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False Positive Tests

By Catherine Salveson, PhD

Q: I had a routine blood test for HIV before donating blood and received a letter saying that the test was inconclusive and I couldn't be considered as a donor. How can this be?

A: As you prepare to see your doctor for a re-test I just wanted to reassure you that you shouldn't be thinking you have HIV. To make that determination you'll need to review your sexual history and decide if you could have come in contact with infected sexual fluids through unprotected sex, or shared IV drug needles.

The thing to remember when getting one of these letters from the Red Cross is that as a cost saving measure some blood donor programs use only the sensitive Western Blot test, and not the specific Elisa test. This means they cast a broad net for anything that could be HIV (that false positive already mentioned). Without the second test, the Elisa for being really specific that whatever was detected is really HIV, no diagnosis can be made.

When screening tons of blood every day, blood banks do their best, but those using only the Western Blot test do scare alot of people who find out later that it wasn't an HIV antibody that tipped the test.

Let's assume this is you, so in the meantime, don't be overly concerned unless you have real risk factors. In all probability you are just fine.