Understanding Primary vs. Confirmatory Types of HIV Tests

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Testing for HIV is done in two stages. First, a primary test is done. If it is positive, then the test is repeated to check for false positives. If the primary test is positive twice, then the sample is tested with a second, more specific confirmatory test. This test is used to make certain that the primary test isn't reacting to an infection other than HIV.

How HIV Test Types Are Used in Combination

There are a number of ways that you can think about the types of HIV tests.

One way is to look at the ways the different test types work. The other way is to look at how the different types of HIV tests are used. Primary tests are used to look for any sign of HIV. Confirmatory tests are used to check the results of primary tests. The goal of this process is to minimize the risk of incorrectly diagnosing anyone with HIV.

Tests Used for HIV Primary Testing

  • EIA or Enzyme Immune Assay: This test looks for antibodies to HIV. However, it has its limits. Most people start producing anti-HIV antibodies within 6-12 weeks of infection. But some HIV positive people can take up to 6 months to produce enough antibodies to test positive on an EIA test. Therefore this test is not useful for newly infected individuals. With this test, someone recently infected with HIV may not test positive. However, they can still pass HIV onto other people. They may, in fact, be very infectious.
  • ELISA or Enzyme-Linked Immunosorbent Assay: This test is similar to an EIA. It looks for the same things, but it uses different techniques and ingredients.
  • PCR or Polymerase Chain Reaction: This test uses technology to amplify viral RNA from the blood. In other words, it makes more copies of RNA. This allows the RNA to be more easily detected. Therefore, PCR can help detect even small amounts of virus in a newly infected person. However, it is expensive, time-consuming, and not readily available

    Tests Used for HIV Confirmation

    • Western Blot: This is the main method used to confirm the results of an initial positive test. It is an antibody test that is more specific for HIV-antibodies than the EIA/ELISA tests. However, it is substantially more expensive. That is why it is used for confirmation instead of initial testing.
    • RIPA: This blood test is done when antibody levels are low. It can also be done when Western Blot results are unclear. This test is expensive, difficult to perform and not often used.
    • Immunofluorescence Assay. This is another type of confirmatory test used when Western Blot results are unclear. Sometimes it is used instead of a Western Blot after an ELISA test.
    • DNA/RNA Amplification Tests: These tests are similar to PCR. They are occasionally used when the result of a western blot is unclear. Amplification tests have become much less expensive over time.

    Tests can also be used to determine a person's viral load. This is the amount of virus present in their bloodstream. Viral load tests are not usually used to diagnose HIV. Instead, these tests are used to monitor how effective treatment is. However, there is some research that suggests that checking viral load at the same time as HIV is diagnosed may help to connect people to a doctor for care.​

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