All the tests described so far depend on having a laboratory with trained technicians to operate them. They often take more than an hour to carry out and some Western Blot tests have to be incubated overnight.

A number of companies have developed 'Rapid Tests' supplied as single-use kits that do not require laboratory facilities, although most of them do need to be kept in a refrigerator until they are used. Most of them use whole blood, not serum, which makes them much easier to use by health staff and, sometimes, untrained personnel.

These kits are a lot more expensive, per test, than ELISAs. They are therefore not suitable for use in mass screening, for example, of blood donors. They have hardly been used in most Western countries. Only one has been licensed by the US Food and Drug Administration: the SUDS test made by Murex/Abbott [off the market at the time of writing, due to manufacturing problems].

However, evaluations by the World Health Organization and by the UK's Public Health Laboratory Service suggest that the best of these tests are comparable in performance to most ELISA tests. Individual tests give a comparatively high rate of false positives, but when used in combination these may still be a realistic alternative to ELISA and Western Blot tests.

The biggest advantage of Rapid Tests is, as their name implies, that they give results very fast - often as little as 10 to 20 minutes. This means that people who may have travelled many miles to reach a test centre need not make another journey to get their results.

In the USA, it has been shown that a Rapid Test used as an initial screen can greatly increase the proportion of positive people who return for their test results. Unfortunately, the cost where confirmation is delayed is that many people are given an initial result that later turns out to be negative, causing avoidable distress.

How rapid tests work

One system uses antigen coated on the inside of a small tube through which the blood sample flows. If antibodies bind to the antigen, they block up the tube.

Another system uses a membrane on which the antigen is coated. If antibodies in a blood sample bind to this, they will hold a colloidal suspension of gold which makes them visible as a dark spot on the membrane.

Risks and limitations

One disadvantage of Rapid Tests is their potential for misuse, if a single test is used without confirmation. The temptation to do this in a community where HIV prevalence is high, so that most positive results are genuinely positive, and where even $2 or $3 is a lot of money, must be considerable.

People working in the UK with migrant and refugee communities may come across individuals who have been tested in this way without proper confirmation of their test results. It is standard practice in the UK is to re-test any HIV positive person who is seen for the first time by an HIV treatment service. If any test negative, they must obviously be treated with great sensitivity as they will be at risk of losing confidence in treatment services which they or their family may still need at a later date.