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Several medical tests exist to detect the presence of HIV, the virus that causes AIDS. These HIV tests are often called AIDS tests, although they actually measure HIV, rather than AIDS. However, one medical test, the CD4 T-cell count although not an HIV test, is so commonly used in HIV/AIDS patients (in addition to its use in other diseases) that it is also described here.
HIV tests are used for different purposes:
- to test whether an individual is infected with HIV
- to screen all donated blood/blood products (plasma) organ and sperm donors
- to measure the amount of free virus in a patient's blood in order to monitor the progress of the disease under antiretroviral drug therapy
Terminology
This article uses some specialist medical testing terminology:
- the window of a test is the time period after initial infection during which the test cannot yet detect the presence of HIV. The shorter the better.
- the sensitivity of a test is the percentage of HIV positive cases that are correctly identified as positive by the test.
- the specificity of a test is the percentage of HIV negative cases that are correctly identified as negative by the test. The higher the better.
Antibody tests
When infected with HIV, the body usually produces antibodies that specifically bind to the virus. Antibody tests detect the presence of these rather than the presence of HIV itself.
In 1996, the CDC described the case of an HIV-infected man who persistently tested negative on the antibody tests [1] (http://www.cdc.gov/mmwr/preview/mmwrhtml/00040569.htm). This is considered to be extremely rare.
The ELISA test was the first screening test commonly employed. It has a high sensitivity but not a very high specificity. The test proceeds by the general ELISA method: the patient's 400-fold diluted serum is poured over a plate to which known HIV particles have been attached. The HIV-antibodies in the serum (if present) will bind to the HIV particles. The plate is then washed to remove all other components of the serum. Then a specially prepared "secondary antibody"—an antibody that binds to other antibodies, in this case human antibodies—is applied to the plate, followed by washes. This secondary antibody is chemically linked in advance to an enzyme. Thus the plate will contain enzyme in proportion to the amount of HIV-antibodies in the serum sample. A substrate for the enzyme is applied, and catalysis by the enzyme leads to a change in color or fluorescence. Measurement of this change yields an estimate of the quantity of HIV-antibodies in the serum.
The low specificity of the test comes from the fact that sometimes there are cross-reacting antibodies: these are antibodies which attach to HIV particles "by accident", even though the body has never encountered HIV. If an ELISA test is positive, the result is commonly confirmed with a Western-Blot test.
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 levelled 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: the patient's diluted serum is poured over the membrane and HIV-specific antibodies attach to those proteins the persons immune system has synthesized them. Others are washed away, and enzyme-linked antibodies with the capability to attach to first detect the presence of bound antibody. Since the proteins were separated, it is possible to see exactly to which HIV proteins the patients has antibodies (which the regular ELISA test cannot accomplish). The test is considered positive if antibodies to several major HIV proteins are present.
The combination of ELISA and Western Blot is the standard test employed for diagnosing HIV infection. All blood and organ donations are also screened with these tests. The window for the newer versions of these tests is stated in the literature to be on average 22 days. The patients gets the results usually 2 days to 2 weeks after the blood withdrawal.
OraQuick is another antibody-based test; it needs only one drop of blood from the finger and gives results in 20 minutes. Here the blood is mixed with a solution and then a measuring strip is inserted; the solution slowly travels upwards on the strip until it reaches a region where HIV proteins are attached. If the blood contains HIV antibodies, they will bind here, later causing a color reaction.
Orasure is an HIV test which uses mucosal trasudate 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 an approved home test: the patient collects a drop of blood and mails the sample to a laboratory; the results are obtained over the phone.
There are also several unapproved immediate home-test products on the U.S. market.
Antigen tests
Antigen tests directly detect the presence of a part of the virus (an antigen), by applying specific antibody.
The p24 antigen test detects the presence of the p24 protein of HIV (also known as CA), a major core protein of the virus. It uses a variant of the ELISA strategy: a well is coated with monoclonal antibodies specific to the p24 protein. The patient's blood is poured over the plate, and p24 protein will stick to the antibody; the rest is washed away. Then enzyme-linked antibody to p24 is applied to the plate; the enzyme action then causes a color change if p24 was present in the sample.
