Are there false positive HIV tests

Monthly unstatistics

"You are likely to be HIV positive"

Non-statistics from January 8, 2019

December 1st was World AIDS Day. The motto in 2018 was “Know your status”. The aim is for 90 percent of all people to know their immune status by 2020. Since October, HIV rapid tests have been allowed to be freely sold in Germany, which you can order in pharmacies, drug stores or via the Internet and carry out anonymously yourself. All you have to do is read the instructions for use, pierce the skin on the fingertip with a lancet and pour the blood into a test tube. Then you wait 10 to 15 minutes and finally you get the result: positive or negative. Unfortunately, there is no clear explanation of what the result means. However, this would be especially important for all those who do an HIV self-test without a doctor.

Assuming you want to know your status and order online the “autotest VIH” recommended by the German AIDS Federation, which is provided with the CE mark of the EU, which confirms suitability for lay people (www.autotest-sante.com). The test result is positive. Read the instructions for use to see what it means. It says: “You are probably HIV positive.” The same wording can be found in other certified rapid tests such as “INSTI” and “Exacto.” Is a positive test a death sentence? How likely is likely? Many people think this means they are more likely than not to be infected. The instructions for use also state how good the test is in numbers: sensitivity: 100%, specificity: 99.8%.

Out of 13 people who test positive for HIV, only one is infected

Sensitivity is the likelihood that a person will test positive if they are infected with HIV. Specificity is the likelihood that a person will test negative if they are not infected with HIV. In other words, the false alarm rate is only 0.2 percent. So this impressive accuracy proves that a positive result is almost certain. It seems so.

How high is the probability of being infected with HIV if the test result is positive? This is neither 100 percent nor 99.8 percent. It is also not found in the instructions for use, nor is it explained how to determine it.

A rough calculation can give the answer. According to the Federal Statistical Office, around 69 million people over the age of 18 live in Germany. An estimated 11,400 of them are infected without knowing it: 2,700 of them through heterosexual contact, the others through sex between men or intravenous drug use (www.rki.de). So about one out of every 6,000 Germans is infected (69 million divided by 11,400). This will test positive with certainty (100 percent). However, among the 5,999 people who are not infected, we expect another 12 who also test positive. This follows from the false alarm rate of 0.2 percent. This means that out of a total of 13 people who test positive, only one is actually infected. So "you are likely to be HIV positive" means that there is only about an 8 percent chance that you will be infected. In other words, if you test positive in the rapid test, the probability that you are not infected is 92 percent. In the case of heterosexuals without risk behavior, the majority of Germans, the probability of being infected is even lower, it is less than 5 percent.

Comprehensible education saves lives

The instructions for use say that if you test positive you should see a doctor as soon as possible. That makes sense. Only studies in Germany show that, for example, the majority of AIDS advisors at health authorities themselves have not learned to understand health statistics and believe that there are no “false positives” (https://europepmc.org/abstract/med/26149159 ). It would therefore be all the more important to clearly state in communication and, above all, in the instructions for use what a positive result in the rapid test really means. In similar situations, people have thought about and committed suicide - even though they were not actually infected - to avoid the stigma and social discrimination that are still linked to AIDS.

Rapid HIV tests can be useful. However, dealing responsibly with them includes explaining to people in an understandable way what a positive test result really means.

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Contact Person:

Prof. Dr. Gerd Gigerenzer, Tel .: (030) 82406-0
Sabine Weiler (RWI Press Office), Tel .: (0201) 8149-213

With the “Unstatistics of the Month”, the Berlin psychologist Gerd Gigerenzer, the Dortmund statistician Walter Krämer and RWI Vice President Thomas K. Bauer scrutinize both recently published figures and their interpretations every month. You can find all “non-statistics” on the Internet at www.unstatistik.de.

Press release (PDF)