Hepatitis A is a highly contagious liver infection caused by the hepatitis A virus (HAV). When the body is infected with HAV, it defends itself by producing two types of antibodies in sequence: first, IgM, which appears 2 to 4 weeks after infection and is present for 2 to 6 months, then IgG, which appears a few weeks after IgM and is present for the rest of the person’s life. IgG provides long-term immunity (protection), whether due to a prior infection by the virus or due to vaccination. Testing for HAV antibodies Total (IgG plus IgM) is especially useful in confirming an individual’s long-term immunity and the appropriateness of vaccination. The specific anti-HAV IgM test is used mainly to find the cause of an acute or very recent liver disease.
A positive HAV-Ab total test (without isolated testing for IgM) indicates prior HAV exposure but does not rule out acute or very recent hepatitis. About 30% of North American adults over the age of 40 are positive for HAV-Ab total. A negative HAV Ab Total test means there has been no prior exposure to the virus and the hepatitis A vaccination should be administered. A positive IgM test suggests acute or very recent hepatitis A.