Protein 24 Capsid Antigen

Other names: P24 Antigen Test

P24 is a protein ’shell’, which is found on the surface of HIV cells; the test is used to detect HIV cells in the blood. P24 protein cannot be detected until around a week after the individual has been exposed to the HIV virus; levels are then detectable until the body has produced antibodies to fight off the virus. Once the body has produced antibodies, the p24 test will produce negative results, even when the individual has been infected by the HIV virus.

Why is the test used?

The test is primarily used to detect HIV in the early stages but it may also be used to screen donor blood for HIV. The test can also be used to monitor the condition of the patient and assess the efficacy of treatment.

The test may be ordered if a patient is worried that they may have been exposed to the HIV virus and wishes to know if they have been infected.

The test may also be ordered for patients who have started therapy to monitor their condition and assess the progress of the infection.

How is the test performed?

The test is usually carried out by taking a sample of blood from a vein in the patient’s arm; a needle is inserted into the vein and the blood is drawn out and collected in a syringe. Once the sample has been collected, the blood will be bottled, labelled with the patient’s name and sent away to the laboratory for testing.

In some cases, a sample of blood may be collected from the patient’s finger; the sample is obtained by pricking the finger with a needle.

What do the test results show?

If the test is positive, this indicates that the patient has been infected by the HIV virus; the higher the result, the more HIV particles in the blood. If the test result is negative, this may not rule out the possibility of HIV, as it may be too early for the test to detect p24; p24 is also not detectable once the body has started to produce antibodies, so the test will be negative even if the patient has got HIV. If the test is negative, further testing will usually be ordered.

Specific Blood Tests

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