Six people who should have been higher on a recipient list for kidney transplants were not selected because of a computer programming error, the Japan Organ Transplant Network said Sunday.

Network officials said they have telephoned the patients to apologize and explain the mistake.

The error is "a serious issue affecting the basic structure of our system, which is designed to distribute the limited supply of organs for transplants in a fair and equal manner," said Kikuo Nomoto, vice director of the network.

A health ministry official in charge of organ transplants described the error as "something that should absolutely not have happened."

Selecting kidney recipients is handled by a transplant coordinator, who examines a short list prepared by a computer screening program when the prospect of a transplant comes up.

Various factors are taken into account, including how long a patient has been waiting as well as certain types of human leukocyte antigen.

The computer software included antigen types that should not have been negative factors in the selection process, thus barring some patients from the short list, officials of the Japan Organ Transplant Network said.

The network introduced the computer system in January 2002. The error occurred as staff failed to input new information following a revision of the human leukocyte antigen criteria in October 2001.

They found the error this month when a person linked to a local branch of the network made an inquiry about the input of antigen data.

About 130 kidney transplants have been performed in Japan since 2002, but the six patients, who should have been high on the waiting list, have yet to receive new kidneys.

According to the network, there are currently some 12,600 patients on the waiting list for kidney transplants.