Dr. Susan Houseman stopped marching with her picket sign yesterday just long enough to write a prescription for one of her patients who stood on the sidewalk outside the Group Health Association medical center in Northwest Washington.

"We care about our patients and the quality of care," said Houseman. "We want to have time for quality care . . . . That's why we're on strike."

Houseman, 41, an internist who treats about 2,000 patients, is among the 160 unionized GHA doctors who went on strike yesterday in a dispute over workloads, office-hour schedules and the direction of the 145,000-member health maintenance organization.

GHA, one of the nation's oldest and largest HMOs, kept its seven Washington area health centers open with 13 management and several nonunion physicians, canceled hundreds of nonemergency appointments and referred many patients to private doctors and other health care facilities.

Striking doctors treated only their hospitalized patients and emergency cases. Late yesterday, with no new negotiations scheduled, the union representing GHA nurses filed notice that if the strike continues, nurses may honor doctors' picket lines and walk out, GHA officials said.

Like the doctors, Harold Wool cares about quality care: he is a GHA patient and the elected president of the member-owned GHA. "The doctors have said we are trying to have 'assembly-line medicine.' Unfortunately that is a gross misstatement," he said.

"We are not interested in profits. Our mission is simply to deliver high-quality care -- and we have to do it at a competitive cost," said Wool, a retired government labor economist who joined GHA in 1938.

The doctors' strike, according to both sides involved, reflects the intensifying competition for the health-care dollar in the Washington area and the difficult question of how to deliver quality medical care to working families and retirees while curbing costs.

The strike began at 12:01 a.m. yesterday after GHA management and the Capital Alliance of Physicians, one of the small number of doctors unions in the country, deadlocked over proposed changes that would have required doctors to devote several more hours per week to office visits and to see more patients. Doctors contend the proposed changes would force them to speed up care, leaving less time for visiting hospital and nursing home patients and for other duties.

GHA had four group-health competitors in 1983 but expects to have as many as 15 competing HMOs by 1987. In an extensive national survey of health-care practices, GHA found that its doctors spend less time on office visits and see fewer patients yearly than most competitors, according to Dr. Robert Rosenberg, GHA's executive director.

To remedy that, he said, GHA is proposing to increase office hours and to begin an incentive pay system with 1 percent to 4 percent pay bonuses linked to doctors' attaining certain numbers of "encounter units," which are patient-doctor contacts. A simple office visit is one unit, while more serious cases are assigned greater weight.

"This is a very dangerous concept," said Dr. Nieves Zaldivar, a GHA pediatrician who is the union president. "The most important thing is a philosophical change . . . and we are going to have to care more about the numbers we see than the quality of care if we accept it."

"I already have to shortchange some duties," said Dr. Bill Silverman, an internist. "I get 15 minutes for most visits. That's occasionally enough time, but often it's not. I run over that time, because I think it's important to sit and talk to patients, and not just tear them off a prescription."

But Rosenberg and Virginia Dollard, GHA's director of medical management, said the new system would set minimum numbers of office hours that many doctors are already achieving and that therefore would not result in speedups. Some HMOs already use "capitated" pay systems in which physician pay is linked to numbers of patients, "and we were trying to have an incentive-reward system for being more productive," Rosenberg said.

The incentive pay would be in addition to a pay raise of 4 percent over three years, and the customary 2 percent to 3 percent annual step increases many doctors receive, Rosenberg said. GHA doctors average $91,000 in salary, and pay has been a secondary issue in the talks.

Houseman said increased office hours or incentive plans may subtly cause doctors to overlook health problems. "I had a young patient with chest pains that radiated to her arms . . . . Cardiogram was negative. And it was only after I sat down and spent time with her that I learned she'd had an acquaintance who had died with the same symptoms," Houseman said.

The patient's anxiety turned out to be the apparent cause, she said, "but if you want to get to the bottom of problems, you sometimes have to take time with a patient."

Wool said he believes union "propaganda" has exaggerated GHA's intent and that doctors can still take adequate time with patients under the new proposals.

GHA patients interviewed yesterday generally appeared sympathetic to picketing doctors. "The doctors are really pushed, and I think they should really have time to find out about a patient's condition," said Arlene Eddy, a D.C. public school secretary.