Diagnostic errors, not surgical misadventures, obstetrical mistakes or improperly delivered medications, are the main source of successful malpractice claims. However, little is being done to identify such errors and measure their effects.

Those are among the conclusions of a study that examined 25 years of malpractice payments, which by law have to be listed in the National Practitioner Data Bank maintained by the federal government.

Incorrect, missed or delayed diagnoses accounted for 29 percent of successful malpractice claims. They accounted for 35 percent of the total amount of money paid out and caused 39 percent of malpractice-related deaths.

“Diagnostic errors are the most common, the most costly and the most deadly of all medical errors,” said David E. Newman-Toker, a neurologist at Johns Hopkins University School of Medicine. He and Ali S. Saber Tehrani led the research.

Autopsy studies suggest that 10 to 20 percent of deaths are from causes not diagnosed when the patient was alive. About half could have been successfully treated.

By one estimate, 40,000 to 80,000 Americans die from missed diagnoses each year.

A few examples of serious diagnostic errors include mistaking an ectopic pregnancy for appendicitis, an aortic dissection for severe heartburn or a brainstem stroke for an episode of dizziness. In some cases, the correct diagnosis is eventually made — after complications or disability have occurred.

The research team examined about 350,000 malpractice allegations from 1986 to 2010 in which payments were made. Each allegation was assigned to one of several broad categories. Besides diagnostic errors, categories of mistakes included treatment errors (27 percent), surgical mishaps (24 percent), obstetrical problems (7 percent), medication errors (5 percent), anesthesia disasters (3 percent) and several smaller groups.

The claims listed in the National Practitioner Data Bank almost certainly represent the most serious cases in each grouping.

In 41 percent of the cases in the diagnostic error category, the patient died, compared with 39 percent for medication errors and 26 percent for treatment errors. The average payment for a successful diagnostic error claim was $389,000 — second only to obstetrical claims, which averaged $695,000.

The study didn’t address why diagnostic errors are so common. But Newman-Toker said in an interview, “Diagnosis is hard. It’s complicated. It happens under quite a lot of uncertainty and time pressure.”

Diagnostic errors are harder to catch than medication and surgical errors because there’s often a lag before it’s clear the mistakes have occurred. Partly for that reason, “there are no public reporting requirements for measuring diagnostic accuracy or error,” Newman-Toker said.

The researcher said the solution to the problem is not necessarily more diagnostic testing. He said that about 40 percent of people who go to emergency rooms complaining of dizziness get CT scans of their heads. The total cost is about $500 million, and “almost every one of those scans is useless” in determining the cause, he said.

The study is published online by BMJ Quality & Safety, a publication of the British Medical Journal.