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Blood Test Tracker

Cytomegalovirus (CMV) IgM

Looks for IgM antibodies that often appear early in a new CMV infection, but can also stay positive for months or come back falsely positive.

AI-verified against cited sources. Awaiting expert reviewAlso called CMV IgM, CMV-IgM, CMV Ab IgMLOINC 5126-8

What it is

Cytomegalovirus (CMV) is a very common virus that most people catch at some point, often without knowing. IgM is the type of antibody the body tends to make first. The test looks for it in a blood sample. A positive result can point to a recent infection or a flare-up of an old one. The result is printed as negative, equivocal or positive.

Why it matters

Doctors may order it when someone has a mononucleosis-like illness, or when a pregnant person has a possible new CMV infection. It is not a stand-alone answer. IgM can stay positive for months and can be a false alarm, so it is usually read together with IgG, IgG avidity and sometimes CMV DNA.

If it’s low

A negative result means no CMV IgM was found. This makes a very recent infection less likely, but IgM can be missed, and it may not have appeared yet if the infection is only a few days old. In one study of pregnant women with a first infection of known timing, about 5 in 100 had already lost detectable IgM within 3 months on one assay type.

Common causes

  • No recent CMV infection
  • Testing very early, before IgM has appeared
  • IgM that cleared early, which can happen in some people

What can help

  • If you are worried about a recent exposure, ask whether IgG or a repeat test makes sense

If it’s high

A positive result means IgM antibodies to CMV were found. This can happen in a recent first infection, but IgM can also stay positive for weeks to months and can be positive when an old infection flares up. False-positive results are well known. Your clinician usually confirms what a positive IgM means with other tests, such as IgG avidity or CMV DNA, before drawing conclusions.

Common causes

  • A recent first CMV infection
  • IgM left over from an infection months ago, because IgM can persist
  • Low levels of IgM when an old infection reactivates
  • A false-positive result, which is more common with IgM than with IgG

What can help

  • Ask whether IgG, IgG avidity or CMV DNA testing would help show if the infection is recent
  • If you are pregnant, ask how soon you can be seen to discuss the result and any next steps
  • Ask whether the test should be repeated or confirmed on a second sample

What can affect your result

  • Time since infection: IgM rises early and then fades, but how quickly differs a lot between people
  • Assay: IgM results vary between kits much more than IgG results do
  • Reactivation of an old infection, which can cause low levels of IgM
  • Other infections or immune conditions, which can occasionally cause cross-reacting false positives
  • Pregnancy: IgM is often tested together with IgG and IgG avidity to judge how recent an infection is
  • IgM can stay positive for weeks to months after an infection, so a positive result does not prove the infection is recent.
  • False-positive IgM results occur, and different kits can disagree with each other more often than they do for IgG.
  • Each lab uses its own kit, scale and cut-offs, so numbers printed in AU/mL or as an index cannot be compared between labs. The number above the cut-off does not show how much antibody is present.

IgM usually appears early in a new infection and fades over weeks to months, but in some people it fades within 3 months and in others it stays positive for more than 6 months.

When to talk to a clinician

Talk with a clinician soon if you are pregnant and have a positive CMV IgM, or if you develop a flu-like or mononucleosis-like illness (fever, tiredness, sore throat, swollen glands) while pregnant. If you have a weakened immune system, contact your care team promptly for fever, new vision changes or other new symptoms. CMV in a newborn is checked with a PCR test on saliva or urine in the first 2 to 3 weeks of life, not with antibody tests.

Questions you could ask

  • Which other CMV tests should be read together with this one?
  • Could this be an older infection or a false-positive IgM?
  • Would IgG avidity or CMV DNA testing help show whether it is recent?
  • Does this result matter for my pregnancy or treatment?
  • Does a negative IgM rule out a recent exposure in my situation, or should we repeat it?

Questions people ask

Does a positive CMV IgM mean I was infected recently?

Not always. IgM often appears early in a new infection, but it can stay positive for months, can reappear when an old infection flares up, and sometimes shows up as a false positive. Your clinician usually pairs it with IgG avidity or CMV DNA to judge how recent an infection is.

Why did two labs give me different CMV IgM results?

IgM kits differ from one another much more than IgG kits do. In one 2026 comparison of four platforms, the share of samples called IgM-positive ranged from about 9% to about 19%. If results disagree, ask your clinician whether IgG avidity or a repeat sample would settle it.

Show the science and sources

For clinicians and researchers

CMV IgM is run on automated immunoassays. Mayo uses an electrochemiluminescence immunoassay with a reference of Negative, and Labcorp used a chemiluminescent immunoassay with Negative below 30.0 AU/mL on its 2020 page; an equivocal result should be retested, with a second sample collected if it stays equivocal. Labcorp states that the test is qualitative only and that the value above the cut-off does not reflect the amount of IgM, that IgM may persist for weeks to months, and that low levels may be detectable during reactivation. Result codes are LOINC 5126-8 (Labcorp) and 30325-5 (Mayo). In a 2026 four-platform comparison (237 IgM samples), IgM positivity ranged from 8.9% to 18.6% against 5.5% by the composite standard, with only fair to moderate agreement (kappa 0.34-0.58), and IgG avidity showed that most isolated IgM-positive results did not reflect a recent infection. In a 2021 study of pregnant women with dated primary infection, about 5% cleared IgM within 3 months, about 35% stayed positive for around 6 months and about 60% stayed positive for longer than 6 months on a chemiluminescent assay (an ELISA IgM was negative more often), and about 11% had atypical IgM clearance or IgG avidity maturation, which can lead to misdating. IgM is therefore interpreted with IgG, avidity and, where relevant, CMV DNA. CDC states that congenital CMV cannot be diagnosed with IgG or IgM.

LOINC 5126-8 (primary), 7853-5, 30325-5, 24119-0. Specimen: serum.

Sources

  1. Clinical Overview of CMV and Congenital CMV, Centers for Disease Control and Prevention
  2. Cytomegalovirus Antibody, IgM, Serum (test CMVMS), Mayo Clinic Laboratories
  3. Cytomegalovirus (CMV) Antibodies, Qualitative, IgM (test 096727), Labcorp (2020)
  4. Consistent performance of cytomegalovirus IgG assays across platforms contrasts with marked IgM variability, supporting IgG avidity-based infection classification in a high-prevalence population, Scientific Reports (2026)
  5. Pitfalls in the Serological Diagnosis of Primary Human Cytomegalovirus Infection in Pregnancy Due to Different Kinetics of IgM Clearance and IgG Avidity Index Maturation, Diagnostics (2021)
  6. LOINC codes verified with the NLM Clinical Tables LOINC search (e.g. 5126-8), 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.

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