HIV testing and Inside Health


Here are some of the references I used for todays’  Inside Health on HIV testing.

One thing I didn’t get a chance to talk about was how prevalence – the number of cases within a community – influences the false positive rate in the people being tested.

The bottom line is that false positives in a low prevalence community are more common than in a high prevalence community.

So, for example, one 4th generation HIV test has a false positive rate of 3 per 1000. So for every 1000 HIV negative people, 3 will test as positive.


If you are running the test in a low prevalence area, where the rate – as in the UK – is 0.1%, 1 in 1000, this means that in 1000 people, 4 will test positive – 3 will be false positives, and one a true positive. So if you are in a low prevalence community and test positive, you only have a 1 in 4 chance of being a true positive.


If you run the test in a high prevalence area, say for example the rate of HIV infection in men who have sex with men in London is usually given at 1 in 10, it’s quite different. The false positive rate of the test is still 3 in 1000, but there will be 100 true positives in this group. So there will be 103 positive tests altogether. The chance of a false positive test is just under 3%, at 3/103. So 100 positive tests in this group will be true positives, 3 will be false positives.


This sort of knowledge I think very important. Here are some of the references I used.

2 Responses to “HIV testing and Inside Health”

  1. Guy Chapman September 5, 2012 at 9:53 pm #

    Any idea what the false positive rate is for coeliac TTG testing? I understand the disease prevalence is around 1% (sounds high to me) and I am also told that there is a 5% false negative rate with the test but don’t know the false positive rate.

    It’s out of purely academic interest, biopsy confirms coeliac in my case, but I’d be really interested to know!

  2. Margaret McCartney
    margaretmccartney September 8, 2012 at 4:10 pm # here you go!