You must lose weight, a doctor told Sarah Bramblette, advising a 1,200-calorie-a-day diet. But Bramblette had a basic question: How much do I weigh?

The doctor’s scale went up to 350 pounds, and she was heavier than that. If she did not know the number, how would she know if the diet was working?

The doctor had no answer. So Bramblette, 39, who lived in Ohio at the time, resorted to a solution that made her burn with shame. She drove to a nearby junkyard that had a scale that could weigh her. She was 502 pounds.

One in three Americans is obese, a rate that has been steadily growing for more than two decades, but the health care system — in its attitudes, equipment and common practices — is ill prepared, and its practitioners are often unwilling, to treat the rising population of fat patients.

The difficulties range from scales and scanners, like MRI machines that are not built big enough for very heavy people, to surgeons who categorically refuse to give knee or hip replacements to the obese, to drug doses that have not been calibrated for obese patients. The situation is particularly thorny for the more than 15 million Americans who have extreme obesity — a body mass index of 40 or higher — and face a wide range of health concerns.

Part of the problem, both patients and doctors say, is a reluctance to look beyond a fat person’s weight. Patty Nece, 58, of Alexandria, Virginia, went to an orthopedist because her hip was aching. She had lost nearly 70 pounds and, although she still had a way to go, was feeling good about herself. Until she saw the doctor.

“He came to the door of the exam room, and I started to tell him my symptoms,” Nece said. “He said: ‘Let me cut to the chase. You need to lose weight.’”

The doctor, she said, never examined her. But he made a diagnosis, “obesity pain,” and relayed it to her internist. In fact, she later learned, she had progressive scoliosis, a condition not caused by obesity.

Dr. Louis J. Aronne, an obesity specialist at Weill Cornell Medicine, helped found the American Board of Obesity Medicine to address this sort of issue. The goal is to help doctors learn how to treat obesity and serve as a resource for patients seeking doctors who can look past their weight when they have a medical problem.

Aronne says patients recount stories like Nece’s to him all the time.

“Our patients say: ‘Nobody has ever treated me like I have a serious problem. They blow it off and tell me to go to Weight Watchers,’” Aronne said.

“Physicians need better education, and they need a different attitude toward people who have obesity,” he said. “They need to recognize that this is a disease like diabetes or any other disease they are treating people for.”

The issues facing obese people follow them through the medical system, starting with the physical exam.

Research has shown that doctors may spend less time with obese patients and fail to refer them for diagnostic tests. One study asked 122 primary care doctors affiliated with one of three hospitals within the Texas Medical Center in Houston about their attitudes toward obese patients. The doctors “reported that seeing patients was a greater waste of their time the heavier that they were, that physicians would like their jobs less as their patients increased in size, that heavier patients were viewed to be more annoying, and that physicians felt less patience the heavier the patient was,” the researchers wrote.

Other times, doctors may be unwittingly influenced by unfounded assumptions, attributing symptoms like shortness of breath to the person’s weight without investigating other likely causes.

That happened to a patient who eventually went to see Dr. Scott Kahan, an obesity specialist at Georgetown University. The patient, a 46-year-old woman, suddenly found it almost impossible to walk from her bedroom to her kitchen. Those few steps left her gasping for breath. Frightened, she went to a local urgent care center, where the doctor said she had a lot of weight pressing on her lungs. The only thing wrong with her, the doctor said, was that she was fat.

“I started to cry,” said the woman, who asked not to be named to protect her privacy. “I said: ‘I don’t have a sudden weight pressing on my lungs. I’m really scared. I’m not able to breathe.’”

“That’s the problem with obesity,” she said the doctor told her. “Have you ever considered going on a diet?”

It turned out that the woman had several small blood clots in her lungs, a life-threatening condition, Kahan said.

For many, the next step in a diagnosis involves a scan, like a CT or MRI. But many extremely heavy people cannot fit in the scanners, which, depending on the model, typically have weight limits of 350 to 450 pounds.

Problems do not end with a diagnosis. With treatments, uncertainties continue to abound.

In cancer, for example, obese patients tend to have worse outcomes and a higher risk of death — a difference that holds for every type of cancer.

The disease of obesity might exacerbate cancer, said Dr. Clifford Hudis, chief executive of the American Society of Clinical Oncology.

But, he added, another reason for poor outcomes in obese cancer patients is almost certainly that medical care is compromised. Drug doses are usually based on standard body sizes or surface areas. The definition of a standard size, Hudis said, is often based on data involving people from decades ago, when the average person was thinner.

For fat people, that might lead to underdosing for some drugs, but it is hard to know without studying specific drug effects in heavier people, and such studies are generally not done. Without that data, if someone does not respond to a cancer drug, it is impossible to know whether the dose was wrong or the patient’s tumor was just resisting the drug.

One of the most frequent medical problems in obese patients is arthritis of the hip or knee. It is so common, in fact, that most patients arriving at orthopedists’ offices in agonizing pain from hip or knee arthritis are obese. But many orthopedists will not offer surgery unless the patients first lose weight, said Dr. Adolph J. Yates Jr., an orthopedics professor at the University of Pittsburgh School of Medicine.

“There are offices that will screen by phone,” Yates said. “They will ask for weight and height and tell patients before they see them that they can’t help them.”

But how well grounded are those weight limits?

“There is a perception among some surgeons that it is more difficult, and certainly some felt it was an added risk,” to operate on very obese people, Yates said. He was a member of a committee that reviewed the risks and benefits of joint replacement in obese patients for the American Association of Hip and Knee Surgeons. The group concluded that heavy patients should first be counseled to lose weight because a lower weight reduces stress on the joints and can alleviate pain without surgery.

But there should not be blanket refusals to operate on fat people, the committee wrote. Those with a body mass index over 40 — like a 5-foot-5-inch woman weighing 250 pounds or a 6-foot man weighing 300 — and who cannot lose weight should be informed that their risks are greater, but they should not be categorically dismissed, the group concluded.

Yates said he had successfully operated on people with body mass indexes as high as 45. What is behind the refusals to operate, he said, is that doctors and hospitals have become risk-averse because they fear their ratings will fall if too many patients have complications.

A lower score can mean reductions in reimbursements by Medicare. Poor results can also lead to penalties for hospitals and, eventually, doctors.

A recent survey of more than 700 hip and knee surgeons confirmed Yates’ impressions. Sixty-two percent said they used body mass index scores as cutoffs for requiring weight loss before offering surgery. But there was no consistency in the figures they picked.

“The numbers were all over the map,” Yates said. And 42 percent who picked a body mass index cutoff said they had done so because they were worried about their performance score or that of their hospital.

“It’s very common to pick an arbitrary BMI number and say, ‘That is the number we won’t go above,’” Yates said. Yet a person with an index of, say, 41 might be healthy and active, he said, but in terrible pain from arthritis. A knee replacement could be life transforming.

“It’s a zero-sum game, with everyone trying to have the lowest-risk patient,” Yates said. “Patients who may be at a marginally higher risk may be treated as a class instead of individuals. That is the definition of discrimination.”

© 2016 The New York Times Company