Doctors fail to take nearly half the recommended steps for treating common illnesses such as high blood pressure and diabetes, suggesting that health care in the United States isn’t nearly as good as many people thought, researchers say.

Treatment guidelines, many written by medical specialty organizations, outline recommended approaches to many common ailments, ranging from painkillers and exercise for arthritis to surgery for breast cancer.

However, the guidelines are often ignored, indicating that even people who have good insurance and doctors they like don’t always get the best care, said Elizabeth McGlynn, a researcher with the Rand Corp. think-tank who led a study published in today’s New England Journal of Medicine.

“I think most people assume the big problem is, ‘Can you get in to see a doctor if you’re ill?”‘ she said. But that turned out to be just the first step: “Once you’re in the door, there is no guarantee you will get the full mix of services you need to protect your health.”

Her study documents a broad range of lapses in treating and preventing run-of-the-mill illnesses. For instance, patients studied did not receive one-third of the recommended immunizations, one-third of the standard medicines for heart disease or half of the recommended care for diabetes.

The report, based on a review of medical records for 6,712 people in 12 cities, looked only at how often their doctors followed the rules, not whether they suffered any ill effects from the lapses.

“This study showed that there is substantial discrepancy between what we say doctors ought to be doing and what they are doing,” said Dr. Earl Steinberg, an adjunct professor at Johns Hopkins University and head of a company that develops computer programs for doctors.

The presidents of the American Academy of Family Physicians and of the American College of Physicians said their organizations have been discussing the problem for some time.

“I don’t think it surprises anybody in the health profession to know there is no real uniformity of health care,” said Dr. James Martin, AAFP president.