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From Alive & Well Emailer, July 2002
How a 99.99% Accurate Test Can Be Wrong Half the Time
By Christine Maggiore
“A new article in Popular Science explains how allegedly
accurate HIV tests can be dead wrong when performed on people with
no risk factors…”
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An article in the July 2002 issue of Popular Science magazine gives
readers some surprising food for thought: the allegedly accurate
HIV tests can be dead wrong when performed on people with no risk
factors. Here’s the text from page 78:
"HIV testing is 99.99 percent accurate, a doctor might tell
his patient. That suggests that if you receive a positive result,
you almost certainly have HIV. But this is not necessarily the case.
The chance of a straight man with no known risk factors contracting
HIV is roughly one in 10,000. That is also the rate at which an
HIV test returns an incorrect result. So if 10,000 men in this low-risk
group get tested for HIV, an average one positive will come back
from the man with HIV, and another man will test positive even though
he is not infected. Thus, in our statistically perfect world, only
one of the two men who test positive actually has HIV. 50 percent
of positive HIV tests in the low-risk group turn out to be false."
The article incorrectly assumes that HIV tests can diagnose HIV
infection and that the claimed accuracy rate of 99.99% has been
established through careful scientific studies. In fact, HIV tests
have not been approved by the US Food and Drug Administration for
use in diagnosing actual HIV infection, and all claims of test accuracy
are based on estimates and assumptions.
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