Just before Katie Marsh dropped out of college, she began to worry that she might have attention deficit hyperactivity disorder.

“Boredom was like a burning sensation inside of me,” said Ms. Marsh, who is now 30 and lives in Portland, Ore. “I barely went to class. And when I did, I felt like I had a lot of pent-up energy. Like I had to just move around all the time.”

So she asked for an A.D.H.D. evaluation — but the results, she was surprised to learn, were inconclusive. She never did return to school. And only after seeking help again four years later was she diagnosed by an A.D.H.D. specialist.

“It was pretty frustrating,” she said.

A.D.H.D. is one of the most common psychiatric disorders in adults. Yet many health care providers have uneven training on how to evaluate it, and there are no U.S. clinical practice guidelines for diagnosing and treating patients beyond childhood.

Without clear rules, some providers, while well-intentioned, are just “making it up as they go along,” said Dr. David W. Goodman, an assistant professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine.

This lack of clarity leaves providers and adult patients in a bind.

“We desperately need something to help guide the field,” said Dr. Wendi Waits, a psychiatrist with Talkiatry, an online mental health company. “When everyone’s practicing somewhat differently, it makes it hard to know how best to approach it.”

Can A.D.H.D. symptoms emerge in adulthood?

A.D.H.D. is defined as a neurodevelopmental disorder that begins in childhood and is typically characterized by inattention, disorganization, hyperactivity and impulsivity. Patients are generally categorized into three types: hyperactive and impulsive, inattentive, or a combination of the two.

The latest data suggest that about 11 percent of children ages 5 to 17 in the United States have been diagnosed with A.D.H.D. And about 4 percent of adults are estimated to have the disorder. But as recently as two decades ago, most mental health providers “didn’t really believe in adult A.D.H.D.,” Dr. Goodman said.

Now, for the most part, that’s no longer the case. And during the pandemic, stimulant prescriptions, primarily used to treat A.D.H.D., “sharply increased,” particularly among young adults and women, according to a study published in JAMA Psychiatry in January.

When diagnosing the condition, providers rely on the D.S.M.-5., the American Psychiatric Association’s official manual of mental disorders, which contains a somewhat arbitrary requirement: In order to meet the diagnostic criteria for A.D.H.D., significant symptoms, such as continual forgetfulness and talking out of turn, should be present in at least two settings before age 12.

But sometimes, older patients either do not recall childhood symptoms or say that those symptoms were mild.

Judy Sandler, 62, who lives in Lincolnville, Maine, was not diagnosed with A.D.H.D. until her mid-50s, after retiring from her job as a teacher: It was the first time in her life she felt like she couldn’t get anything done. She wanted to write, but when she would sit down to focus, she immediately had the urge to get up and do something else: “I’ll just do the laundry,” she would think. “And then go walk the dog.”

During her working years, she benefited from a “hyper-structured” schedule — up until retirement. “All of a sudden, I felt like the rug had been pulled out,” she said.

Patients like Ms. Sandler fall into a gray area. She did not recall having significant symptoms in school or at home, rather she indicated that her symptoms became most problematic later in life. Her husband of 33 years, however, had noticed symptoms for years: She was often forgetful, for example, and found it challenging to slow down.

“There’s a lot more subtlety in making this diagnosis — especially in high-functioning, bright people — than just a symptom checklist,” Dr. Goodman said.

Is the D.S.M. missing symptoms?

The D.S.M. lists nine symptoms of inattention and nine symptoms of impulsivity-hyperactivity that are used to evaluate whether an adult or a child has A.D.H.D.

The D.S.M. does not formally include symptoms related to emotional dysregulation, which is when someone has difficulty managing their mood. It also does not officially mention deficits of executive functioning, or problems with planning, organization and self-regulation. But studies have found that these are some of the most common symptoms that adults with A.D.H.D. experience, said Russell Ramsay, a psychologist who treats adult A.D.H.D.

When the D.S.M.-5 was published in 2013, there was not enough high-quality research to support the addition of these symptoms, Dr. Goodman said. But experts say they are still useful to consider when assessing someone.

Dr. Goodman is working with Dr. Ramsay and other A.D.H.D. specialists from around the world to develop the first U.S. guidelines for diagnosing and treating adults with A.D.H.D., in collaboration with the American Professional Society of A.D.H.D. and Related Disorders.

There is an urgency to do so, in part because of new research that has emerged in the last decade. In addition, while adult A.D.H.D. is often undiagnosed and untreated, some people might be getting diagnosed who don’t actually have the disorder — and given medication they don’t truly need, Dr. Goodman said.

The new guidelines, which are expected to be available for public comment later this year, will aim to create a more uniform process for diagnosing adults, but the D.S.M. will continue to be the “gold standard” for providers, Dr. Ramsay said.

“It’s not wrong,” he added. “It’s just incomplete.”

Is it A.D.H.D or something else?

For adults, a proper A.D.H.D. diagnosis typically requires several steps: an interview with the patient, a medical and developmental history, symptom questionnaires and, if possible, conversations with other people in the patient’s life, like a spouse.

“There are no shortcuts,” said Dr. Lenard A. Adler, a professor of psychiatry at the N.Y.U. Grossman School of Medicine, while speaking to hundreds of providers at the American Psychiatric Association conference in early May. “This isn’t easy.”

While everybody has some trouble paying attention or is restless from time to time, he added, it’s really how pervasive and significant the symptoms are and how consistent and impairing they’ve been throughout the patient’s life that helps doctors decide if an A.D.H.D. diagnosis is appropriate.

But several factors can make it tricky.

People who consider themselves heavy users of digital technology are more likely to report A.D.H.D. symptoms, research suggests.

There’s a “chicken or the egg” dilemma, Dr. Waits said. Are people with A.D.H.D. drawn to using digital technology more than the average person? Or did their A.D.H.D. develop because of their technology use?

People with A.D.H.D. are also likely to have another coexisting condition, like substance use disorder, depression or anxiety, which can make it challenging for both doctors and patients to understand if their symptoms are a result of A.D.H.D., particularly if the symptoms overlap.

Ms. Marsh, who had been diagnosed with depression as a teenager and took up to 10 different medications to treat it without much success, finally received an A.D.H.D. diagnosis after visiting a psychologist in her hometown. This time, the practitioner took the time to talk with her parents and her partner, and then did a fresh analysis of the test results that had been deemed inconclusive four years earlier.

After Ms. Marsh began therapy and started taking the stimulant Focalin, the difference in how she felt was “insane,” she said. Her depression improved as well.

“I could keep track of things in my brain easier,” she added. “I’ve just been able to do a lot more things because I have the motivation for it.”