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EBV viral capsid antigen (VCA) IgM

An early antibody to Epstein-Barr virus that is often present during a first infection and fades within weeks to a few months.

AI-verified against cited sources. Awaiting expert reviewAlso called VCA IgM, EBV VCA IgM, anti-VCA IgMLOINC 24115-8

What it is

Epstein-Barr virus (EBV) is a very common virus that causes infectious mononucleosis (mono) in some people, mostly teens and young adults. VCA stands for viral capsid antigen, a protein in the shell of the virus. IgM is the first antibody the body makes against a new infection. VCA IgM usually shows up early and then fades. Labs report the result as negative, equivocal or positive, and many also print a number.

Why it matters

VCA IgM helps a clinician work out whether an illness could be a recent EBV infection. A positive VCA IgM with a positive VCA IgG and a negative EBNA IgG fits a recent infection. A negative VCA IgM does not rule out EBV, and a positive result can have other causes, so it is read with the other EBV tests, your symptoms and the timing.

If it’s high

A positive VCA IgM may mean a recent or current EBV infection, such as mono. It is not proof by itself. IgM can also be positive when an old infection reactivates, or when another infection causes a cross-reaction. Your clinician reads it with VCA IgG, EBNA IgG and your symptoms.

Common causes

  • A recent first-time EBV infection, including mono
  • A cross-reaction with another infection, such as CMV
  • Reactivation of an old EBV infection, when several EBV antibodies can show at once

What can help

  • Ask your clinician to read this result together with VCA IgG and EBNA IgG
  • Ask about rest, fluids, and when it is safe to return to sports or heavy lifting if mono is suspected
  • Tell your clinician when your symptoms began, because timing matters for these tests

What can affect your result

  • Timing: VCA IgM fades within weeks to a few months
  • Other infections, such as CMV, can cause false-positive results
  • The lab's kit and cut-offs: results from different manufacturers are not interchangeable
  • Cross-reaction: other infections, such as cytomegalovirus (CMV), can make VCA IgM read positive when EBV is not the cause.
  • Labs use different kits and cut-offs. ARUP notes that EBV IgM values from different manufacturers' methods may not be used interchangeably.
  • This test is not meant to stand alone. ARUP says it is used together with other EBV tests.

Appears early in an infection and usually disappears within 4 to 6 weeks (CDC). Quest says it fades within a month in young children and within 3 months in others.

When to talk to a clinician

A VCA IgM result on its own is not urgent. Talk with a clinician about a fever, sore throat, swollen glands or tiredness that does not go away. Get urgent care for trouble swallowing or breathing, or sudden severe belly pain, especially on the upper left side. If you cannot breathe well, call 911 or your local emergency number.

Questions you could ask

  • Which EBV tests should be read together for my situation?
  • Was my blood drawn at a point where these tests are reliable?
  • Does my VCA IgM, together with my other EBV results, point to a recent infection?
  • Could another infection, such as CMV, explain this result?
  • Do I need any follow-up testing?

Questions people ask

Does a positive VCA IgM mean I have mono?

It can fit a recent EBV infection, but it is not proof. Other infections can cause a positive result, and an old infection can reactivate. Your clinician looks at the full set of EBV tests, your symptoms and the timing.

How long does VCA IgM stay positive?

The CDC says it usually disappears within 4 to 6 weeks. Quest says it fades within a month in young children and within 3 months in other people.

Can VCA IgM be positive when I do not have EBV?

Yes. Cross-reactions with other infections, such as CMV, can cause a false-positive result. In one study of sick children, some positive results were thought to be cross-reactions with other infections. That is why this test is read together with the other EBV tests.

Show the science and sources

For clinicians and researchers

VCA IgM is measured by immunoassay. Quest reports U/mL with under 36.00 negative, 36.00 to 43.99 equivocal and over 43.99 positive; ARUP (semi-quantitative chemiluminescent immunoassay) reports 35.9 U/mL or less not detected, 36.0 to 43.9 indeterminate (repeat in 10 to 14 days may help) and 44.0 U/mL or more detected; Mayo reports negative or positive by ELISA. Per Quest, VCA IgM is typically detectable at clinical presentation, then declines to undetectable levels within a month in young children and within 3 months in other individuals. The CDC says anti-VCA IgM appears early and usually disappears within 4 to 6 weeks. ARUP describes the test as an aid in diagnosing primary EBV infectious mononucleosis in conjunction with other serologic tests and not a stand-alone test. VCA IgM with VCA IgG and no EBNA IgG indicates acute infection; all three markers can appear together in recent infection or during reactivation (De Paschale and Clerici 2012). Cross-reactivity is a known cause of false positives: in a retrospective study of febrile children aged 0 to 12 years, about one-third of VCA IgM-positive samples (12 of 36) were judged false positives due to cross-reactivity, mainly with dengue and CMV, and VCA IgM alone had 100% sensitivity but 93.2% specificity (study-specific, pediatric population; false positives were defined by antibody pattern and co-positive CMV or dengue tests, without molecular confirmation). The CDC notes that heterophile antibodies (Monospot) are often absent in children with mono, and that paired samples do not help separate recent from past infection.

LOINC 24115-8 (primary), 7886-5, 5159-9, 30340-4, 20491-7. Specimen: serum.

Sources

  1. Laboratory Testing for Epstein-Barr Virus (EBV), US Centers for Disease Control and Prevention
  2. About Epstein-Barr Virus (EBV), US Centers for Disease Control and Prevention
  3. Epstein-Barr Virus Viral Capsid Antigen (VCA) Antibody (IgM) (test code 8426), Quest Diagnostics
  4. Epstein-Barr Virus Antibody to Viral Capsid Antigen, IgM (test 0050240), ARUP Laboratories
  5. Epstein-Barr Virus Antibody Profile, Serum (test EBVAB), Mayo Clinic Laboratories (via testcatalog.org)
  6. Serological diagnosis of Epstein-Barr virus infection: Problems and solutions, World Journal of Virology (2012)
  7. Hidden burden of primary Epstein-Barr virus (EBV) infection in pediatric febrile illness (0-12 years) - A retrospective study, Journal of Postgraduate Medicine (2026)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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