What measles immunoglobulin G (IgG) antibody titer is considered protective, when should it be ordered, and how should non‑protective results be managed in adults such as health‑care workers, travelers, pregnant women, or immunocompromised patients?

Measles IgG Titer Interpretation and Management

Any detectable measles IgG antibody is considered evidence of immunity in immunocompetent adults, and no specific protective titer threshold is routinely used in clinical practice. 1

When to Order Measles IgG Testing

Serologic testing for measles immunity should be reserved for specific high-risk populations where documentation of vaccination or disease history is unavailable:

• Healthcare workers without documentation of two doses of measles vaccine or physician-diagnosed measles should undergo serologic testing as part of pre-employment screening 1
• International travelers born during or after 1957 without proof of two vaccine doses or prior measles infection should be tested before departure 1
• Pregnant women of uncertain immune status identified during prenatal care, though testing should not delay post-exposure prophylaxis if exposure occurs 2, 3
• Immunocompromised patients being evaluated for susceptibility, recognizing that antibody levels may not accurately reflect protection in this population 1

Important caveat: The CDC does not recommend routine serologic testing before or after vaccination in the general population, as vaccination history (two documented doses) is considered acceptable evidence of immunity 1

Interpretation of Measles IgG Results

Positive Results (Any Detectable IgG)

• Any positive measles IgG result indicates presumptive immunity and no further vaccination is needed in immunocompetent individuals 1
• Commercial assays vary significantly in sensitivity, particularly for low-positive samples, but agreement is good for clearly positive results 4
• False-negative results occur in approximately 11% of sera with low neutralizing antibody levels, meaning some immune individuals may test negative 4

Negative or Equivocal Results

Adults with negative or equivocal measles IgG titers should receive MMR vaccination according to the following algorithm:

• Healthcare workers and college students: Two doses of MMR vaccine separated by at least 28 days 1
• Other adults born during or after 1957: At least one dose of MMR vaccine, with consideration for a second dose based on exposure risk 1
• Adults born before 1957: Generally considered immune due to natural infection during childhood, though vaccination should be considered during outbreaks or for healthcare workers with occupational exposure 1

Critical pitfall: Do not delay vaccination while awaiting serologic results in outbreak settings or after known exposure—proceed directly to post-exposure prophylaxis 1

Post-Exposure Management Based on Immune Status

For Seronegative or Unvaccinated Individuals

The management algorithm depends on timing since exposure:

• Within 72 hours of exposure: Administer MMR vaccine immediately to persons ≥6 months of age (excluding pregnant women and severely immunocompromised individuals) 2, 3
• Between 72 hours and 6 days after exposure: Administer intramuscular immune globulin (IG) at 0.25 mL/kg (maximum 15 mL) for immunocompetent individuals 2, 3
• Immunocompromised contacts: Use higher IG dose of 0.5 mL/kg (maximum 15 mL) within 6 days of exposure 2, 3

Special Populations Requiring IG Regardless of Timing

• Infants <6 months: Always use IG at 0.25 mL/kg, as vaccine is not effective in this age group due to maternal antibody interference 2
• Pregnant women: IG at 0.25 mL/kg is the only option, as MMR vaccine is contraindicated 3
• Severely immunocompromised patients: IG at 0.5 mL/kg, as live vaccine is contraindicated 1, 3

Follow-Up After IG Administration

Any person who receives IG for measles prophylaxis must receive delayed MMR vaccination:

• Wait 5-6 months after IG administration before giving MMR vaccine to avoid interference from passively acquired antibodies 2
• The specific interval depends on the IG dose received, with higher doses requiring longer delays (see dose-related intervals in ACIP recommendations) 1
• Do not perform serologic testing after IG administration to assess immunity—simply schedule the delayed vaccination 2

Healthcare Worker-Specific Considerations

Exposed healthcare workers without documented immunity require work exclusion:

• Remove from patient contact from day 5 through day 21 after exposure, regardless of whether they received post-exposure prophylaxis 3
• If measles develops, exclude until at least 4 days after rash onset 3
• Post-exposure vaccination or IG is not completely effective, making work exclusion mandatory even after prophylaxis 1

Common Pitfalls to Avoid

• Do not use IG for outbreak control—it is indicated only for individual post-exposure prophylaxis of high-risk contacts 3
• Do not assume maternal antibodies protect infants of mothers with measles—all unvaccinated household contacts require prophylaxis 2
• Do not confuse measles (rubeola) with rubella (German measles)—IG is not effective for rubella or mumps post-exposure prophylaxis 2
• Do not use inadequate IG dosing—immunocompromised patients require 0.5 mL/kg, not the standard 0.25 mL/kg dose 2, 3
• Do not delay MMR vaccination beyond 72 hours hoping it will still work—switch to IG if the window has passed 2

Quality of Commercial Assays

Recent evidence demonstrates significant variability among commercial measles IgG assays, particularly for samples with low antibody levels 4. However, this variability does not change clinical management, as any positive result is considered evidence of immunity and any negative result warrants vaccination in appropriate populations 1. The plaque reduction neutralization assay remains the gold standard but is impractical for routine clinical use 4, 5.