HIV **prevalence** in the US =
1 in 300 people are HIV positive **(HIVpos = 0.00333)**

{*Note: probability of being HIVneg
is 1 - 0.00333.*}

**Sensitivity** [probability that the genetic test
will be positive when the individual really has the disease] of ELISA =
99.7% **(0.997)**

**Specificity** [probability that
the test will be negative given that the individual does not have the disease]
of ELISA = 98.5% **(0.985)**

{*Note: probability of being
HIVneg but getting a postive ELISA result is 1 - 0.985 .*}

**What is the probability of being
HIV positive (HIVpos) if you test positive on an ELISA (+)?**

**Bayes' Rule for HIV testing**:

Source for testing data: http://en.wikipedia.org/wiki/HIV_test

© Copyright 2007 Department of Biology, Davidson
College, Davidson, NC 28035

Send comments, questions, and suggestions to: **macampbell@davidson.edu**