Jul. 4th, 2012

holdthesky: (Default)
So, I was reading this BBC article about the OraQuick home-test HIV kit.

I thought the statistics were very unclear, and that handing these kits out without more careful thought in the accompanying literature is liable to cause a lot of harm and distress to people.

The manufacturer claims that the test is 99% accurate in the case of a false-positive, and 92% in the case of a false negative. I'm sure they're honest folk.

On the face of it, that sounds very reassuring, but look at it like this.

I guess there are about 150,000 people with HIV in the UK out of a population of 60m (say). If everyone were to test (without any prior bias with respect to their status) then 1% of the non-infected population, about 600,000 people, would get a false positive diagnosis. Now, it's estimated that 20% of the HIV-positive population don't know that they're infected, and that means about 14,000 people would potentially learn of a positive status with this kit.

If you are someone with a positive test result, you could be one of the 14,000 who are HIV positive, or one of the 600,000 who false positives. Not everyone will take the test but the proportions will stay the same if there's no testing bias.

In other words, if you get a positive test with this case, there's around a 2% chance you are HIV positive, 98% of positive tests will be false alarms.

Even more sadly, I suspect this kit will be widely used in the wake of sexual assault or reckless sexual encounters to provide reassurance and I'm not sure that something for which 98% of alarms are false will really help.

There will be election bias which will level those percentages a little (people who are HIV positive will more likely seek the test) but I find it hard to believe it will help much and when you're relying on "priors" so much you're really doing little other than testing peoples fears.

I hope that this test doesn't cause too much unnecessary distress.

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holdthesky

July 2013

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