ELISA = Enzyme-linked Immunosorbent Assay (checks for antibodies)

After adopting the World Health Organisation’s protocol for HIV screening in Developing Countries, the 3-ELISA protocol has become a standard phrase in the context of HIV testing for insurance purposes in South Africa.

These methods utilize manufacturing techniques that create highly sensitive assays. An early seroconversion will be best detected by these assays. They do, however, in 0.5 – 2% of cases, give false positive reactions because of their great sensitivity.

To circumvent this problem the WHO developed a system whereby three distinctly different ELISA-reagents are used in tandem to confirm any reactivity found in the initial screening assay. If any one of the three does not agree with the result of the other two, the result is expressed as ‘discordant’ and a delay is introduced to allow a potential seroconversion to take place before a second sample is assayed.

In a true false-reactive the result will still be discordant after 3-4 weeks. Although the possibility exists, the potential for false-negative results is minimal. These assays are relatively cheap and automatable in high-volume assay equipment. This minimises the opportunity for human technical error.

Rapid Tests (checks for antibodies)

In the past most clinics have been making use of the Eliza Test and will probably continue to do so for quite some time as it remains a very reliable test which is relatively cheap. The major disadvantage of the Elisa Test is the fact that the test is usually done off-site. In the case of South Africa the test is done for the clinics by the SA Medical Research Institute. It takes about one week for the results to come back. Often staff and students fail to come back for the results. This may be because they have had time for a rethink, and then they refuse to hear the outcome of the test. Another disadvantage for clinics is that the test is not free. Many people cannot afford to pay for it. Paying for employees, students etc. can be quite expensive if you have a very large institution.

Recently we have seen new tests which still check for the presence of antibodies but are able to detect them within a very short period of time. The Rapid Test takes a few minutes to show up HIV antibodies. The advantage of this is that the patient does not have to leave the clinic. The added advantage is that these rapid tests are also very affordable.

P24 Antigen Detection (checks for the virus)

This method can detect a component of the virus within a few weeks after infection. Although not dependent on antibody production, the sensitivity of this test is such that in most cases an ELISA antibody assay would provide the answer at less expense.

PCR Antigen Detection (checks for the virus)

This method is known as the ‘three week test”. This is an extremely sophisticated method which can detect very small numbers of HIV virus in blood soon after infection. The virus usually becomes detectable 3-4 weeks after exposure and the test may be positive some time before antibodies are found (i.e. in the window period). This test will remain positive for the duration of the illness. In further sophistication, the assay is used to quantify the amount of virus in the blood to help in management and drug therapy of infection with HIV.

Although continuous improvement towards more user-friendly methods, the molecular technologies are still:

  • Ultra-sophisticated and must be performed in a super-specialist environment
  • Are very expensive and for the moment are limited to only very few laboratories;
  • Not suitable for mass screening