UPDATE 2-US urges genetic test before using Glaxo AIDS drug

(Adds details on changes to drug label, rates of reactions)

By Lisa Richwine

WASHINGTON, July 24 (Reuters) - AIDS patients should have a genetic test before treatment with GlaxoSmithKline Plc's (GSK.L: Quote, Profile, Research, Stock Buzz) drug Ziagen to see whether they face a higher risk of a potentially fatal reaction, U.S. regulators said on Thursday.

For patients who test positive for a specific gene variation, Ziagen treatment "is not recommended and should be considered only under exceptional circumstances when the potential benefit outweighs the risk," the Food and Drug Administration said in a notice on its website.

Studies found patients with one specific version of an immune system gene had a higher chance of a serious and at-times fatal reaction to Ziagen, the agency said.

Glaxo funded one study that showed testing for the gene variation reduced the incidence of reactions, company spokeswoman Mary Anne Rhyne said. In the study, 5.6 percent of patients tested positive for the variation.

The drugmaker gave the information to the FDA and urged its inclusion in Ziagen's prescribing instructions, Rhyne said.

The recommendation for the screening test is now included in a "black box" on Ziagen's label. A black box is the strongest type of warning for prescription drugs.

The step is another move toward personalized medicine, a drive to determine which drugs work best, or may be harmful, for individual patients.