ELISA is an abbreviation for 'enzyme-linked immunosorbent assay'. ELISA tests are relatively accurate tests, highly sensitive and specific. They compare favorably with other methods used to detect substances in the body such as Radio immune assay (RIA) tests. ELISA tests have an added advantage in that there is no need for radioisotopes or costly radiation counter. An HIV ELISA test is also called HIV enzyme immunoassay (EIA). It is the first and basic test to determine if an individual is positive for a selected pathogen such as HIV. The test is performed in a plastic plate of 8 cm x 12 cm which contains 8 x 12 matrix of 96 wells, each of which is about 1 cm high and 0.7 cm in diameter.
A patient's serum contains certain antibodies. If the patient is HIV positive, then the serum will contain antibodies to HIV. Those antibodies will bind to the HIV antigens on the plate. Sometimes, even in some individuals not infected with HIV antibodies, positive result is given in HIV ELISA. This is called false positive. One reason for false positive is that in women who have had multiple pregnancies, may possess the antibodies directed against human leukocyte antigens (HLA) which are present in host cells used to propagate HIV. As HIV buds from the surface of the host cell, it incorporates some of the host cell HLA into its envelope. False negatives can also occur during the window between infection and an antibody response to the virus called seroconversion. A person will be retested if the serum gives positive result. If the ELISA retests are also positive, then the patient will be retested by western blot analysis.
Urine based HIV test
This was approved in 1996 by FDA as a screening test for HIV. This test uses urine samples to detect HIV antibodies using ELISA method. This test is not considered as accurate as blood test in detecting HIV infection. Urine based test could give false positive result in one or two persons out of hundred. Therefore a positive ELISA screening test must be followed by a confirmatory western blot test or IFA or RIPA and then results confirmed.
CD4 test: The CD4 count in individuals not affected by HIV is normally above 500 cells per cubic milliliter of blood. In HIV infected people, CD4 count is less than 200 cells per milliliter of blood. These patients are referred to as immunosuppressed. HIV people do not run the risk for complications until CD4 cells are fewer than 200 cells per milliliter of blood. A declining number of CD4 count indicates the advancement of the disease. A low CD4 cell count signals that the person is at risk for one of the many unusual infections that occur in individuals who are immunosuppressed. CD4 count is also indicative of the type of therapy the person should undergo to prevent 'opportunistic infections'.
Viral load: The viral load predicts whether or not the CD4 cells will decline in the coming months. Knowledge of the amount of viral load can be instrumental in predicting the development of the disease. Those persons with high viral loads are more likely to experience a decline in CD4 cells and progression of the disease than those with lower viral loads. The viral load is also a vital tool for monitoring the effectiveness of the new therapies and determining when the drug stops working. The greater the decline of viral load after beginning therapy, the longer it will remain suppressed. In general it can be said that a poor response to HIV therapy or treatment failure would include individuals who fail to experience a decline in viral load of approximately 100 fold in the first 8 weeks and have a viral load of greater than 500 copies per ml by week 12 or have levels greater than 50 -75 copies per ml by week 24.
Drug resistance testing: This test is being used in individuals who are experiencing poor response to HIV therapy or treatment failure. The US department of health and human services (DHHS) in their recent guidelines have suggested that 'resistance testing' can be considered in individuals who have never been on therapy particularly in the first months or even years of infection to determine if they might have acquired HIV that is resistant to drugs. DHHS even formally recommend such testing to be performed in individuals starting therapy for the first time.
The Western Blot test
The western blot test uses the general western blot procedure. A slab of gel with high voltage speed is taken and the proteins contained in the HIV infected cells are opened. The separated proteins are transferred to another plate and a procedure similar to ELISA is performed. The patient's diluted serum is poured over the plate and the HIV antibodies bind to certain proteins on the plate while others are washed away. The enzyme linked antibodies detect the presence of bound antibody. With this procedure, the proteins are separated and it is possible to see exactly to which HIV proteins the patients have antibodies. The test is considered positive if the antibodies to several major HIV proteins are present. Normally a combination of ELISA and Western blot tests are employed for diagnosing HIV infection. Blood donors are also screened with these tests.