PHILADELPHIA - A sperm donor passed an extremely rare and dangerous genetic ailment to five children born to four couples, doctors reported Friday in a case that exposes a gap in the screening process.

The disease, severe congenital neutropenia, can be fatal in children if untreated but is so rare that sperm banks do not test for it. Moreover, the family medical history that sperm donors must provide would not necessarily reveal such a defect, especially if the man is only a carrier and has no symptoms.

The four couples used the same sperm bank, said Dr. Lawrence A. Boxer, lead author of report in The Journal of Pediatrics, a Philadelphia publication.

Boxer, director of pediatric hematology and oncology at the University of Michigan and an expert on the disease, would not say where the sperm bank is located, or where the donor or the recipients are from.

"The bottom line is, when you use a sperm donor you really don't know what you're getting," Boxer said.

The researchers did not have any donor sperm to test but connected the children's cases to one man because they all had the same version of the defective gene and because all the couples used the same sperm bank.

Severe congenital neutropenia occurs in about one in 5 million births; Boxer said only about 500 children in the United States have the disease. Children with the disorder lack the type of white blood cell that kills bacteria, and as a result develop severe infections shortly after birth.

American Society of Reproductive Medicine guidelines require anonymous sperm donors to provide a full family medical history going back at least three generations. The guidelines say a complete chromosome screening is not required if a proper family history is taken concerning potential hereditary disorders.

"The question is whether a donor would even be aware that a great-grandparent had this disease," said Scott Brubaker, policy officer for the American Association of Tissue Banks, which accredits sperm banks. "Was this disease even known about three generations ago?"

Sperm donors are routinely screened for more prevalent genetic disorders like cystic fibrosis, Tay-Sachs disease and sickle cell anemia, but not for rare genetic diseases.

It was not clear whether the sperm donor in this case knew he was a carrier. Boxer said the sperm bank reported that the donor was healthy.

The children are doing well through daily injections of a drug that helps build up white blood cells and fight germs, Boxer said. But they will always have an increased risk of leukemia and a 50 percent chance of passing the disease to their own children, he said.