How they work

These tests measure HIV genetic material called RNA from virus particles called virions in the blood plasma. There are several different techniques used.  

HIV-1 RNA PCR (polymerase chain reaction) multiplies the amount of HIV in a blood sample through use of an enzyme. The resulting chemical reaction marks virus and markers are measured to calculate the amount of HIV-1 RNA. This is also known as viral nucleic acid amplification testing (NAAT). Tests in this format include:

• the Amplicor HIV-1 Monitor Test 1.5 assay
• the COBAS AmpliPrep/COBAS Amplicor HIV-1 Monitor Test, version 1.5
• the Abbott RealTime HIV assay
• the COBAS TaqScreen MPX test, version 2.0.

Transcription mediated amplification (TMA) technology uses two primers and two enzymes (RNA polymerase and reverse transcriptase) to amplify virus particles. A TMA test, the Aptima HIV-1 Qualitative Assay, is the only one of these tests which is licensed in the USA for use in the diagnosis of HIV.

Samples may be pooled for testing – this is often the case when blood donations are tested for HIV, but is also the practice for diagnostic testing in some parts of the United States. In this approach, rather than run separate tests for each patient’s blood sample (which would be expensive), the tests are run on pooled samples of blood. Samples are stored until there are at least ninety specimens to pool. This pool is then amplified for evidence of HIV RNA. If the pool tests negative, then all the specimens are deemed to be negative. However, if the pool tests positive, then smaller pools of 10 are tested until each of the HIV RNA positive specimens are isolated.¹

A branched DNA (bDNA) assay uses a substance that produces light when it combines with HIV particles. The amount of light is measured and converted to a viral count. Tests in this format include the Versant HIV-1 RNA 3.9 Assay by Bayer.

Nucleic acid sequence-based amplification (NASBA) assays amplify viral proteins to generate a viral load count. To do an HIV viral culture, blood is separated into blood cells (with lymphocytes containing HIV) and the non-cellular part of blood (plasma containing virions of HIV). Both parts can be introduced into donor human cells to grow HIV artificially in the laboratory and the quantity of new HIV produced after two weeks can be estimated with fair accuracy. Both cell and plasma cultures produce measurements with roughly the same significance when used as prognostic markers. The NucliSens HIV-1 QT by bioMerieux uses this technique.