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Knowing about HIV Antibody Test

About HIV testing

HIV testing shows whether a person is infected with HIV.

HIV antibody testing - when a person is infected by HIV, the body responds by producing antibodies. However, these antibodies are not protective in nature. An HIV antibody test detects the presence of such antibodies. A positive test result shows that a person has been infected. A negative HIV antibody test result after the window period means that a person has not been infected. Nowadays we can have HIV antibody test with blood, oral fluid (oral mucosal transudate; it is not the same as spitting out saliva) or urine.

HIV antigen testing - the p24 antigen test detects the presence of the p24 (a protein component of the HIV viral core). High levels of p24 are present in the blood of newly infected person during the short period between infection and seroconversion (production of antibodies), making p24 antigen test useful in diagnosing early HIV infection. However, p24 antigen test is not always reliable for diagnosing HIV infection after its early stage.

A conventional HIV test (blood taking from vein) takes around 1 week for the result, whereas rapid test (finger prick/oral fluid) only requires 20 minutes. Rapid test is only a screening test (a preliminary test) . Any positive rapid test result should be followed by confirmatory test of venous blood conducted by a laboratory.

Sensitivity and Specificity of a screening test

Sensitivity - the sensitivity of a screening test is its ability to show positive results in infected people, e.g., if a test is done on 100 infected people and show positive results for 99 of them, then the test is said to have a sensitivity of 99%. Tests with low sensitivity will result in more false-negatives, which means missed diagnosis for infected people causing false sense of security.

Specificity - the specificity of a screening test is its ability to show negative results in non-infected people, e.g., if a test is done on 100 non-infected people and show negative results for 99 of them, then the test is said to have a specificity of 99%. Tests with low specificity will result in more false-positives, causing unnecessary anxiety for non-infected people.

Precautions

Biotin, also named vitamin H, vitamin B7 or coenzyme R, is commonly used in dietary supplement products that improve hair, skin and nail conditions. Taking high dose (10 mg or more per day) of biotin may affect many laboratory test results, including false negative result of HIV antibody and antigen tests. If you are taking dietary supplements containing biotin, please stop taking it 48 hours before the test. In addition, biotin amount in ordinary food products do not affect the test results.

For details, please refer to the Important Safety Alerts of the Medical Device Division of the Department of Health:
https://www.mdd.gov.hk/en/safety-alerts-communications/safety-alerts-special-warnings/archive/important-safety-alerts/index-id-1646.html