Immunoglobulin M (IgM) by mass concentration (serum/plasma)

What IgM Does in Your Body

IgM is produced early in an infection and can remain in your system for months or even years after the initial illness. It’s particularly important for defending against certain bacteria and some viruses. Your body produces higher levels of IgM when it encounters new pathogens, helping to neutralize them before other immune responses kick in.

When Your Doctor Might Order This Test

Doctors may order an IgM test to diagnose or monitor conditions like chronic infections (e.g., hepatitis B or C), certain autoimmune diseases, or to check for past exposure to specific pathogens. It can also help track how well your immune system is responding to treatment for infections.

Common Uses and Conditions

This test is often used alongside other blood tests to confirm or rule out infections. For example, it may be part of a broader evaluation for conditions like Lyme disease, Epstein-Barr virus (EBV), or other viral/bacterial infections where IgM levels rise during active infection.

What to Expect Before the Test

No special preparation is usually needed. You’ll have a small needle inserted into your arm to draw blood, similar to other routine blood tests. If you’re on blood thinners or have a history of bleeding, let your doctor know before the test.