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The test, called "HIV antiviral resistance testing by genotyping," involves genetic analysis of the virus to detect mutations associated with drug resistance.
The results of the test can help physicians prescribe the most effective antiviral drug regimen for each patient, said Dr. Ann Warford, chief virologist at Stanford's Diagnostic Virology Laboratory.
Potent combinations of three or four different drugs are now the standard treatment for HIV infection. Physicians today choose from 11 different FDA-approved drugs to combat the virus. A typical drug regimen includes two drugs designed to block HIV's reverse transcriptase (RT) enzyme and one drug that counters the virus' protease enzyme.
Combinations of RT inhibitors and protease inhibitors have proved highly effective at suppressing HIV replication and improving clinical outcomes. Nevertheless, drug-resistant strains of the virus undermine the clinical progress of many HIV-infected patients. Early identification of drug resistance allows physicians to make timely changes in the drug regimen before a patient's condition deteriorates.
Stanford's antiviral resistance testing is easy to get. A blood sample -- drawn at any clinic, hospital or doctor's office -- can be transported to Stanford by courier service. The courier service is provided free of charge by Stanford. (See "Getting the test" below.) Test results are returned to the referring physician within two to three weeks.
Using advanced gene-sequencing equipment and the most accurate techniques available, the Stanford lab amplifies viral genes present in the patient's blood sample, determines the genetic sequences of the RT and protease genes, and analyzes them for known resistance mutations. The results are then correlated with the patient's drug history and clinical findings.
Antiviral resistance testing should be used in combination with tests that measure the amount of virus in a person's bloodstream, or viral load, Warford said. When patients begin drug therapy, their viral load frequently drops from hundreds of thousands of copies of virus per milliliter of blood to fewer than 50 copies per milliliter. After several months of suppression, however, the viral load may suddenly start to climb -- a trend that often signals the emergence of a drug-resistant strain.
"The primary reason for drug failure is drug resistance due to mutations in the reverse transcriptase or protease genes," said Dr. Robert Shafer, a clinical assistant professor of medicine (infectious diseases) at the Stanford University School of Medicine.
Other factors also can cause drug treatments to lose effectiveness, including a patient's failure to stick to the drug regimen (which may involve taking 20 pills every day) or poor absorption of one or more drugs into the bloodstream. Testing for resistance mutations can help clinicians distinguish drug resistance from other causes of increasing viral load, Shafer said.
The drug resistance pattern revealed by the test also may suggest specific changes that can be made in the patient's drug regimen to improve its effectiveness.
Antiviral resistance testing is also valuable for determining the baseline genetic characteristics of a patient's virus before treatment begins. Approximately 10 to 15 percent of newly infected individuals have acquired a strain of HIV that is already resistant to an antiviral drug, usually AZT, Shafer noted.
"If a person has been infected with a resistant strain, that would certainly influence the choice of initial therapy," he said.
Considering the high cost of antiviral drug therapy (up to $15,000 per year for each drug), resistance testing to identify drugs that are unlikely to help a particular patient is a cost-effective tool in addition to being clinically useful, Shafer said.
Knowledge of resistance mutations is critically important in choosing drugs to prevent HIV transmission to newborn children of infected mothers and in cases of accidental needle sticks in hospitals, Shafer added. In these situations, drug treatment is intended to completely block the establishment of an infection.
Shafer maintains a large computer database linking HIV gene sequences to the treatment history of each patient from whom the sequences were obtained and to other technical data. Eventually, he plans to link the gene sequences to data on the drug susceptibility of each viral strain and the clinical outcomes of the patients.
Shafer and other experts in Stanford's Center for AIDS Research are in the forefront of research on drug resistance mutations in HIV. Their knowledge and input are vital to the Virology Laboratory's resistance testing program and help ensure the reliability of test results and interpretations, Warford said.
In addition to highly accurate antiviral resistance testing, the Diagnostic Virology Lab offers the most sensitive type of viral load test available, capable of detecting as few as 20 viral copies per milliliter of blood.
The cost of testing -- $300 per gene or $650 for a package including the RT and protease gene tests and a viral load test on the same sample -- is reimbursable by insurers.
For further information on HIV antiviral resistance testing at Stanford, contact Ann Warford, Diagnostic Virology, by phone at 650/725-8031 or by e-mail at warford_A@hosp.stanford.edu
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Getting the Test
1. The test must be ordered by a physician. Contact the Diagnostic Virology Laboratory, Stanford University Hospital, at 650/723-5706 to obtain a requisition/order form.
2. Have blood drawn at any clinic, hospital or doctor's office. This service is available 24 hours a day at Stanford University Hospital. If you want to see a Stanford physician, call Dr. Andrew Zolopa, director of the Stanford Positive Care Clinic, at 650/498-5013.
3. Call a courier service for free transport of the blood sample. Courier services available in the Bay Area include:
Medical Courier: 800/652-1147 Spectrum Courier: 415/640-7481 NCM Courier: 408/929-3339
For further information on HIV antiviral resistance testing at Stanford, contact Ann Warford, Diagnostic Virology, by phone at 650/725-8031 or by e-mail at warford_A@hosp.stanford.edu
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CONTACT: Stanford University Medical Center Tim Stephens, 650/725-8047 or 723-6911 tims1@leland.stanford.edu or Patient/Physician Contact: Stanford Diagnostic Virology Laboratory, 650/723-5706
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