Ask Kim: What Causes a False Positive?

Dear Kim,

I know it’s not super common, but I have heard that it’s occasionally possible to get a false positive or negative on a rapid HIV test. What causes that? Is there any way to avoid it?



Dear Curious,

You’re right – it’s not very common, as rapid HIV tests are over 99% accurate, but it is possible to get a false negative or false positive in very rare circumstances. There are a few reasons this can happen.

The first, and most common, is human error. In order for a rapid test to work properly, it has to be unexpired, and clinics must run controls on the tests regularly. Controls tell us whether a batch of HIV tests are working properly. 

The environment in which HIV tests are stored also affects its accuracy. If it gets too hot or too cold, that may affect the quality of the tests. Clinics have sensitive temperature control equipment that includes alarms to let staff know if the tests have become to warm or too cold, which could render them invalid.

Certain infections or disorders can also lead to inaccurate rapid HIV test results. Autoimmune disorders like lupus, viral infections such as the Epstein-Barr virus, and recent vaccinations like the Hepatitis B vaccine can on rare occasions cause false positive results. 

False negative results, on the other hand, most often happen during the “window period” – the time between when the person is infected and when the virus reproduces enough to be detected by the test.

False negative results are much less common than false positives, although both are exceedingly uncommon. To ensure that a positive rapid test result is a true positive, a confirmatory test – a blood sample analyzed in a lab – is performed. If this is also positive, the person is diagnosed with HIV.

I hope this answers your question! Again, keep in mind that the rapid HIV test is highly accurate, so your change of getting a false positive or negative is very low.

Be well, be yourself.


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