Tests to detect the human immunodeficiency virus (HIV) should be performed in patients with a suspected diagnosis or in risk situations (unprotected penetrative sexual intercourse, intravenous drug use, or blood-to-blood contact).

There are different methods to carry out the diagnosis, but to simplify we could summarise it as indirect tests and direct tests.

Direct tests are those that directly look for the virus or parts of it in the body, and indirect tests are those that analyse the body’s reaction to the presence of the virus.

What types of HIV tests are there?

Detection of antibodies by immunoassay:

  • It is an indirect test, which reacts to the presence of antibodies against HIV-1 and HIV-2.
  • Antibodies are proteins that are generated by the immune system to identify and fight certain diseases. In this case, those that specifically react against HIV are detected.
  • The time it takes for the body to create these antibodies depends on each organism, but a window period of up to 90 days after infection is established.
  • They can be done in blood or saliva.

Detection of antibodies and p24 antigen

  • It is a combination of a direct and an indirect test. Detects antibodies (as above) and HIV p24 antigen in blood.
  • An antigen is any substance responsible for being recognized by antibodies to activate our immune system. In this case, the p24 antigen is part of the HIV virus and is detected by this test.
  • After exposure to HIV, the antigens will show up in your blood before the antibodies, so the window period is reduced to 4 to 6 weeks after exposure.
  • It is done in blood.

HIV detection by nucleic acid amplification

  • It is a direct screening test. It is in charge of searching for and amplifying the genetic material of the virus, so that if it is detected in the test and compared with the already known HIV genes, the test will come out positive.
  • HIV infection can be detected from 10 to 21 days after infection (most studies give it practically 100% reliability from day 21)
  • This test is used for diagnosis in a qualitative way and is also used as a method to assess the efficacy of treatment, in patients with already diagnosed HIV, in a quantitative way, analyzing what we know as Viral Load (it offers the number of copies of the virus present in the organism)
  • It is done in blood.

Western blot

  • It is a direct test that is used only in a confirmatory way and not as a diagnosis. It is responsible for detecting virus-specific proteins in the sample provided.
  • Since it is a confirmatory test, a specific window period is not established for it (there must be a previous positive test).
  • It is done in blood

What HIV tests are available at Open House?

  • We have the three main tests and we perform all of them on a blood sample
  • Detection of antibodies by immunoassay, we perform it in blood that can be venous or capillary extraction, and its cost is €40
  • Detection of antibodies and p24 antigen, we perform it in blood that can be venous or capillary extraction, and its cost is €80
  • Detection of HIV through nucleic acid amplification, we perform it in blood by venous extraction and its cost is €140

Dr. Julieta Domingorena Córdoba

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