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How is HIV/Aids diagnosed?

HIV infection and Aids are classed in Australia as Group D notifications. Initial diagnosis involves careful history and physical examination looking for risk factors and clinical manifestations of immunodeficiency are necessary.

Diagnostic testing can only be done with your consent and includes:

• Detection of HIV antibody by the ELISA screening test, with confirmation carried out by Western blot analysis.
• Detection of the viral p24 antigen in serum.
• PCR tests to detect pro-viral DNA sequences.
• HIV culture, although this is only performed in certain special clinical situations.

Pre-test expectations
Generally, before you’re HIV-test your healthcare worker or doctor will run through the following with you and ask:

• If you understand what the testing entails.
• If you comprehend the implications of a positive or negative result.
• What your behaviour has been like since any previous testing to gauge your HIV exposure risk.
• How much you understand about HIV, how it’s transmitted and how to prevent acquiring it.
• What forms of support are available to you after you receive the result.
• If you understand what the “window period” of the test refers to and the implications of this.

Rapid HIV testing
Rapid HIV tests are available in certain parts of Australia, but not everywhere. So if you require one of these you need to check with your local Aids council or clinic.

A rapid HIV test is generally performed at a clinic and the results are available in 30 minutes. However, it’s important to note that even if you receive a preliminary positive result this can often be a “false positive” (one of the drawbacks of rapid testing). Results of rapid testing always need to be confirmed by laboratory tests.

Standard HIV testing
Ordinary HIV tests do not detect the virus, but rather the specific antibodies that are produced by the immune system in response to HIV infection.

Antibodies are produced from about three weeks after infection and usually become detectable by testing by four to six weeks after infection. This four- to six-week period between infection and a positive test is called the window period. In some people the window period is longer; it may take up to three months for an antibody test to become positive after they have been infected, but this is unusual.

People who think that they might have been exposed to infection are usually asked to wait at least four weeks before having the HIV test. Also, even if the first test is negative (no antibodies detected), a follow-up test should be done three months after the suspected exposure.

The most widely used and best antibody test is called an ELISA test. These days, most laboratories screen with an ELISA that tests for antibody as well as a viral protein (p24 antigen). The p24 antigen appears in the blood a week before antibody. This shortens the window period (time from exposure to development of a positive test). If a positive result is obtained on an ELISA test the laboratory will confirm the result by testing with at least one different type of ELISA test. As an additional check a second blood specimen is usually taken from the person for repeat testing.

Currently, home HIV tests are not available in Australia. Most health care officials do not favour them as the quality of the test cannot be regulated, so that there may be a greater risk of false positive or false negative results. Also, a person testing themselves or someone else will probably not have the information or psychological support that is gained through pre- and post-test counselling.

HIV testing in babies
Mothers who suspect they may have HIV should first consult their midwife or doctor regarding HIV testing.

If you are HIV+ there is a one-in-four chance of your baby being infected if you don’t receive the right treatment which can reduce the risk of transmitting HIV to your baby.

In babies younger than 18 months old the mother’s antibodies in the baby’s blood can interfere with the HIV antibody test. Therefore, to test whether a baby is infected with HIV, it is necessary to detect the virus itself. This is commonly done with a PCR test.

Your baby will be tested for HIV at birth and at intervals for up to two years. If your baby is found to be infected with HIV, paediatricians are able to foresee which illnesses are more likely to affect your baby and can offer early treatment.

You'll be advised not to breastfeed as this can transmit HIV to your baby.

CD4 count
CD4 cells are a critical part of your immune system. These are the cells which become infected and destroyed by HIV.

A CD4 count test is used to determine how advanced a person's HIV disease is. It indicates what reserves of T-helper lymphocytes the person has and, therefore, the remaining strength of their immune system.

This basically means the CD4 count is a measure of the damage already done.

A normal CD4 count is 800 or more cells per microlitre of blood. This level drops gradually in the blood over time and the immune system progressively weakens. Once the level drops below 350 cells per microlitre of blood, the patient should be assessed for anti-retroviral drug therapy.

Viral load
The viral load test measures the amount of virus in the blood, which shows how rapidly HIV is multiplying and, therefore, how quickly the disease is likely to progress.

It is mainly used for monitoring response to antiretroviral drug treatment. In patients on anti-retroviral therapy the viral load should be lower than detectable limits. If the viral load is detectable it means that the therapy is not working well enough. This could be because of non-compliance (patient is not taking the medication reliably) or the virus has developed resistance to the drugs.
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