamination or cross-reaction, but this is uncommon with modern techniques 1, 3.

Strep B Test Results & Follow-Up

Receiving and understanding your Strep B test results is crucial for making informed decisions about labor, delivery, and newborn care.

Interpreting the Results

• Positive: Means GBS was found in your sample—you are considered a carrier. This does not mean you are ill, but you can pass the bacteria to your baby during delivery 5.
• Negative: No GBS detected; routine care continues. However, a negative result only reflects your GBS status at the time of testing 5.
• Indeterminate or Inconclusive: Rare, but may occur due to technical issues or poor sample quality. A repeat test or risk-based management may be recommended 5.

What Happens If You're Positive?

If you test positive for GBS:

• You’ll receive intravenous antibiotics (usually penicillin or ampicillin) during labor to reduce the risk of passing GBS to your baby 5.
• Antibiotics are not needed before labor, as GBS can recolonize.
• Intrapartum antibiotics have been shown to greatly reduce the risk of early-onset GBS disease in newborns 5.

Special Situations

• If you go into labor before being tested: Rapid PCR or immunoassay may be used, or antibiotics may be given based on risk factors (e.g., previous baby with GBS disease, fever during labor) 3, 4.
• If your baby shows signs of infection: Newborns may have urine or other body fluids tested for GBS using rapid antigen-based or PCR tests, which can help guide urgent treatment decisions 1, 3.

Understanding Sensitivity and Specificity

• Culture and PCR tests provide high accuracy. PCR may even detect slightly more cases than culture 3, 5.
• Immunoassays are faster but may miss mild cases, so they are best used as adjuncts rather than replacements for culture 2, 4.

Follow-Up for Newborns

If a mother is GBS positive and did not receive antibiotics, or if the baby shows signs of infection (such as fever, irritability, difficulty feeding), the newborn will be closely monitored and may receive prompt antibiotic treatment 1, 3.

Conclusion

Strep B testing is a simple but vital part of prenatal and neonatal care, protecting newborns from potentially life-threatening infections. Here’s a quick recap of the key points:

• Purpose: To identify GBS carriers (mainly pregnant women) and prevent severe neonatal infections through timely intervention 1, 3, 5.
• Testing Methods: Includes culture (gold standard), PCR (fast and highly accurate), and immunoassays (quick but less sensitive) 2, 3, 4, 5, 6.
• Results: A positive result prompts intrapartum antibiotics to protect the baby; negative results require no special action 3, 5.
• Limitations: Immunoassays may miss mild cases; PCR and culture are more reliable but vary in availability and speed 2, 3, 4, 5.

In summary:

• Group B Strep is a common bacterium with serious implications for newborns.
• Routine, evidence-based screening and timely management have dramatically reduced GBS-related complications in newborns.
• Always discuss your results and any follow-up steps with your healthcare provider to ensure the safest outcome for you and your baby.