In an annex behind Doctors' Hospital in Lanham is a private blood-drawing center that owes its success to widespread fears about AIDS contamination of national blood supplies.

On Tuesday and Thursdays, people facing surgery, or their relatives and friends, give their blood to the Maryland Blood Center, which then transports it by courier or taxi to the hospital where the surgery will be performed.

"Because of the AIDS scare, we are seeing tremendous growth," said Dr. Menira Rifat, owner of the two-year-old for-profit center and director of laboratories for Doctors' Hospital.

The Maryland center, and others throughout the country, are part of new challenges to the nation's volunteer blood donor system.

Because of the fears about public blood supplies, hospitals and doctors across the country are being forced to change longstanding policies on blood donation.

Many hospitals that have long discouraged "directed donations," where relatives or friends give blood for a specific patient, now allow them because patients are refusing surgery without them.

The Red Cross and the American Association of Blood Banks refuse to endorse directed donations, for fear that relatives with medical problems will conceal them. Several local hospitals, which Rifat would not name, will not accept blood from the center.

Dr. Ron Sacher, director of Georgetown University Hospital's blood bank, said the hospital has expanded its program to draw a patient's own blood before surgery and "reluctantly" accepts directed donations "if asked and pressured by patients."

Patients are charged more for blood donated by friends and relatives, rather than from the general pool, because of clerical costs, he said.

Sacher noted: "We deal with a lot of influential people who put pressure on us. As a service hospital, it's very difficult to say we won't do it."

Other hospitals, ones that operate their own blood banks, began selecting donors before national tests were instituted in the spring of 1985 to test for the acquired immune deficiency syndrome virus.

Fairfax Hospital is one such case. When AIDS contamination of blood supplies first surfaced in 1983, its blood bank began stocking blood for premature infants from "prime" donors, according to Dr. Freydoon Arhari, director of the hospital's blood bank. Donors whose medical history was known to the hospital were asked to donate again.

Blood officials say AIDS caused the changes in blood practices, although other blood problems have caused far more death and disease.

According to a study published by the American Enterprise Institute last year, more than 200,000 people -- or 7 to 12 percent of the 3 million Americans who receive blood transfusions each year contract hepatitis, a serious liver infection, from the blood.

"Some recover, but others develop chronic liver disease or spread hepatitis to family and friends," noted one of the authors, Ross Eckert of Claremont McKenna College.