Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
The Western blot tests for antibodies to a particular protein(s). In HIV diagnosis a set of HIV proteins are laid out on a nitrocellulose film and then a sample of blood serum from a patient is added to the film. If the serum contains any antibodies to HIV proteins then the antibodies will latch on at the appropriate position on the film.
As in the ELISA test, we then need to be able to see whether antibodies have attached - and just as in the ELISA test, we do that by washing the film gently and adding an enzyme which will attach to antibodies present and which can catalyse a chromogenic reaction.
Again we wash the strip, and then add the ingredients for that chromogenic reaction. Where antibody has bound, we see colour produced.
There are different standards for interpreting the Western blot. The US Multicenter AIDS Cohort Study 1983-1992 suggests that three weak bands or any strong band indicates a positive result. Other standards require that there must be at least one band representing antibodies to envelope protein before a positive result can be reported.
You may also want to read about the ELISA test and Tests of HIV progression.