Indications for Testing
Laboratory testing in connection with ART for HIV is appropriate for:
- Patients newly diagnosed with HIV
- Patients previously diagnosed with HIV who delayed ART (testing should be repeated if previously performed)
- Patients who experience treatment failure during ART
Following an established diagnosis of HIV and before beginning treatment with ART, recommended laboratory tests include the HIV RNA level, CD4 T-lymphocyte cell count, tests for other infections (eg, hepatitis B virus [HBV], hepatitis C virus [HCV], tuberculosis), and tests to assess general health, such as CBC, kidney and liver function, lipid level, and glucose level tests. A pregnancy test should be given to women of childbearing age before beginning treatment. ART initiation should not be delayed by these test results unless kidney or liver damage is discovered or transmitted drug resistance is strongly suspected.
Drug Resistance Testing
Drug resistance testing involves genotypic or phenotypic testing to evaluate viral strains and guide the selection of therapy regimens. These tests are designed to detect resistance to the various classes of ART drugs. Resistance testing should be performed in early HIV infection even if the patient has opted to postpone ART so that the results will be available when the individual begins therapy. However, repeat testing at the start of therapy should be considered because of the possibility that a drug-resistant virus (superinfection) has been acquired between initial diagnosis and ART initiation.
Genotypic is preferred over phenotypic testing in most contexts because of its faster turnaround time, lower cost, and greater sensitivity for combinations of wild-type and resistant virus. Test interpretation is more straightforward than with phenotypic tests. Genotypic testing is also preferred in patients with known drug resistance variant patterns and in those anticipated to have uncomplicated variant patterns. Combined genotypic and phenotypic tests are also available, as are tests that quantify HIV RNA and then reflex to genotypic or phenotypic tests to investigate possible drug resistance.
Most genotypic assays use Sanger sequencing to identify variants in the reverse transcriptase (RT), protease resistance (PR), and integrase genes of circulating HIV RNA in plasma. These assays can identify resistance to nucleoside reverse transcriptase inhibitors (NRTIs), non-NRTIs (NNRTIs), integrase strand transfer inhibitors (INSTIs), and protease inhibitors (PIs). Testing is also available to detect variants in the gp41 gene, which are associated with resistance to fusion inhibitors (ie, enfuvirtide). Test interpretation requires awareness of the variants that might result from various ART drugs and the connection between those variants and possible resistance to other drugs. The International AIDS Society-USA has a list of resistance-associated variants, and the Stanford University HIV Drug Resistance Database is a useful resource to aid in genotypic test interpretation.
Genotypic testing for variants in RT and PR genes is indicated in patients who have not previously been treated with ART. If transmitted resistance to INSTIs is suspected, testing for variants that cause INSTI resistance (ie, variants in integrase genes) should be considered.
In patients currently being treated with ART, resistance testing is recommended in those with an unsatisfactory decrease in viral load and in those with treatment failure, as indicated by virologic failure and HIV RNA levels >1,000 copies/mL. In patients with HIV RNA levels between 500 and 1,000 copies/mL, drug resistance testing may be inconclusive but should still be considered.
In cases of virologic failure, resistance testing should be performed while the patient is still on the regimen or within 4 weeks of treatment discontinuation, if possible; testing during this period increases the likelihood that resistance variants will be detected. If treatment failure occurs during therapy with INSTIs, testing for INSTI resistance should be performed to evaluate whether the patient can be given INSTIs in future regimens.
Phenotypic assays are used to evaluate whether the virus is able to replicate in the presence of various ART drug concentrations. Phenotypic testing should be performed in addition to genotypic testing in patients who are known or anticipated to have complex drug resistance variant patterns.
Other Testing for Resistance to Specific Drugs
Coreceptor Tropism Testing for CCR5 Antagonist Resistance
Coreceptor tropism assays are recommended if a CCR5 antagonist is under consideration, as well as for patients who experience virologic failure while receiving a CCR5 antagonist (ie, maraviroc). Phenotyping coreceptor tropism assays are preferred to genotyping assays. Coreceptor tropism usage “shifts” can occur during the disease course, so testing must be repeated at treatment failure. This testing is also useful to guide treatment in ART-experienced patients with significant drug resistance or consistently high levels of HIV RNA.
In patients with an undetectable viral load who are being considered for treatment with a CCR5 antagonist, a proviral DNA tropism assay can be used, but caution is advised because the clinical efficacy of this test is still being explored.
HLA-B*5701 Testing for Abacavir Hypersensitivity
The HLA-B*5701 genotyping test is required by the U.S. Food and Drug Administration (FDA) before treatment with abacavir to identify patients at risk for abacavir hypersensitivity. Abacavir should not be administered if the patient is a carrier of HLA-B*5701, regardless of age. Risk of hypersensitivity is the same whether the patient carries one or two copies of the variant allele.
After ART initiation, patients should be evaluated within 6 weeks to assess their ability to tolerate the therapy and adherence to treatment. Genotypic or phenotypic resistance testing should occur at treatment failure and also when a regimen switch is planned. In patients who experience virologic failure, resistance testing should be performed while the patient is still receiving ART or within 4 weeks of discontinuation to increase the likelihood that the resistant virus will be detected if present.