1. Antibody tests - ELISAs are the most frequently used method for screening of blood samples for HIV antibody. The sensitivity and specificity of the presently available commercial systems approaches 100% but false positive and false negative reactions occur. Other test systems available include passive particle agglutination, immunofluorescence, Western blots and RIPA bioassays. Western blots are regarded as the gold standard and seropositivity is diagnosed when antibodies against both the env and the gag proteins are detected. The sensitivity of the test systems are currently being improved by the use of recombinant antigens.
2. Antigen tests - HIV antigen can be detected early in the course of HIV infection before the appearance of antibody. It is undetectable during the latent period (antigen-antibody complexes are present) but become detectable during the final stages of the infection. It was argued that the routine use of antigen screening tests in the blood transfusion service may result in earlier cases of HIV infection being identified. However a large scale study carried out in the US failed to show any benefit.
3. Virus isolation - virus isolation is accomplished by the cocultivation of the patient's lymphocytes with fresh peripheral blood cells of healthy donors or with suitable culture lines such as T-lymphomas. The presence of the virus can be confirmed by reverse transcriptase assays, serological tests, or by changes in growth pattern of the indicator cells. However virus isolation is tedious and time consuming (weeks) and is successful in only 70 to 90% of cases. Therefore virus isolation is mainly used for the characterization of the virus.
4.. Demonstration of viral NA - this can be accomplished by probes or by PCR techniques. The latter may be useful because of its extremely high sensitivity.
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