Health Secrets: HIV Positive..? Maybe Not

 Health Secrets You're Not Supposed To Know series

 FOR LOW-RISK PEOPLE, A POSITIVE RESULT FROM AN HIV TEST IS WRONG HALF THE TIME

 

  Although a lot of progress has been made in improving the length and quality of life for people with AIDS, getting a positive result from an HIV test must still rank as one of the worst pieces of news a person can get.
  It's not uncommon for people to kill themselves right after hearing the results, and those who don't commit suicide surely go through all kinds of mental anguish.
 But the accuracy of these tests is lower than generally believed.

   In fact, if you test positive but you're not a member of a high-risk group (such as non-monogamous gay men and intravenous drug users), the odds are 50-50 that you actually have the virus.

 To be declared HIV-positive, your blood goes through three tests — two ELISA tests and one more sensitive and costly Western Blot test.
  Makers of the tests trumpet a 99.99 percent accuracy rate when all three are used.
   Many AIDS counselors even tell people that the tests never give a false positive (that is, the tests don't indicate that someone is HIV-positive when he or she really isn't).

  The test manufacturers' claim is misleading, and the counselors' claim is flat-out BS.
Cognitive scientist Gerd Gigerenzer — who specializes in risk and uncertainty — explains the reality in plain English:
  Imagine 10,000 men who are not in any known risk category.
  One is infected (base rate) and will test positive with practical certainty (sensitivity). Of the 9,999 men who are not infected, another one will also test positive (false positive rate). So we can expect that two men will test positive.


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  *references:
   Gigerenzer, Gerd.
 Calculated Risks: How to Know When Numbers Deceive You. Simon & Schuster, 2002.