Blood work:
A complete guide for monitoring HIV

May 2007     View PDF     En español

Resistance tests

HIV can change itself so that anti-HIV drugs do not work as well. This is called drug resistance, and it’s one of the most common reasons why therapy fails. There are two different types of resistance tests. Genotypic tests look for changes in the genes of HIV that are linked to drug resistance. Phenotypic tests assess which drugs can stop HIV growing in a lab setting. Several different labs offer these tests.

People who use resistance test results to help make treatment decisions face a better and more sustained reduction in HIV levels than those who make decisions based on their treatment history and viral load results. To accurately test drug resistance, people should have a viral load over 1,000 copies. Otherwise, the results may not be accurate or the test cannot be done.

A replication capacity test measures whether HIV can reproduce and how fast. This is sometimes called viral fitness. The test is usually done along with a resistance test. It is thought to be most useful for people with fewer treatment options who are trying to put the best possible regimen together.

Another test that looks at HIV is called a tropism assay. It looks at which of two common proteins—called CCR5 (R5) and CXCR4 (X4)—HIV uses to enter CD4+ cells. This test will be used for people wanting to block R5 with anti-HIV drugs, like maraviroc. Four results are possible: uses R5 only (R5 tropic), uses X4 only (X4 tropic), can use either (dual tropic), or some combination of the first three (mixed tropic).