Cytomegalovirus (CMV): Reading IgG, IgM and Avidity

"Cytomegalovirus IgG positive" — and immediately worry, especially in pregnancy. Relax: cytomegalovirus (CMV) is a very common herpesvirus carried by most adults. In a healthy person it usually lives in the body for life without symptoms. The test is needed to tell whether the infection is RECENT or PAST — and here it is not only IgG/IgM that matters, but avidity.

What Cytomegalovirus Is and Why the Test Is Done

CMV is a herpesvirus transmitted via saliva, blood, sexually, and from mother to child. In most people the primary infection passes unnoticed or as mild malaise; sometimes it resembles infectious mononucleosis. After infection the virus persists for life. The test matters above all for pregnant women and people with weakened immunity — for them, determining the stage of infection is critical.

IgG and IgM: What They Mean

• IgG — appear later and persist FOR LIFE; their presence means contact with the virus occurred (at some point).
• IgM — appear in a fresh infection, but for CMV this is NOT a reliable sign of recent infection: IgM can persist for a long time and rise again with reactivation. So a single IgM does not prove a recent infection.

IgG Avidity: Past or Recent Infection

It is avidity (the binding strength of IgG to the virus) that reliably separates recent from past infection:

• Low avidity (usually < 0.6) + positive IgM → likely PRIMARY (recent) infection.
• High avidity (> 0.7) → infection occurred long ago (more than ~3–4 months), which essentially rules out recent infection. So with positive IgG/IgM, the doctor often orders exactly the avidity test.

What a Positive CMV IgG Means

Most often "IgG positive, IgM negative" simply means a long-past infection and established immunity (like most adults). This is NOT a disease and outside pregnancy usually needs no action. What warrants attention is a positive IgM with low avidity — a sign of recent infection assessed in context.

CMV and Pregnancy (Why Avidity Matters)

In pregnancy a primary CMV infection can be dangerous to the fetus, so avidity is especially important here: high IgG avidity early in pregnancy is reassuring (old infection), while low avidity + IgM calls for further work-up with a doctor. The test is often taken alongside other infections — for example HIV and EBV, and with symptoms a complete blood count is also assessed.

When to See a Doctor

See a doctor for a positive IgM with low avidity, when planning or managing pregnancy with unclear CMV status, for prolonged fever with swollen lymph nodes, and with weakened immunity. Do not interpret the result yourself: only the combination of IgG + IgM + avidity in context is meaningful, and a doctor assesses it.

This article is for informational purposes only and does not replace a doctor's consultation.