Resistance testing

Resistance testing is recommended at the time of HIV diagnosis, regardless of whether the patient has recently seroconverted or presents with established infection. Testing does not need to be repeated at the time of ARV initiation, but may be considered in some cases of possible re-infection. Considerations for repeat testing would include a sudden increase in viral load, a sudden CD4 cell count decline, a seeming repeat of seroconversion illness, or engagement in high-risk behaviour. 

Current patients who are antiretroviral naive and did not undergo resistance testing at the time of their diagnosis should be tested prior to initiating ARV therapy. If a plasma sample from the time of diagnosis has been stored, it should be retrieved for retrospective testing. In antiretroviral-naive patients, genotypic testing is recommended.

When a treatment-experienced patient has detectable viral load, and their virus was previously suppressed, a second viral load test should be quickly given to help distinguish viral rebound from a transient viral blip. If both viral load tests show detectable virus, a resistance test should be given to guide selection of subsequent therapy.

In treatment-experienced patients, genotypic testing is generally recommended because it is widely available and relatively cost-effective. However, in patients with complex resistance patterns, phenotypic testing can be more informative.

Resistance testing should take place while the patient is still on therapy. Most resistance tests perform reliably if the viral load is over 1000 copies/ml. Below this level, reliability would depend on the test used and the expertise of those administering and interpreting it.

Phenotypic testing is recommended to identify a change in co-receptor use or modifications in gp120. (At present, the Trofile assay is the only licensed test for this use.) 

Current recommendations for HIV clinics and laboratories are to permanently record resistance test results; forward all test results to the next treatment centre if patients move or transfer care; and store sequences in case they are ever needed for re-analysis.