A Staff Report from the Straight Dope Science Advisory Board

Are blood tests ineffective in detecting HIV?

Dear Straight Dope:

Recently, a co-worker and I were discussing HIV tests. She disputed the validity of testing based on the fact that antibodies to the HIV virus may not show up in the blood for 10 years or more after the virus is contracted. This was news to me. It was my understanding that the antibodies could be detected after six months, and that *symptoms* of the virus might not show for up to 20 years. My fiance and I did the responsible '90s thing and were tested after six months of monogamous, condom-infused dating. When we both turned up negative (as expected), we celebrated by having unprotected sex. That was a year ago. Are we to assume now that we aren't really safe? Do we have to have 20 years of negative test results to be 100 percent sure we're clean? Tell me it ain't so!

It ain't so. Most HIV tests are really two tests: the initial test (the Eliza) and a confirmatory test (the Western Blot or I.F.A.). HIV testing is extremely accurate. The vast majority of persons will have a positive test three months after infection with the virus. Over 99% of infected persons will have a positive test six months after contracting the virus. During this "window period"--the 3-6 months between becoming infected and showing up positive on the test--an infected person can still spread the virus to others (usually through sex or sharing syringes). People with other, unrelated immune deficiency problems sometimes don't react to the HIV antibody test at all, but this is extremely rare.

There are a few people who have been infected with HIV for 20 years or more who are still healthy (but testing HIV positive), but that isn't common. The average time between infection and symptomatic disease is 10 years, though that seems to be decreasing for some reason.

So, as long as you are monogamous, unprotected sex isn't a risk for you ... unless you want to talk about pregnancy (or other possible pre-existing diseases, like human papilloma virus or hepatitis B).

Good luck!

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