False positives on HSV tests?
January 25, 2006 8:58 AM   Subscribe

False positives on HSV tests?

My girlfriend tested positive a few years ago to genital herpes, though she's never had any symptoms. After switching to a new ObGyn clinic, her new doctor mentioned in passing that she probably got a false positive if she's never had any symptoms, as the HSV test is notorious for spitting out positives if you've ever had chicken pox as a kid. Is this the case? What's the history of this test in terms of accuracy, and are there new tests that are actually accurate?
posted by anonymous to Health & Fitness (5 answers total)
Older tests can sometimes be less accurate. There's a wealth of information here.

I wonder if the doctor specified HSV 1 or 2? If it's 1, it could just be the regular old coldsore on the lip variety.

However, realistically: A lot of people do have HSV2. Some of them are silent carriers.

This doesn't answer all your questions, but I hope it's helpful.
posted by veronica sawyer at 9:35 AM on January 25, 2006

The answer to any question like this is going to be complicated.

If the accuracy of a given test is 95%, that (normally) means that:

(1) IF you actually have HSV, THEN it will come out positive 95% of the time (or for 95% of people). Call this prob(+|H), or positive given H.
(2) IF you actually do NOT have HSV, THEN it will give a false positive only 5% of the time. Call this prob(+|C), or positive given clear.

But that's backwards. What most people care about is:

IF the test comes out positive, THEN what is the probability that I actually have HSV? That is, people care about prob(H|+), or the probability of H given a positive result.

The answer to this depends on the prevalence of HSV in the population -- if everyone gets a test for a rare condition, than 5% of a whole damn bunch can be more than 95% of a relative few, so you end up with more false positives than true positives. It also depends on why your girlfriend got a test a few years ago, since the relevant "population" for her might be higher- or lower-risk than the population as a whole. Broadly, prob(H|+) will be lower if the test was a routine screen done without prompting by any particular event.

I dunno if this is helpful to you. My point is that you have to interpret accuracy of tests, especially routine tests, rather cautiously.
posted by ROU_Xenophobe at 11:25 AM on January 25, 2006

I'm surprised that this question hasn't had more answers by now. If you don't get many more, you might consider re-posting, but with the word "herpes" visible somewhere from the front page (i.e. not only in the "more inside" part).
posted by unknowncommand at 1:46 PM on January 25, 2006

Anecdotal: had childhood chicken pox. Consistently negative on repeated blood tests for both HSV1 and HSV2.
posted by orthogonality at 2:09 PM on January 25, 2006

Here's an answer from Go Ask Alice to just about the same question. They say it's about 5% false positive, and old tests can't differentiate between HSV1 and 2 (genital herpes vs. coldsores). New tests can differentiate between the two.

Chickenpox is actually a member of the herpes virus family, so yes, that could have triggered a false positive result, particularly if the test was a Tzanck test.

Still, if you're worried, it doesn't hurt to get checked out again!
posted by orangskye at 4:40 PM on January 25, 2006

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