AIDS tests cover a number of different procedures used to diagnose and treat HIV patients. Some AIDS tests are used to diagnose patients or confirm a diagnosis; others are used to measure the progression of the disease or the effectiveness of specific treatment regimens.
Some AIDS tests can also be used to screen blood donations for safe use in transfusions.
Diagnostic blood tests for AIDS are usually given to persons in high-risk populations who have been exposed to HIV or who have the early symptoms of AIDS.
Most persons infected with HIV will develop a detectable level of antibody within three months of infection.
It is possible to diagnose HIV infection by isolating the virus itself from blood sample or by demonstrating the presence of HIV antigen in the blood. Isolating the virus itself is expensive, not widely available, and slow.
More common are blood tests that work by detecting the presence of antibodies to the HIV virus. They are as follows:
Enzyme-linked immunosorbent assay (ELISA):
ELISA (also sometimes called EIA) is often used as the first screening tool for HIV. HIV antibody tests are the most appropriate test for routine diagnosis of HIV among adults.
It is inexpensive and very sensitive for detecting the presence of HIV antibodies. In most cases, a blood sample is tested, but other types of ELISAs that use saliva and urine have also been developed. The actual ELISA takes 3.5 to 4 hours.
ELISA test involves HIV antigens that are attached to a plastic surface. A sample of the patient’s blood serum is added. If there are any HIV antibodies in the blood, they will attach to the HIV antigens. After rinsing off excess blood factors, a second antibody is added.
This antibody binds with any HIV antibodies that are attached to the HIVantigens. The second antibody also contains a chemical that changes color, indicating the presence of HIV antibodies in the patient’s blood.
Occasionally the result may be a false positive. Western blot or immunoblot test is used to confirm the diagnosis of AIDS.
Western blots (WB) testing:
It is only performed if an ELISA or rapid test is positive. The WB can be positive, negative, or indeterminate. Indeterminate tests are neither positive nor negative.
An indeterminate result usually means that a person has just begun to seroconvert at the time of their test. In the rare cases in which this occurs, the person will need to be retested, usually about one month later.
False positive results are extremely rare with the WB, so it confirms (proves) that HIV antibodies are present.
In Western blot testing procedure HIV antigens are suspended in a flat slab of gel. An electric current run through the gel causes the proteins to separate from one another and move varying distances depending on their molecular size.
Afterwards, the gel is pressed against a nylon or nitrocellulose filter, transferring the proteins to the filter. The patient’s blood is made to react against the filter, followed by treatment with developing chemicals. If HIV antibodies are present, they show up as a colored patch or blot on the filter.
Immunofluorescent assay (IFA):
The IFA can be used instead of the WB to confirm ELISA results. Like the WB, IFA tests for the presence of antibodies in a blood sample. The exact strategy is slightly different in that it uses a microscope. It can be faster than a WB.
Instead of Western blotting sometimes this test is used to confirm ELISA results. An IFA test detects the presence of HIV antibody in a sample of the patient’s serum by mixing HIV antigen with a fluorescent chemical. If antibodies are present in the test sample their binding with HIV antigens can be observed under a microscope with ultraviolet light.
Polymerase chain reaction (PCR):
A PCR test is used to evaluate the very small number of AIDS patients with false-negative ELISA and Western blot tests. PCR testing is based on present knowledge of the genetic sequences in HIV. It can detect the genetic material of HIV rather than the antibodies to the virus, and so can identify HIV in the blood within two or three weeks of infection.
This test works by amplifying the presence of HIV nucleic acids in a blood sample. Numerous copies of a gene are made by separating the two strands of DNA containing the gene segment, marking its location, using DNA polymerase to make a copy, and then continuously replicating the copies.
The test is also known as a viral load test and HIV NAAT (nucleic acid amplification testing). Since the virus is continually generating new variants, PCR testing could yield a false negative in patients with these new variants.
Antigen test (P24 test):
The antigen on HIV that most commonly provokes an antibody response is the protein P24. Early in HIV infection, P24 is produced in excess and can be detected in the blood serum (although as HIV becomes fully established in the body it will fade to undetectable levels).
P24 antigen tests are not usually used for general HIV diagnostic purposes, as they have a very low sensitivity and they only work before antibodies are produced in the period immediately after HIV infection. They are now most often used as a component of ‘fourth generation’ tests.
Some of the most modern HIV tests combine P24 antigen tests with standard antibody tests to reduce the ‘diagnostic window’. Testing for antibodies and P24 antigen simultaneously has the advantage of enabling earlier and more accurate HIV detection.
Blood tests to evaluate patients already diagnosed with HIV infection are as important as the diagnostic tests.
Because AIDS has a long latency period, some persons may be infected with the virus for 10 years or longer before they develop symptoms of AIDS. These patients are sometimes called antibody-positive asymptomatic carriers. Prognostic tests also help drug researchers evaluate the usefulness of new medications in treating AIDS.