Reports of inaccuracies suggest that new federal rules to ensure reliable directories are having little effect. Starting this year, all plans sold through the marketplaces are required to “publish an up-to-date, accurate and complete provider directory” or be subject to penalties or removed from the marketplace portal.

But so far no plans have been fined or kicked off the enrollment sites for having poor doctor directories, said Aaron Albright, a spokesman for the Centers for Medicare and Medicaid Services, which would enforce the rules. A Health and Human Services Department survey of Medicare plans for those 65 and older that was released in October found errors in nearly half of the listings in doctor directories.

Staci Doolin, a co-owner of a radon-testing company in Forsyth, Ill., consulted the Blue Cross Blue Shield of Illinois physician directory in January to make sure her primary care physician was in the network and even called the insurer to double-check.

The directory was wrong. The doctor was not in the plan.

“I thought I was good to go, and then I get this bill and it says my insurance didn’t cover anything and I owe $503,” Ms. Doolin said.

It took until September to resolve the matter — but not before the office threatened to summon a bill collector. She never recovered $100 she spent on a dermatologist who was listed in the directory but who also was not part of the plan.

No comprehensive data exists on doctor directory accuracy. The health law and the Health and Human Services Department set standards for network adequacy but leave most enforcement up to states. States rarely test the lists for accuracy and often rely on consumers to report problems.

But third-party surveys frequently reveal big discrepancies. One recently published study showed as many as a fourth of the doctors listed in California directories last year for marketplace plans were not accepting new patients. About one doctor in 10 was not working for the listed practice.