HIV tests detect the presence of the human immunodeficiency virus in serum, saliva, and sometimes also urine. Mainly antibodies against the virus, viral antigens, or the viral RNA are detected by the investigations. The tests are described in terms of their sensitivity and specificity. Tests may produce false positive and false negative results depending on the method of the test and taking into consideration human errors.

 

All these aspects of HIV testing are also cumulatively defined as Performance of the test. Some autoimmune diseases and in some other conditions false results can be produced.

 

Window Period

Every investigation to detect HIV in body has some limitations, like some tests can detect the HIV only after a gap of some days once an individual is infected. This timeframe after which a test will come positive or a investigation will be able to determine the qualitative result correctly. In simple words the test which an individual will conduct will come positive only after some days if at all an individual is infected. The window period will also vary from an investigation to investigation. The window periods for few of the commonly performed investigations are –

• HIV-1 antibody tests - 22 days for subtype B
• Antigen testing - 16 days
• Nucleic Acid Testing - 12 days 

Principles of HIV Testing

• For Screening blood and cellular products antibody, antigen and nucleic acid tests are used as a minor mistake can render numerous individuals infected with HIV
• For Diagnosis of HIV infection two antibody tests are routinely used. If a person is positive for ELISA method, then a second test using the Western blot technique is performed before declaring the patient HIV positive.

• For all patients undergoing HIV test, strict confidentiality is maintained. Also patient is consented first

• HIV testing is done anonymously which means that none of the patients name will be written on the samples sent for the testing, only patient number will be mentioned.

 

Investigations for HIV Determination

 

• Antibody Tests - Antibody tests include ELISA i.e. Enzyme Linked Immunosorbent assay, Western Blot Test and Rapid antibody Tests. In routine clinical practice ELISA is performed and if it is found to be positive then a patient is prescribed western blot test which is a gold standard in detecting the HIV infection in a patient, combined the two tests will give a accurate result. For rapid screening of antibodies many quick kits are available like Oraquick and Clearview which can provide the results in minutes.
• Antigen Tests - In this test the patient is tested for P24 protein of the HIV which serves as antigen for the test. The test is not used routinely in clinical practice due to its low sensitivity.
• Nucleic Acid Test for HIV - This test includes a complex process call polymerase chain reaction (PCR) and is targeted towards the particular sequence of the bases over the gene of the virus. The test reduces the window period significantly but the cost is the only limiting factor for the widespread use of this test.
• CD4 cell count is also sometimes helpful in determining the prognosis and gives an idea about the patient’s condition. It is not a specific test for the diagnosis of the HIV
• Duo tests kits are now a day’s available in the market. These tests combine the benefits of the antibody and antigen tests and are more accurate.