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Understanding the test results

Jan 5, 2007

My sister recently donated at a blood drive. She got a letter back stating that she testing positive for hiv-1 and hiv-2, but negative on the western blot and negative on some other test, I can't remember what it was. The letter stated that she was NOT considered to have HIV. My question is: if she tested positive for those two and negative for the other two does she really have HIV? What would be the reason for the false positive? Thank a million! -Worried Sister-

Response from Dr. Frascino

Hello Worried Sister,

Your worries are completely unwarranted. Your sister is unquestionably HIV negative. You can read about many similar cases in the archives. (Have a look!)

Basically, the HIV testing procedures for blood screening are developed to absolutely protect the blood supply. They are designed to be, in a sense, overcautious, which means they pick up more "false positives" in an effort to make sure they don't miss a "false negative." That your sister's confirmatory tests Western Blot and, presumably, a NAT are negative is definitive and conclusive. HIV is not her problem. No way. No how. There are a variety of reasons for false-positive ELISA tests. You can read about these in the archives as well.

I'll repost a question from the archives that discusses HIV testing for blood donations.

Dr. Bob

Rejected blood donation Jun 5, 2006

I donated blood for the first time a few weeks ago. I received a letter from the Red Cross saying they obtained "inconclusive" results for HIV. While it does say it is usually the testing process rather than donor health problems. I plan on getting tested, but wanted to make sure if the Rapid test is what i need or if i need to do the Orasure which can take up to 2 wks for results. Do I need to get tested again in the 3months? I just want to minimize any anxiety.

After reading many of the posts/responses i'm fairly confident that i'm at extremely low risk. I do have HSV2 and rarely have sex. The last time was around October and the condom did come off. Worst part.. i thought the guy took it off after sex.. he didn't. It was still inside of me and the jerk didn't bother to tell me.

thanks tk

Response from Dr. Frascino

Hey tk,

A negative rapid test will give you a definitive result and no additional testing will be needed thereafter. You'll have your reassurance in just 20 minutes!

I'll repost a question from the archives that addresses the HIV screening for blood donations.

Good luck! Stay well.

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

Re Re Post: HIV from using a Dildo on Female Escort [ A Petrified Doc, Have Donated will donated again]
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