Gary Sergott felt weary all the time. “I’d get tired, short of breath, a sort of malaise,” he said. He was cold even on warm days and looked pale with dark circles under his eyes.

His malady was not mysterious. As a retired nurse-anesthetist, Mr. Sergott knew he had anemia, a deficiency of red blood cells. In his case, it was the consequence of a hereditary condition that caused almost daily nosebleeds and depleted his hemoglobin, the protein in red blood cells that delivers oxygen throughout the body.

But in consulting doctors about his fatigue, he found that many didn’t know how to help. They advised Mr. Sergott, who lives in Westminster, Md., to take iron tablets, usually the first-line treatment for anemia.

But like many older people, he found a daily regimen of four to six tablets hard to tolerate. Some patients taking iron complain of severe constipation or stomach cramps. Mr. Sergott felt “nauseated all the time.” And iron tablets don’t always work.

After almost 15 years, he found a solution. Dr. Michael Auerbach, a hematologist and an oncologist who is the co-director of the Center for Cancer and Blood Disorders in Baltimore, suggested that Mr. Sergott receive iron intravenously instead of orally.

Now Mr. Sergott, 78, gets an hourlong infusion when his hemoglobin levels and other markers show that he needs one, usually three times a year. “It’s like filling the gas tank,” he said. His symptoms recede, and “I feel great.”

His story reflects, however, the frequent dismissal of a common condition, one that cannot only diminish older adults’ quality of life but lead to serious health consequences, including falls, fractures and hospital stays.

Anemia’s symptoms — tiredness, headaches, leg cramps, coldness, decreased ability to exercise, brain fog — are often attributed to aging itself, Dr. William Ershler, a hematologist and researcher said. (Some people with anemia remain asymptomatic.)

“People say, ‘I feel weak, but everybody my age feels weak,’” Dr. Ershler said.

Even though hemoglobin levels are likely to have been included in their patients’ records, as part of the C.B.C. (complete blood count) routinely ordered during medical visits, doctors often fail to recognize anemia.

“The patients come to the clinic and get the blood tests, and nothing happens,” he said.

Anemia affects 12.5 percent of people over 60, and the rate rises thereafter, according to the most recent survey data from the National Health and Nutrition Examination Survey.

But that may be an underestimate.

In a study published in the Journal of the American Geriatrics Society, Dr. Ershler and his colleagues examined the electronic health records of almost 2,000 outpatients over 65 at Inova, the large health system in Northern Virginia from which he recently retired.

Based on blood test results, the prevalence of anemia was much higher: About one in five patients was anemic, with hemoglobin levels below normal as defined by the World Health Organization.

Yet only about a third of those patients had anemia properly documented in their medical charts.

Anemia “deserves our attention, but it doesn’t always get it,” said Dr. George Kuchel, a geriatrician at the University of Connecticut, who wasn’t surprised by the findings.

That’s partly because anemia has so many causes, some more treatable than others. In perhaps a third of cases, it arises from a nutritional deficiency — usually a lack of iron, but sometimes of vitamin B12 or folate (called folic acid in synthetic form).

Older people may have decreased appetites or struggle to shop for food and prepare meals. But anemia can also follow blood loss from ulcers, polyps, diabetes and other causes of internal bleeding.

Surgery can also lead to iron deficiency. Mary Dagold, 83, a retired librarian in Pikesville, Md., underwent three abdominal operations in 2019. She remained bedridden for weeks afterward and needed a feeding tube for months. Even after she healed, “the anemia didn’t go away,” she said.

She remembers feeling perpetually exhausted. “And I knew I wasn’t thinking the way I usually think,” she added. “I couldn’t read a novel.” Her primary care doctor and Dr. Auerbach both advised that oral iron was unlikely to help.

Iron tablets, available over the counter, are inexpensive. Intravenous iron, becoming more widely prescribed, can cost $350 to $2,400 per infusion depending on the formulation, Dr. Auerbach said.

Some patients find a single dose sufficient, while others will need regular treatment. Medicare covers it when tablets are hard to tolerate or ineffective.

For Ms. Dagold, a 25-minute intravenous iron infusion every five weeks or so has made a startling difference. “It takes a few days, and then you feel well enough to go about your daily life,” she said. She has returned to her water aerobics class four days a week.

In other cases, anemia arises from chronic conditions like heart disease, kidney failure, bone marrow disorders or inflammatory bowel diseases.

“These people don’t lack iron, but they’re not able to process it to make red blood cells,” Dr. Kuchel said. Since iron supplements won’t be effective, doctors try to address the anemia by treating patients’ underlying illnesses.

Another reason to pay attention: “Loss of iron can be the first harbinger of colon cancer and stomach cancer,” Dr. Kuchel pointed out.

In about a third of patients, however, anemia remains frustratingly unexplained. “We’ve done everything, and we have no idea what’s causing it,” he said.

Learning more about anemia’s causes and treatments might prevent a lot of misery down the road. Besides its association with falls and fractures, anemia “can increase the severity of chronic illnesses — heart, lung, kidney, liver,” Dr. Auerbach said. “If it’s really severe and hemoglobin goes to life-threatening levels, it can cause a heart attack or stroke.”

Among the unknowns, however, is whether treating anemia early and restoring normal hemoglobin will prevent later illnesses. Still, “things are happening in this field,” Dr. Ershler said, pointing to a National Institute on Aging workshop on unexplained anemia held last year.

The American Society of Hematology has appointed a committee on diagnosing and treating iron deficiency and plans to publish new guidelines next year. The Iron Consortium at Oregon Health and Science University convened an international panel on managing iron deficiency and recently published its recommendations in The Lancet Haematology.

In the meantime, many older patients can gain access to their C.B.C. results and thus their hemoglobin levels. The World Health Organization defines 13 grams of hemoglobin per deciliter as normal for men, and 12 for nonpregnant women (though some hematologists argue that those thresholds are too low).

Asking health care providers about hemoglobin and iron levels, or using a patient portal to check the numbers themselves, could help patients steer conversations with their doctors away from fatigue or other symptoms as inevitable results of aging.

Perhaps they’re signs of anemia, and perhaps it’s treatable.

“Chances are, you’ve had a C.B.C. in the last six months or a year,” Dr. Kuchel said. “If your hemoglobin is fine, great.”

But, he added, “If it’s really outside the normal boundaries, or it’s changed compared to a year ago, you need to ask questions.”