For example, should more weight be given to making difficult matches for recipients who are particularly prone to rejection, or to achieving as many transplants as possible? Should a patient’s health status or age or time on the federal transplant waiting list be considered? The answers now vary from registry to registry.

“This is organ allocation,” said Dr. Dorry L. Segev, a transplant surgeon at Johns Hopkins Hospital, which has a registry that caters to hard-to-match patients. “They’re deciding who lives and who dies.”

With black-market organ brokers and Internet matching in the headlines, pioneers in kidney-paired exchange recognize that they may be one misplaced shipment or medical mishap away from deeper scrutiny. But they also continue to stretch boundaries.

Garet Hil, founder of the National Kidney Registry, which is based on Long Island, N.Y., said he hoped to attempt a transoceanic swap some day soon. Others have discussed starting chains with deceased donor kidneys, adding magnitudes of logistical complexity.

“It is a little bit like the Wild West,” said Dr. Robert A. Montgomery, another transplant specialist at Hopkins. “There has to be some regulation, and it has to be fair, because if people don’t think it’s fair, they’re not going to donate organs.”

The paired exchange registries have fought the perception that kidney swaps unfairly steer the benefits of altruistic donations to transplant candidates who have willing donors. A Good Samaritan who wanted to give a kidney to a stranger could otherwise donate to someone on the waiting list for a deceased organ, although doing so would not start a chain. The wait for a deceased kidney can take years, and they typically do not function as well or survive as long.

There is a racial dimension because African-Americans, who have high rates of kidney disease, are underrepresented among those with willing donors and overrepresented on the waiting list for cadaver kidneys. Researchers point to a variety of possible reasons: lower incomes, lower rates of insurance coverage and less access to high-level care; fewer suitable donors because of the prevalence of diabetes and hypertension; and perhaps a cultural reluctance to ask family members to donate.