The latest pills from Pfizer and Schering-Plough may stop working in some patients because a test identifying who should get the medicines isn’t always accurate, Bloomberg News reports. The meds work work by blocking a chemical entryway known as CCR5 that the virus uses to infect cells. In about 10 percent of cases, a Monogram Biosciences test incorrectly identifies patients who will benefit, according to scientists at an AIDS meeting.

New research on Pfizer’s Selzentry and Schering’s vicriviroc, as well as the test’s reliability, will be presented today at the Conference on Retroviruses and Opportunistic Infections in Boston. While the pills promise to fight HIV in patients who can’t take older medicines, the new drugs’ effectiveness depends on accurate screening, Bloomberg notes.

“The test is wrong in about 8 to 10 percent of patients initially screened to see if they are candidates for a CCR5 antagonist,” David Hardy, director of the division of infectious disease at Cedars Sinai Medical Center in Los Angeles, tells Bloomberg. “We’re waiting to see if the next-generation test from Monogram will eliminate the errors.”

Selzentry was approved in August for patients who stopped responding to older medicines, and is the only approved CCR5 inhibitor, the first new family of AIDS meds in a decade. Vicriviroc, a similar drug, is in the third and final stage of testing generally required for FDA approval.

As many as 90 percent of previously untreated HIV patients will have a strain of the virus that enters healthy cells through the CCR5 doorway, Howard Mayer, executive director of clinical research and development for Pfizer, tells Bloomberg. After five years of drug therapy, about half of patients still have that strain, and by then, he says, most patients have higher levels of another version of the virus known as X4 that infects cells through a different route, making drugs such as Selzentry and vicriviroc ineffective.

A new test to determine better who can benefit from the Pfizer and Schering drugs is about six months away from reaching the US market, Chris Petropoulos, Monogram’s chief scientific officer, tells Bloomberg. “We will improve our ability to identify the patients who shouldn’t go on these drugs, but not at the cost of screening patients who should get the drugs and benefit from it,” says Petropoulos.

Monogram’s test, called Trofile, is designed to detect which patients have strains of HIV that will latch on to the CCR5 receptor to break into healthy cells, Bloomberg writes. Patients can have more than one strain of the virus at once, at different levels in the blood. Sometimes, the test doesn’t detect the presence of a virus strain that uses a different doorway, known as the X4 receptor.

“In patients with X4, Selzentry or another CCR5 antagonist is not likely to have a significant effect on the virus,” says Pfizer’s Mayer. “It would be helpful to identify those patients who had X4 variance with the first screening so those patients would not inappropriately be put on Selzentry.”