THE EVALUATION OF AIDS therapies is a tricky business. Waiting until a drug regimen shows a clear-cut effect in extending life or reducing secondary infections may take years. And measures of a treatment's effectiveness now in use are less predictive than doctors would like. For example, it's clear that a big drop in the numbers of an immune system cell called CD4 is a bad sign. But a small, drug-induced rise in CD4 counts doesn't necessarily help. Doctors and AIDS activists think there may be a better gauge of a patient's progress: directly measuring the amount of virus--the viral load--in the body.

Now there's firm proof that it works. To evaluate a new AIDS drug, Pharmacia & Upjohn Co. is using a sensitive test, polymerase chain reaction (PCR), to measure viral load. The preliminary results, announced on Jan. 17, are striking. Subjects who started the trial with less virus took longer to get sick than those harboring more HIV. And those in whom the drug cut viral levels 90% were five times less likely to die or progress to more serious stages of the disease. That means the amount of virus is "a very powerful predictor," says AIDS activist Ben Cheng of Project Inform. Clinicians and activists are now pressuring insurers and HMOs to pay for PCR viral load tests, which cost between $125 and $250 each, as part of normal patient care.

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