over21

Cut-n-Paste=====

In theory, the body produces immunoglobulin M (IgM) antibodies first, to a variety of infections, and immunoglobuin G (IgG) antibodies later. Over time, IgM antibodies tend to stop being produced entirely. Therefore, a test for IgM antibody to any particular infection may be positive before a test that detects IgG antibody. And in a person infected a long time previously, IgM antibody is absent but IgG persists.

That's the theory. In general, it holds up pretty well in young children, but not so well in adults. In adults with new HSV infection, IgM antibody against the virus doesn't actually get produced all that much faster than IgG antibody. And many people with longstanding HSV-1 or HSV-2 infection continue to produce IgM, especially when they have a new symptomatic outbreak. Thus, for HSV, IgM doesn't necessarily mean a new infection; and absence of IgM doesn't necessarily mean a longstanding infection.

On top of that, there are 2 other main problems with IgM testing for HSV. First, false positive tests are common--that is, apparent IgM antibody that simply isn't present at all, despite what the test says. This is NOT generally due to cross reaction with related viruses, i.e. HSV-1, varicella (herpes) zoster, and the like. It has to do mostly with the physical chemistry of the test.

Second, there are no type-specific IgM tests on the market--nary a one. Any true-positive IgM test for HSV is detecting antibody to HSV-1, HSV-2, or both, and can never distinguish between them.

Why do labs continue to offer such a lousy test? Several reasons there too, some reasonable, some not. Some providers still request the test; they were taught the theory, they assume it applies accurate to HSV, and don't understand the limitations of the test. (The pediatricians generally are right; IgM testing for HSV in fact remains useful in diagnosing neonatal herpes in newborns. Their immature immune systems don't crank out IgG so rapidly.) Also, some labs just keep offering the test out of habit, in the belief the providers want the test; while at the same time, the docs just assume that if the lab does the test, it must be worthwhile. (That is, a communication issue.) Also, there is simple profit motive for labs: IgM testing is very cheap but gets good pay-off from insurance companies, so the profit margin is high.

Bottom lines: Sometimes a new HSV infection in an adult indeed will be positive by IgM before IgG. But this is pretty infrequent, and it outweighed by the downsides: high risk of false positive result; and even when truly positive, there is no distinction between HSV-1 and HSV-2, which is pretty important to most patients and providers.