This test is now used routinely to screen blood donations, thus reducing the window to about 16 days. It is not useful for general diagnostics, as it has very low sensitivity and only works during a certain time period after infection; once the body produces antibodies against HIV, the test becomes useless.
A variant of the p24 test first applies heat to denature the p24 protein and thus separate it from its antibody. This test is used to monitor the viral load and disease progression, as a cheaper alternative to the nucleic acid based methods described next.
Nucleic acid based tests
In the Amplicor 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. The polymerase chain reaction (PCR) is applied, using two primers thought to be unique to the virus's genome. After the PCR amplification process is completed, which takes some time, the resulting amplified segments bind to specific oligonucleotides bound to the vessel wall and are then made visible with a probe that's bound to an enzyme. The amount of virus in the sample can be quantified.
In the Quantiplex bDNA or branched DNA test plasma is
centrifugated to concentrate the viruses, which are
then opened to release the RNA. Special oligonucleotides are
added which bind to viral RNA and to certain oligonucleotides bound to
the wall of the vessel. In this way, viral RNA is fastened to the
wall. Then new oligonucleotides are added which bind at several
locations to this RNA; and other oligonucelotides which bind at
several locations to those oligonucleotides. This is done to amplify
the signal. Finally, oligonucleotides that bind to the last set of
oligonucleotides and that are bound to an enzyme are added; the enzyme
action causes a color reaction which allows to quantify the viral RNA
in the original sample. Currently, version 3.0 of the Quantiplex is in use; it is claimed to detect viral loads as low as 50 per millilitre.
Nuclisens NASBA test still missing
Nucleic acid based tests are now routinely used to determine the viral load, the amount of free virus in the blood. This is an important variable when monitoring a drug therapy.
In the U.S. donated blood is also screened with nucleic acid based tests, shortening the window to about 12 days. Since these tests are relatively expensive, the blood is screened by first pooling some 10-20 samples, testing these together, and if the pool tests positive, each sample is retested individually.
Other forms of HIV test
Other forms of HIV test include:
- Radioimmunoprecipitation assay (RIPA): A confirmatory blood test that may be used when antibody levels are very low or difficult to detect, or when Western blot test results are uncertain. An expensive test, the RIPA requires time and expertise to perform.
- Dot-blot immunobinding assay: A rapid-screening blood test that is cost-effective and that may become an alternative to standard EIA and Western blot testing.
- Immunoflourescence assay: A less commonly used confirmatory blood test used on reactive ELISA samples or when Western blot test results are uncertain.
Other tests used in HIV/AIDS treatment
The CD4 T-cell count is not an HIV test, but rather a procedure where the number of CD4 T-cells in one microlitre of blood are counted in a standard medical lab test after a blood draw.
This test does not check for the presence of HIV. Instead, it is used to estimate viral activity and immune system function in HIV+ people. This test is also used occasionally to estimate immune system function for people whose CD4 T cells are impaired for reasons other than HIV infection, which include several blood diseases, several genetic disorders, and the side effects of many chemotherapy drugs.
Generally speaking, the lower the number of T cells, the lower the immune system's function will be. Normal T4 counts are about 1000 CD4+ T cells per microliter, although the counts may fluctuate by 10% or more even in healthy people, depending on recent infection status, nutrition, exercise and other factors -- even the time of day. Women tend to have somewhat lower counts than men.
Symptoms of T4 cell immune collapse are almost never seen until the number drops below 500. In HIV+ people, AIDS is officially diagnosed when the count drops below 200 cells.
Similar symptoms of immune collapse are generally seen in people with very low T4 cell counts, no matter whether this immunosuppression is caused by HIV or by cancer or by any other disease. However, the long-term treatment differs substantially, because it needs to address the cause of the immunosuppression.
External links
Caution: The FDA jurisdiction is within the United States only. Contact your national health department if you are not from that country
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