In a competitive ELISA test the HIV antigen is exposed at the same time both to the test serum and to enzyme-linked antibodies chosen to bind to the HIV antigen.

What happens next depends on the outcome of a 'competition' between the antibodies, to see which binds more closely to the antigen. Weakly binding, 'cross-reactive' antibodies should not give a positive result, so this test should be more specific (see below) than basic ELISA tests. However, there is a risk that weakly binding genuine HIV antibodies might also be displaced, giving rise to false negatives and making the test less sensitive than basic ELISA tests.

This test should also detect antibodies, no matter what class of antibodies they are. Or, to put it the other way, it cannot distinguish between antibodies of different classes.

If the test serum has antibodies that bind to the antigen as well as or better than the test antibodies, these will prevent the enzyme-linked antibodies from binding. Otherwise the test antibodies, with their attached enzymes, will be bound.

The system is washed in the normal way for an ELISA test before the indicator chemical solution is added.

In this case, a colour change means a negative result and no change is a positive result.

The accuracy of this system does not depend on having highly purified antigen, because the only antigens that matter are the ones recognised by the enzyme-linked antibodies. The system has the further advantage that control tests are easy to run and will quickly show if the test system is working properly, as a check on operator error.