Performed by: Labcorp: What It Is, What It Tests, and How to Get Your Lab Results

Streptococcus Pneumoniae IgG Antibody Test

This test measures IgG antibody levels to Streptococcus pneumoniae to evaluate how well a person has responded to pneumococcal vaccination. Streptococcus pneumoniae is a bacteria responsible for most cases of pneumonia, as well as other serious conditions like bronchitis, septicemia, and meningitis. Pneumonia is especially dangerous for young children, adults over 65, and individuals with weakened immune systems.

There are several pneumococcal vaccines available, with the two most common being Prevnar13 (PCV13) and Pneumovax23 (PPSV23). PCV13 provides protection against 13 pneumococcal serotypes, while PPSV23 covers 23 serotypes. Depending on age and risk factors, a person may be recommended to receive one or both vaccines. After vaccination, this blood test helps verify if sufficient IgG antibody levels have been developed to protect against infection.

Antibodies Tested in the PPSV23 Vaccine

This test specifically measures antibodies to the 23 serotypes included in the PPSV23 vaccine, which are classified as: 1, 2, 3, 4, 5, 6B, 7F, 8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19A, 19F, 20, 22F, 23F, and 33F. It typically takes 3–4 weeks after vaccination for IgG antibodies to fully develop.

Testing is usually ordered after vaccination to confirm adequate antibody levels for protection. It's important to review test results with a healthcare provider, as low antibody levels may not always be flagged on the report but could indicate insufficient immunity.

Understanding Protection and Antibody Levels

While the presence of antibodies suggests some level of immunity, the exact serologic correlates of protection against pneumococcal disease have not been rigorously established for all patient populations. Published data and expert consensus, including recommendations from the World Health Organization (WHO), suggest the following:

Healthy children vaccinated with pneumococcal conjugate vaccines (e.g., PCV13) are generally considered protected against invasive pneumococcal disease when antibody levels reach ≥0.3–0.50 mcg/mL.
Higher antibody levels may be needed for protection against non-invasive infections like pneumonia, otitis media, or sinusitis.
For individuals vaccinated with pneumococcal polysaccharide vaccines (e.g., PPSV23) or those who are immunocompromised, a cutoff of ≥1.3 mcg/mL may be a more relevant benchmark for evaluating immune response.

In addition to the quantity of antibodies, other factors such as antibody avidity (strength of binding to the antigen) and opsonophagocytic activity (ability of antibodies to promote bacterial clearance) contribute to protection.

Post-Vaccine Response

After vaccination, a 4–6 week period is typically needed to assess the immune response. A normal post-vaccine serologic response is often defined as either:

IgG seroconversion or a 2- to 4-fold increase in IgG titers for at least 50%–70% of the vaccine serotypes.

Key Considerations

Low antibody levels: While low antibody levels may not necessarily be flagged on the report, they might not guarantee sufficient protection against pneumococcal infections. This is particularly critical for individuals with higher risk factors, such as those who are immunocompromised or have other underlying health conditions.
Consult with your doctor: All test results should be interpreted in the context of clinical history and discussed with a healthcare professional to determine if further vaccination or other interventions are necessary.

This test plays a vital role in ensuring the effectiveness of pneumococcal vaccination, especially for at-risk individuals, by identifying whether adequate protection has been achieved.