WASHINGTON — New rules this week are bringing changes to the nation’s kidney transplant system that will give some patients a better shot at a longer-lasting organ, and move others up the waiting list.

The goal is to get the maximum benefit out of a scarce resource: kidneys from deceased donors.

“Not everyone’s going to get the same benefit out of a kidney transplant,” said Dr. Matthew J. Ellis of Duke University Medical Center, who is optimistic the new policy will help. “We’re trying to apply commonsense principles to address those issues.”

The long-awaited changes from the United Network for Organ Sharing, or UNOS, come as the gap between who needs a new kidney and who gets one widens. Nearly 102,000 people are on the national waiting list for a kidney, while fewer than 17,000 transplants are performed each year.

Roughly 11,000 transplants are thanks to kidneys donated from someone who just died rather than living donors. How long you’ve been waiting is the main factor in deciding who gets the next available cadaver kidney.

But starting Thursday, the fittest of those kidneys — based on donor age and medical history — will be offered first to patients who are expected to survive a transplant the longest.

Called longevity matching, the hope is these typically younger adults won’t outlive their new organ and need another that could have gone to someone else.

Nearly 15 percent of transplant candidates are awaiting a repeat operation, and “the potential here is that we will reduce return to the waiting list,” said Dr. Richard N. Formica of Yale University, who chairs UNOS’ kidney transplant committee.

Another big change: People who started dialysis before getting in line for a transplant will be credited for that dialysis time, moving them up on the list.

The less time spent on dialysis, the better the prognosis after a transplant. Yet how quickly people are put on the transplant list varies around the country.

Recalculating waiting lists and estimating patients’ life expectancy has been a big job for transplant centers.

“We’re scrambling to get this done,” said Dr. John Roberts, transplant chief at the University of California, San Francisco, one of the nation’s largest kidney programs.