Some medical testing laboratories in the United States failed to follow rules for screening samples of semen, human eggs and stem cells for viruses, including HIV and hepatitis C, according to federal officials.

The labs, which were not named publicly, should retest the samples or at least tell doctors which ones were not screened properly, the Food and Drug Administration said in a notice to be published in today's Federal Register.

Some labs have tested pooled specimens, instead of individual donors as required, or have not used FDA-approved tests, violating rules implemented in May 2005, the agency said. The use of the samples may spur the spread of disease, the FDA said, urging the labs to identify and quarantine any improperly tested specimens and act to prevent lapses.

Samples should be retested or labs should seek new specimens from donors if too much time has passed, the FDA said. In cases where retesting is not possible, the agency plans to use discretion in allowing labs to distribute samples whose risks are indicated to doctors and on the labeling.

"Patients may wish to accept the small increased risk presented by these testing deficiencies, provided the labeling is appropriate and accurately reflects the increased risk," the regulators said.

Donated semen and eggs are used by fertility clinics.

The FDA did not say how many labs may have violated screening regulations or how many samples may be affected. The agency's guidelines, which are nonbinding, are intended for immediate implementation.