Simple and non-invasive tests, confirmatory tests, follow-up tests

Simple anti-HIV screening tests have been developed for use in clinics, in unfavourable laboratory conditions and close to the patient. When results are needed urgently, for instance before transplantation procedures and to select a blood donor in the field, they are quick and practical. Saliva (oral fluid) and urine can conveniently be used as specimens to investigate for anti- HIV when venepuncture is difficult, hazardous or unacceptable to the patient. These simple rapid and non-invasive tests are attractive options and may lead to developments such as home testing. However, few of these tests are quite as accurate as the conventional assays on serum, and follow-up confirmatory tests are essential before a positive diagnosis is made by these means.

In many countries, including the UK, formal procedures have been put in place to secure accurate testing. The most important is that when there is a positive anti-HIV finding the test is repeated and the implicated specimen is tested by other, methodologically independent, anti-HIV assays. Another specimen should then be sought. Although this may cause some delay in confirming a positive finding, anti-HIV testing is as a consequence more precise. A few infected individuals may have little or no detectable anti-HIV when first tested or there may have been technical or clerical mistakes, including specimen misidentifications and transcription errors. Follow-up at an interval of one to four weeks greatly diminishes the chance of either a false negative or a false positive anti-HIV result, and follow-up specimens are the most important element in the accurate laboratory diagnosis of HIV infection. When newly infected individuals are followed up, they show an increase in the titre and range of HIV antibodies. By contrast, persistently weak anti-HIV reactions are usually non-specific. Sometimes PCR (see below) will resolve a difficult-to-confirm antibody reaction. Follow-up procedures also guard against specimen misidentification and transcription errors.