HIV Tests (AIDS Tests)

HIV test kits used both to screen donor blood, blood components and cellular products, and to diagnose, treat and monitor persons with HIV and AIDS are regulated in the United States by the FDA.

HIV tests to detect antibodies, antigens or RNA in serum, plasma, oral fluid, dried blood spot or urine have been approved by FDA for donor screening, diagnosis, prognosis and patient monitoring.

The window period is the time from infection until a test can detect any change. The average window period with antibody tests is 22 days. Antigen testing cuts the window period to approximately 16 days and NAT further reduces this period to 12 days.

Antibody tests
Antibody tests are specifically designed for the routine testing of HIV in adults, are inexpensive, and are very accurate. If a person does not have a realistic risk of infection, then these tests are not necessary. Antibody tests give false negative results during the window period

ELISA
The ELISA test, or the enzyme immunoassay (EIA), was the first screening test commonly employed. It has a high sensitivity. The low specificity of the test is because antibodies attach to antigens in the test kits "by accident", even though the person has never been exposed to HIV. About 80% of positive ELISA tests are followed by a negative Western-Blot test, and therefore regarded as false positive.

Western blot
The Western blot test uses the general Western blot procedure. HIV-infected cells are opened and the contained proteins are entered into a slab of gel to which a voltage is applied. Different proteins will move with different velocities in this field, depending on their size, while their electrical charge is leveled by a substance, called sodium lauryl sulfate. Once the proteins are well separated, they are transferred to a membrane and the procedure continues similar to ELISA.

Rapid or point-of-care tests
Rapid Antibody Tests are qualitative immunoassays intended for use as a point-of-care test to aid in the diagnosis of HIV infection. These tests should be used in conjunction with the clinical status, history, and risk factors of the person being tested. The specificity of Rapid Antibody Tests for in low-risk populations has not been evaluated.

OraQuick is an antibody test that provides results in 20 minutes. The blood, plasma or oral fluid is mixed in a vial with developing solution, and the results are read from a stick-like testing device.

OraSure is an HIV test, which uses mucosal transudate from the tissues of cheeks and gums. It is an antibody test, which first employs ELISA, then Western Blot.

There is also a urine test; it employs both the ELISA and the Western Blot method.

Home Access Express HIV-1 Test is a FDA-approved home test: the patient collects a drop of blood and mails the sample to a laboratory; the results are obtained over the phone.

Antigen tests
The p24 antigen test detects the presence of the p24 protein of HIV (also known as CA), a major core protein of the virus.

Nucleic acid based tests
Nucleic acid based tests amplify and detect a 142 base target sequence located in a highly conserved region of the HIV gag gene.

In the RT-PCR test, the viral RNA is extracted from the patient's plasma and is treated with reverse transcriptase so that the RNA of the virus is transcribed into DNA.

In the Quantiplex bDNA or branched DNA test plasma is centrifugated to concentrate the viruses, which are then opened to release the RNA.

CD4 count significance
The CD4 T-cell count is not an HIV test, but rather a procedure where the number of CD4 T-cells in one microliter of blood are counted in a standard medical lab test after a blood draw.

This test does not check for the presence of HIV. It is used monitor the immune system function in HIV+ people. Declining CD4 T-cell counts are considered to be a marker of the progression of HIV infection. In HIV+ people, AIDS is officially diagnosed when the count drops below 200 cells or when certain opportunistic infections occur.