HIV screening for donating blood (BLOOD DONATION, FALSE POSITIVE TEST 2009)


Dr. Bob,

My sister recently tried to donate blood and was denied. She recieved a letter stating that she was HIV positive. How accurate are the tests that the Red Cross performs? She is getting an HIV test but I would like to know what the accuracy is of their testing is. The person that I spoke to said that she shouldn't worry?

Please help.....thanks.



The purpose of HIV screening at blood donation sites is to protect the blood supply, not to diagnose HIV disease. The tests run at blood donation centers are very sensitive so that they won't miss any true infections. In the process they pick up some false-positives (people who are HIV negative but test positive on these very sensitive screening tests for a variety of reasons). If your sister has essentially no HIV risk, there is no cause for concern. Her follow-up tests will quickly reveal her HIV-negative status confirming that the blood donation screening test was falsely positive. If on the other hand she has been at risk for HIV infection, she shouldn't have been donating blood in the first place. In addition if indeed she is HIV positive, the donation center did her a favor by encouraging further testing to confirm her diagnosis so that she can be appropriate monitored and treated.

I'll reprint below some information from the archives that discusses HIV screening at blood donation sites.

Dr. Bob

Safe blood transfusion v. Window period Mar 25, 2006

Hi Doc! I have heard that places that accept blood (for storage and transfusion) can know if the blood is infected even if the person donating is at his/her window period. Then why don't they use this technology to straight-away determine if a person is Hiv+ or not? My second question: After sustaining a severe hollow-bore needlestick and laceration while performing a medical procedure on a patient, were you on PEP? Thanks.

Response from Dr. Frascino


The answer to your first question is a bit complex and technical, but I'll try to explain.

Blood donors have been screened for HIV since 1985. Although newer generations of the test have been developed that increase the sensitivity for detecting HIV antibodies, in rare instances infections in donors have been missed. In 2002 the FDA licensed the first NAT (nucleic acid test) for screening blood donors. NAT detects viral genes rather than anti-HIV antibodies or antigens (proteins from the virus). Viral gene detection allows for earlier detection, because the production of detectable antibodies, which is an immune response, requires time. With the use of NAT, the HIV window period has been reduced to approximately 16 days.

NAT is FDA-approved only for use in screening donated blood. It is not approved for routine HIV screening for individual patients. There are a number of reasons for this having to do with the sensitivity/specificity of the testing assay and the cost. The purpose of blood-donation screening is to protect the blood supply. Consequently the sensitivity of the screening tests is set so that it will err on the side of protecting the blood supply. In other words, if the test picks up some false positives, no problem. The questionably positive blood can then be discarded. Donors are notified to follow up with their doctors to see if their test was a false positive or if they are truly HIV infected. Again the primary purpose of these tests is not to diagnose patients, but to protect the blood supply. NAT is also extremely expensive. NAT testing is performed on pooled samples. That means usually around 20 donor samples are mixed together and tested as a batch. If the batch tests positive, then each of the 20 donor samples is tested individually by our usual serological testing methods. See, I told you this was a bit confusing.

As for your second question, my occupational exposure was in January 1991. We didn't know much about PEP back then and only a few antiretrovirals were even available. I did take AZT within moments of being stuck and continued it for a full course, but seroconverted nonetheless. The patient I was performing the procedure on had advanced-stage AIDS; consequently, his viral strain was probably resistant to AZT. We didn't have resistance tests back then. I should also point out that PEP, even when taken as directed, isn't always successful in preventing HIV infection.

Dr. Bob