Laboratory Testing for Immunity to MMR, Varicella, Tdap, and TB
For determining immunity status to MMR, Varicella, Tdap, and TB, order serologic testing for IgG antibodies to each disease, with the exception of TB which requires specialized testing such as an interferon-gamma release assay (IGRA).
MMR (Measles, Mumps, Rubella) Immunity Testing
Recommended Tests:
- Measles IgG antibody
- Mumps IgG antibody
- Rubella IgG antibody
Interpretation:
- A positive IgG result indicates immunity from previous infection or vaccination 1
- Any antibody level above the standard positive cutoff value of a licensed assay is considered evidence of immunity 1
- Negative or equivocal results suggest lack of immunity, and vaccination would be recommended regardless of reported history 1
Important Considerations:
- Serologic testing is not necessary if there is documentation of appropriate vaccination (2 doses of MMR) 2
- For persons born before 1957, generally considered immune to measles and rubella, but testing may still be recommended, especially for healthcare workers 2, 1
- Up to 9.3% of healthcare workers born before 1957 may lack immunity to measles 1
Varicella (Chickenpox) Immunity Testing
Recommended Test:
- Varicella-Zoster Virus (VZV) IgG antibody
Interpretation:
- Positive result indicates immunity from previous infection or vaccination
- Commercial ELISA assays may have limitations in detecting vaccine-induced immunity, which can be lower than immunity from natural infection 3
- More sensitive tests like fluorescent antibody to membrane antigen or latex agglutination may be needed for borderline cases 3, 4
Important Considerations:
- PCR testing is the diagnostic method of choice for active infection but not for immunity status 4
- Vaccine effectiveness against varicella (any severity) after two doses is approximately 95% 5
Tdap (Tetanus, Diphtheria, Pertussis) Immunity Testing
Recommended Tests:
- Tetanus IgG antibody
- Diphtheria IgG antibody
- Pertussis antibodies (less standardized and not routinely recommended)
Interpretation:
- For tetanus and diphtheria, specific IgG antibody levels correlate with protection
- Pertussis immunity testing is more complex and less reliable; no well-established serologic correlate of protection exists
Important Considerations:
- According to ACIP guidelines, serologic testing for tetanus and diphtheria can be performed before administering additional doses in persons with documented receipt of ≥3 doses 2
- Alternatively, a single booster dose of Tdap can be administered, followed by serological testing after 1 month 2
Tuberculosis (TB) Testing
Recommended Tests:
- Interferon-Gamma Release Assay (IGRA) - preferred for those with history of BCG vaccination
- QuantiFERON-TB Gold Plus
- T-SPOT.TB test
- Tuberculin Skin Test (TST) - alternative option
Interpretation:
- Positive IGRA indicates TB infection (latent or active)
- IGRA is preferred over TST for BCG-vaccinated individuals due to higher specificity 2
- Indeterminate results may require retesting
Important Considerations:
- Screening for TB is essential prior to starting immunosuppressive therapy 2
- False-negative results can occur in immunosuppressed patients 2
- TB screening should include a combination of clinical risk stratification, chest x-ray, and IGRA 2
Testing Algorithm
Review vaccination records first
- If complete documentation exists (2 doses MMR, 2 doses varicella, Tdap within 10 years), testing may not be necessary 2
If records are unavailable or incomplete:
- Order all appropriate serologic tests
- For MMR: Measles, Mumps, and Rubella IgG antibodies
- For Varicella: VZV IgG antibody
- For Tdap: Tetanus and Diphtheria IgG antibodies
- For TB: IGRA (QuantiFERON-TB Gold Plus or T-SPOT.TB)
Interpret results and take action:
- Positive antibody tests: Immunity confirmed, no vaccination needed
- Negative antibody tests: Administer appropriate vaccines
- For TB: Positive IGRA requires further evaluation for active TB versus latent TB infection
Common Pitfalls and Caveats
- False negatives in varicella testing: Commercial ELISA assays may miss vaccine-induced immunity; consider more sensitive tests if clinical history suggests immunity 3
- Timing of testing: For recent vaccinations, wait at least 4-6 weeks before antibody testing to allow adequate immune response
- Immunocompromised patients: May have false-negative results despite prior vaccination or infection
- Cost considerations: Serologic testing may be more expensive than simply revaccinating; consider cost-effectiveness 2
- TB testing in immunosuppressed patients: Higher rates of indeterminate results; clinical correlation is essential 2
Remember that vaccination is generally safe even in previously immune individuals, so when in doubt about immunity status, vaccination is often the most practical approach 2.