WASHINGTON (AP) - The government has approved the first gene-based test to tell quickly whether an HIV patient's virus is mutating to make a particular drug therapy fail, important to know so the person can switch AIDS medications.

Visible Genetics Inc.'s Trugene is one of the most complex genetic test systems to clear the Food and Drug Administration. FDA officials described it on Thursday as an important tool in helping doctors select the medications most likely to fight each patient's HIV.

The AIDS virus naturally grows resistant to medications through evolution. Experts estimate 60 percent of patients have a virus that is resistant to at least one drug.

Until now, most HIV patients have kept tabs on how well their treatment is working by undergoing tests to see how much of the AIDS virus is in the bloodstream. A spike can mean that HIV is growing resistant to one or more drugs, and it's time to try to a different medicine.

But to specifically check a patient's blood for genetic mutations that mean one of the 15 anti-AIDS drugs won't work has required additional laboratory testing not routinely available.

With Trugene, a doctor sends a patient's blood sample to one of 130 labs where Visible Genetics so far has trained personnel. A computer decodes the HIV genes in that blood, identifying all the genetic mutations present. Then a software program matches those mutations to a list of more than 70 mutations currently linked to resistance in specific drugs.

The lab mails the doctor a report listing the likelihood that each AIDS drug would work according to the viral mutations currently coursing through that patient's blood, said Visible Genetics president Richard Daly.

The test is 98 percent accurate, said FDA medical reviewer Dr. Andrew Dayton. Better, as scientists discover additional mutations that cause drug resistance - a rapidly changing field - the new information can be added to the software promptly so that the test remains useful in real-world practice, he said.

The test takes three days to complete and will cost between $300 and $500 per patient, Daly said.

"It's a major step forward in HIV treatment," said Dr. R. Scott Hitt, president of the American Academy of HIV Medicine.

But "it is only one piece of the puzzle" in picking the best therapy, Hitt cautioned, urging patients to seek treatment from HIV specialists who can properly interpret test results.

FDA researchers set up the computer gene sequencer in an agency lab to understand the novel test fully and approved its sale late Wednesday after a year of review, record review time for such a complex new science, Daly said. Testing just became available.

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