The Western blot - the most commonly used confirmatory test - is a highly specific immunoblot that allows for the visualization of antibodies to the structural polypeptides of HIV. Therefore, to enable the laboratory to select appropriate testing, it is important to provide a clinical history that includes any recent high-risk behaviour or symptoms consistent with seroconversion illness ( 1- 3).Īll HIV diagnostic laboratories should confirm repeatedly positive EIA screen tests with another assay (Figure (Figure1). In a small number of early seroconverters who are still in the 'window period', the p24 antigen may become positive before antibody is detectable. With the highly sensitive HIV-1/HIV-2 enzyme immunoassay (EIA) tests currently on the market, seroconversion can be detected within two to three weeks of infection in the majority of cases. Antibody testing is the method most commonly used to diagnose HIV infection. HIV infection is identified either by the detection of HIV-specific antibodies in serum or plasma or by demonstrating the presence of the virus by nucleic acid detection using polymerase chain reaction (PCR), p24 antigen testing or, rarely these days, by growing virus in cell culture. Laboratory quality assurance programs and the participation in HIV proficiency testing programs are essential to ensure that diagnostic laboratories provide accurate, timely and clinically relevant laboratory results. However, the performance of these tests requires adherence to good laboratory quality control practices, as well as the backup of a licensed diagnostic laboratory to provide confirmation and resolution of positive or indeterminate results. These tests provide rapid, on-site HIV results in a format that is relatively easy for clinic staff to perform. Point of Care tests have become increasingly popular in the United States and some places in Canada over the past several years. Laboratories should also provide specific information on specimen collection, storage and transport so that specimen integrity is not compromised, thereby preserving the accuracy of laboratory results. The choice of assays is guided by the initial screening results and the clinical information provided by the physician both are integral to the laboratory's ability to provide an accurate laboratory diagnosis. Most diagnostic laboratories have complex testing algorithms to ensure accuracy of results and optimal use of laboratory resources. A variety of other assays are essential to confirm positive antibody screens (Western blot, polymerase chain reaction ), provide an adjunct to antibody testing (p24 antigen, PCR), or provide additional information for the clinician treating HIV-positive patients (qualitative and quantitative PCR, and genotyping). Current enzyme immunoassays are sensitive enough to detect antibody as early as one to two weeks after infection. HIV diagnostic testing has come a long way since its inception in the early 1980s.
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