Antigen verses Antibody Tests for HIV


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The detection of the Human Immune Deficiency Virus (HIV) is usually achieved through a blood test. A small sample of blood is taken from the patient and passed through well-established laboratory tests to detect the presence of the virus. HIV blood tests generally work by trying to detect either antigens or antibodies, two unique and distinctly different molecules. This article looks at the methods of antigen and antibody testing used at present, and the pros and cons of each one.

Antibody tests for HIV

Antibodies are part of your body’s defensive response towards foreign bodies that cause infection and disease. Antibodies are specific to particular illnesses, and therefore can indicate the presence of specific viruses and diseases once detected.

The two main methods of antibody testing for HIV are the Western Blot and ELISA. ELISA stands for Enzyme-Linked Immunosorbent Assay, and this test involves adding a sample of a person’s blood to a specially prepared dish equipped with molecules that bind to HIV antibodies. Once the sample has been added to the plate and the HIV antibodies are bound, the rest of the person’s blood is removed, leaving the HIV antibodies behind. These are then treated with a special selection of compounds and ultimately the presence of antibodies to the virus is detected as either a change in color or fluorescence.

The Western Blot is similar in that antibodies are selected, however this method relies on the complete separation of the various components of blood. After a series of different steps, whoever is performing the test can clearly see the presence of HIV antibodies.

Antigen tests for HIV

These tests try to detect the presence of HIV antigens in blood. Antigens, are molecules specific to disease causing pathogens. The antigen which these tests look for on HIV is the p24 protein which forms part of the structure of the virus. While antigen testing can still be used, it is not a procedure which is routinely used in Europe or the United States anymore.

Antigen vs. Antibody Tests

The main criticism of the antigen test is its low sensitivity, which means that it can only detect the virus if it is present in large quantities. It is also only really effective within a certain time after infection as the body’s natural defenses begin to generate antibodies against p24 which disrupts its detection during tests.

The main criticism of the ELISA test is where the cutoff point between a negative and positive result is, so while the test is very sensitive there is always a risk of a false negative and/or positive. Generally speaking ELISA tests are confirmed with a Western Blot, and this usually leads to a fairly conclusive and reliable final result which can accurately report the presence of HIV.


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