Western blot

From AIDS Wiki

The Western Blot is an HIV antibody test often used to confirm a series of positive ELISA tests. The Western Blot has the proteins that are believed to be HIV antigens separated by molecular weight on a strip. When a serum sample is added to a strip reactions may occur between antibodies in the sample and antigens in the test kit. If enough of the right reactions occur then the test is considered reactive and often this means that a person will be declared to be HIV-positive.

The Perth Group has identified 11 different interpretations of the test from different countries and, within the US, by different organizations. That means that you might be positive by one standard, negative by another and 'indeterminate' by a third. The status of 'indeterminate' occurs with Western Blot because there are situations where a sample is reactive on one or more proteins but not enough to qualify as a positive test. Indeterminate tests are almost always treated as negative (uninfected).

This creates a bit of a conundrum. If any one protein can occur in uninfected people it means that none of the proteins in the Western Blot kit are unique to HIV (or at least, that false reactions can occur in the absence of HIV). If one false reaction can occur, then multiple reactions can occur, meaning that a positive Western Blot might be a false positive. An argument can be made that if 1% of people have a reaction to p24 and 1% to p41, that the chances of a reaction to both are 1 out of 10,000, but this assumes that the two antibody-antigen reactions are independent. This is not a valid assumption. In fact, it is not valid to assume that the ELISA tests and Western Blot tests are independent, given that they are both antibody tests.

For detailed scientific information on HIV tests including Western Blot see RethinkingAIDS.com.