IFA stands for 'indirect immunofluorescence assay'.

IFA tests are based on preparations of fixed HIV-infected and uninfected cells, supplied in wells on a microscope slide, to which test sera are added and incubated.

The idea is that anti-HIV antibodies will bind to HIV infected cells but not to uninfected ones. The test requires a skilled operator and an expensive piece of equipment, but gives faster results than Western Blot.

After incubation, the preparation is washed and fluorescein-labelled antibody against human immunoglobulins is added. This is again incubated and washed and then prepared for viewing.

When the slide is viewed under a fluorescent microscope, HIV antibodies are present if the infected cells in the test system glow green and the uninfected ones do not.