F.D.A. Acts on Lab Tests Developed In-House

The Food and Drug Administration announced on Thursday that it would start regulating medical laboratory testing, saying that tests used to make important treatment decisions must be vetted and validated before they go into use.

The decision, long in coming, has been fiercely opposed by some laboratories and pathologists, who have said that regulation by the agency is unnecessary and would significantly increase the cost and time needed to develop tests, stifling innovation and depriving patients of some vital tools.

The issue has also become the subject of jockeying in Congress, with some Democrats pushing for F.D.A. regulation and some Republicans trying to head it off.

In its announcement, the agency tried to address such concerns, saying it would phase in the requirements over nine years and would focus oversight on the tests where a wrong result would pose the highest level of risk to patients. It would exempt tests for which there is no approved alternative and tests for rare diseases, which might be expensive to develop if required to go through F.D.A. review.

Still, the...