White doctors routinely delay treatment of HIV-positive black patients, a new finding that adds to a growing body of research that suggests a sharp racial divide in American medical care.

Researchers didn’t examine why doctors act differently when their patients are black, and it’s not clear that racism is the cause, said study co-author Dr. William D. King, a visiting assistant professor of infectious diseases at the University of California at Los Angeles. However, he said, “we can definitely see there is a huge disparity. There is discrimination.”

King and his colleagues examined the results of a previous study that looked at HIV-positive patients from 1996 to 1999. Of 1,241 adult patients, 61 percent were white with white doctors, 32 percent were black with white doctors, and 6 percent were black with black doctors. Fewer than 1 percent were whites with black doctors.

The researchers report their findings in the November issue of the Journal of General Internal Medicine.

Overall, the black patients received powerful HIV drugs after a median of 439 days - well over a year - compared to 277 days for whites. If blacks had white doctors, they received drugs after a median of 461 days; if they had black doctors, the delay was only 342 days. White patients with white doctors, meanwhile, received drugs after 353 days, similar to black patients with black doctors.

The differences between the races remained even when the researchers took into account factors such as how far the cases of HIV had progressed. “Medically, we couldn’t really find an explanation about why this disparity existed,” King said.

It seems likely the delayed care would have hurt the black patients, although the study didn’t examine how their health changed over time. “If they didn’t get the medicine in a timely manner, they could have gone to be sicker or died earlier from the disease,” King said.

During the years of the initial study, a new generation of drugs began boosting the life spans of AIDS patients, and many of them now are able to live fairly normal lives.

So what’s the explanation for the disparity? The researchers suggest there may be a communications divide between blacks and whites that contributes to a lack of trust. “If patients trust you, they’re more likely to come back for care and to take the regimens that are prescribed,” King said.

The researchers also pointed to previous studies suggesting that white doctors think black patients are more likely to fail to follow instructions about drug regimens.

As for racism, it’s difficult to prove because it’s hard to define, King said.

Paul D. Cleary, a professor of health care policy at Harvard Medical School, agreed. “Different people define it differently,” said Cleary, who studies racial disparities in health care. “A lot of people think of it as an overt, conscious way of treating people of color differently than other people. I think it’s subtler than that.”

For example, he said, doctors “may have a stereotype about an African-American patient that leads them to be more cautious [with treatment]. They may not be trying to harm a patient, they may be trying to do the absolute best job they can, but it’s possible that they’re treating an African-American patient differently in a way that harms them.”

“That,” he added, “is very different than saying, ‘I don’t like African-Americans and I’m not going to treat them well.’”