Antiretroviral Susceptibility Testing – HIV resistance, HIV susceptibility, HIV genotype

Diagnostic Use

It is essential that the blood specimen be collected while the patient is still receiving antiretroviral drug therapy, otherwise wild-type virus will outgrow resistant variants and mutations will not be detected.

Drug resistance is a major factor contributing to the failure of antiretroviral therapy. The high error rate of reverse transcriptase and the rapid turnover of the HIV-1 population contribute to the prompt emergence of resistant variants in the setting of incomplete suppression of virus replication. Data from retrospective and prospective studies suggest that resistance testing can be helpful in guiding the choice of antiretroviral therapy in patients failing their current regimen. Drug resistance evolves through a stepwise accumulation of mutations; early-stage mutants are often only partially resistant and tend to be less fit than wild-type virus in the absence of therapy. Resistance to antiretroviral drugs is determined by mutations in the genes that encode the protease (PR) and reverse transcriptase (RT) genes. Primary mutations are those that alter binding of the drug to its target and result in an increase in the amount of drug necessary to inhibit the enzyme. Secondary mutations increase the level of resistance by improving the fitness of viruses carrying primary mutations. Usually secondary mutations have little or no effect on the level of resistance in the absence of primary mutations.

Resistance mutations are described by the number representing the specific position of the mutant codon, prefixed by a letter indicating the relevant amino acid found at the position in the consensus (?wild-type?) sequence, and followed by a letter indicating the amino acid found at that position in the mutant virus. Thus, the mutation characteristic of 3TC resistance is described M184V, indicating that at codon 184 Valine (V) has been substituted for Methionine (M).

Specimens requiring HIV drug resistance analysis have usually been submitted for HIV viral load determination. If there has been a significant and sustained rise in viral load for no obvious reason the ID physician may request drug resistance analysis. This can be performed on the stored plasma remaining from the HIV viral load assay.

Department

Microbiology - Virology

Delphic Registration Code

HIVG

Laboratory Handling

Phlebotomy

5ml EDTA (purple). Ensure the tube is only for this test.

Laboratory

Samples are referred to LabPlus for testing.

Synonyms

HIV genotype
HIV resistance testing
HIV susceptibility testing

Turnaround Time

6 weeks

Test Code

2291

Specimen Type

• Plasma

Specimen Requirements

Tube/Container Type:

Preferred

• EDTA (Lavender Cap)

Preferred Sample Volume (ml)

5.0

No of Samples Required

2

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

• EDTA (Lavender Cap)

Preferred Sample Volume (ml)

2.5

Pre-testing Requirements

Viral load will be checked before testing for HIV genotyping and drug resistance analysis. A minimum HIV viral load of 1,000 copies/mL is required for successful typing. Genotyping will not be attempted if viral load is <1,000 log copies/mL.

Aliquot Instructions

Transport to Lab

Frozen

Additional Referral/Aliquot Information

Plasma must be separated within 24 hours of collection.
For referral laboratories, please send the sample frozen.

Test Method

Samples are referred to LabPlus for testing. The assay is performed by amplifying HIV-1 reverse transcriptase (RT), protease (PR) and integrase genes. Samples are sequenced and analysed. Mutations over a 5% threshold are assessed for contribution to resistance.
Reports are forwarded to requesting clinician.

Non-Funded Test Cost

$1158.41 (Exclusive of GST)

Billing Code

2291

Contact Information

Enquiry Form