Mumps IgG
Antibodies that show your immune system has met the mumps virus, from the MMR vaccine or a past infection. Results are usually printed as negative, equivocal or positive.
AI-verified against cited sources. Awaiting expert reviewAlso called Mumps Ab IgGLOINC 25418-5
What it is
Mumps is a viral infection that can cause fever, swollen cheeks and jaw, and sometimes complications such as swollen testicles. After the MMR vaccine or an infection, your body makes IgG antibodies against the mumps virus. This test looks for them in a blood sample. Labs print the result as negative, equivocal or positive, and often add a number in the lab's own units, such as AU/mL or an index.
Why it matters
A positive result is accepted by US public health guidance as evidence of mumps immunity. Clinicians sometimes order this test for healthcare workers, students, travelers or people without vaccine records. The test has limits. CDC says mumps IgG does not necessarily predict protection against the disease, and some people with documented vaccination still test negative or equivocal. For that reason, vaccine records are usually the first choice, and testing is not recommended when other acceptable proof of immunity exists.
If it’s low
A negative or equivocal result means not enough mumps IgG was found for your lab to call you immune. When judging immunity, CDC treats an equivocal result as negative. This is common in people who were never vaccinated, but it also happens in some people who were. CDC says people with two documented doses of a mumps-containing vaccine are considered to have presumptive evidence of immunity even with a negative or equivocal result, and no extra dose is recommended on that basis alone.
Common causes
- Never vaccinated and never infected
- Antibody that has dropped below what the test can detect since vaccination or infection
- A weaker response to the vaccine in some people, even with documented doses
- Differences between assays, since each lab sets its own cutoffs
What can help
- Find your vaccine record and bring it to your visit
- Ask your clinician whether any vaccine follow-up makes sense for you, including the timing if you are pregnant or planning a pregnancy
If it’s high
A positive result means mumps IgG was found in your blood. This is accepted as evidence of immunity, from vaccination or from a past infection. The lab's label (positive) matters more than the exact number. A positive result does not guarantee you can never catch mumps, because protection can fade and the test does not predict it exactly.
Common causes
- Mumps vaccination (the MMR vaccine)
- A past mumps infection, including one you did not notice
- A current or very recent mumps infection (uncommon; other tests, such as PCR, are used to look for this)
What can help
- Keep a copy of the result with your vaccine records for school, work or travel
- If you develop swollen cheeks or jaw, fever or testicular pain, contact a clinician, because a positive IgG does not rule out mumps
What can affect your result
- Vaccination history: people with two documented doses may still test negative or equivocal
- Time since vaccination or infection
- Assay brand: cutoffs, units and the equivocal zone differ between labs (Labcorp, for example, prints AU/mL)
- Outbreak setting: a positive IgG can reflect a current infection rather than vaccine immunity
- Each lab's assay has its own cutoffs and units (for example AU/mL or an index), so numbers from different labs cannot be compared directly.
- Grossly hemolyzed, lipemic or icteric samples, and samples with visible particulate matter or gross bacterial contamination, can be rejected (Labcorp).
- During a mumps outbreak, a positive IgG in a close contact may reflect a current infection rather than vaccine immunity, so CDC advises against using IgG testing of contacts to check immunity.
The result reflects past vaccination or infection, not a current change, so there is no trend to wait for.
When to talk to a clinician
A negative or equivocal result is not urgent on its own. Talk with a clinician if you are pregnant or planning a pregnancy, work in health care, or have been around someone with mumps. See a clinician promptly if you develop swollen cheeks or jaw, fever, or testicular pain or swelling, even if your mumps IgG was positive. Seek urgent care for a severe headache, a stiff neck or confusion.
Questions you could ask
- Do I need this blood test if I can find my vaccine record?
- Does my job or school require proof of mumps immunity?
- Does this result count as proof of immunity for my job, school or travel?
- Do I have a vaccine record that counts as proof of immunity?
- Do I need any vaccine follow-up, and when, given my health and any plans for pregnancy?
Questions people ask
Does a positive mumps IgG mean I can't get mumps?
No. A positive result is accepted as evidence of immunity, but CDC notes that the antibody level does not necessarily predict protection. If you develop symptoms such as swollen cheeks or jaw, contact a clinician.
I had two MMR doses but my result is negative. What now?
CDC says people with two documented doses of a mumps-containing vaccine are considered to have presumptive evidence of immunity even if a blood test is negative or equivocal, and it does not recommend another dose on that basis alone. Ask your clinician what applies to you, especially if you are pregnant or work in health care.
What does equivocal mean?
Your result fell in the lab's gray zone between negative and positive. For decisions about immunity, CDC treats an equivocal result as negative. Your clinician may look at your vaccine record or repeat the test.
Show the science and sources
For clinicians and researchers
Mumps IgG is measured by immunoassay. Labcorp uses a chemiluminescent immunoassay reported in AU/mL: negative below 9.0, equivocal 9.0 to 10.9, positive above 10.9. Other labs use different assays, units (AU/mL or index values) and cutoffs. ACIP (2013) accepts mumps-specific IgG detectable by any commonly used serologic assay as laboratory evidence of immunity and does not recommend screening when other acceptable evidence exists. The CDC Pink Book adds that mumps IgG does not necessarily predict protection and that equivocal results should be considered negative when assessing immunity. IgM can be transient, delayed or missed, and a four-fold rise in IgG is hard to see in vaccinated people because acute-phase IgG is often already high, so acute mumps is confirmed by RT-PCR or culture rather than by IgG. A negative PCR in a compatible illness does not rule out mumps. Serologic testing of close contacts during an outbreak is not advised, because a positive IgG may reflect acute infection.
LOINC 25418-5 (primary), 7966-5, 22415-4, 6476-6. Specimen: serum.
Sources
- Mumps Antibodies, IgG (test 096552), Labcorp
- Prevention of Measles, Rubella, Congenital Rubella Syndrome, and Mumps, 2013: Summary Recommendations of the Advisory Committee on Immunization Practices (ACIP), MMWR Recommendations and Reports (CDC) (2013)
- Mumps (Pink Book, Epidemiology and Prevention of Vaccine-Preventable Diseases), CDC
- LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.