U.S. President Joe Biden said Friday that his poor performance in last week鈥檚 debate was a bad night but not evidence of a serious condition.

鈥淚 have medical doctors travelling everywhere. Every president does, as you know. Medical doctors from the best of the world travel with me everywhere I go,鈥 Biden told ABC News鈥 George Stephanopoulos. 鈥淚 have an ongoing assessment of what I鈥檓 doing. They don鈥檛 hesitate to tell me if they think there鈥檚 something else wrong.鈥

Biden said he spoke with his doctor after the debate and was told, 鈥測ou鈥檙e exhausted.鈥 Being president, he said, means 鈥淚 get a full neurological test every day.鈥

But doctors who work with aging patients say there鈥檚 no single moment or test that determines a person鈥檚 mental fitness or ability to function. In-depth evaluations can provide more answers, including ruling out potential problems. Such tests might start with short questionnaires, then expand to physical exams, blood tests, brain imaging, lengthy interviews with the individual and conversations with family members and friends.

In an analysis published Friday, CNN Chief Medical Correspondent Dr. Sanjay Gupta urged the president to undergo detailed cognitive and neurological tests and to make those results public. Such testing 鈥渃an help determine whether there is a simpler explanation for the symptoms displayed or if there is something more concerning,鈥 Gupta wrote.

When asked on Friday whether he鈥檚 had cognitive tests and an exam by a neurologist, Biden said no.

鈥淣o one said I had to. 鈥 They said I鈥檓 good,鈥 the president said.

Biden gave no indication he would be willing to have such tests conducted in the aftermath of his debate performance.

Effects of typical aging

Most people who are aging normally will experience some cognitive changes, but when and how much varies from person to person.

Slow, age-related cognitive decline starts around age 27, studies show, even for healthy and well-educated people. A person鈥檚 reasoning and problem-solving skills, along with their ability to maintain the thread of a conversation, can get tougher with age, but not so much that it impairs what doctors consider normal functioning.

鈥淓very aging researcher will tell you, though, that not everything declines as you get older, and not everything declines at an equal rate,鈥 said Dr. Charan Ranganath, a professor of psychology and neuroscience and the director of the Dynamic Memory Lab at the University of California, Davis.

For example, the ability to remember facts 鈥 an essential skill for a president 鈥 doesn鈥檛 disappear with age, he said.

鈥淚t鈥檚 just that you鈥檙e slower to pull it up, and you鈥檙e more likely to struggle to pull up the facts when you need it,鈥 Ranganath said.

And it鈥檚 not just memory that changes with age, said Dr. John Rowe, a professor of health policy and aging at Columbia University.

鈥淚t鈥檚 reasoning and sociability and attention and orientation, and emotional stability and judgment and so on,鈥 he said. 鈥淪ome get impaired with age, and some actually get better with age, such as vocabulary.鈥

A person鈥檚 moral reasoning and judgment or their capacity for compassion and empathy will not necessarily decline, Ranganath added. Unless there is some kind of cognitive impairment, those aspects of personality can improve in some people with normal aging.

鈥淪o, again, pretty important for a job like president,鈥 Ranganath added.

Cognitive impairment 鈥 a loss of memory and other mental abilities that affect daily life 鈥 is different and is not considered normal or inevitable with aging. It occurs in 12 per cent to 18 per cent of people older than 65 and is more prevalent with advancing age, Rowe said.

By age 80, more than 25 per cent of people may have some mild cognitive impairment. They may start to lose things and miss appointments, 鈥渙ccurrences that could be characterized as impairing your function,鈥 Rowe said.

A minority in this group will go on to develop dementia each year, he said.

And some may adapt, maybe seeming to have improved function. 鈥淭hey always put the key in the same place. They write down all their appointments,鈥 Rowe said.

Screening for cognitive impairment

To screen for cognitive impairment, a person鈥檚 primary care physician will often start with the Mini Mental State Exam, known as the MMSE, or the Montreal Cognitive Assessment, also called the MoCA.

The MMSE is a five-to-10-minute 11-question assessment that tests five areas of cognitive function: orientation, registration, attention and calculation, recall and language.

Former President Donald Trump, who has faced questions about his cognitive function, has repeatedly said he 鈥渁ced鈥 the MoCA 鈥 a 30-point test that takes about 10 minutes. It evaluates the ability to concentrate and tests executive function, memory, language, conceptual thinking and orientation, the ability to do calculations and the ability to organize and manipulate spatial information to create a design.

Both ask for simple information, like what the date is and where the participant is, and ask to count backward from 100 using a particular number. Both tests ask a person to draw. The MMSE asks the participant to identify particular objects in the room and to take a piece of paper in their right hand, fold it in half and put it on the floor. For the MoCA, the person will be told five words and then asked to repeat them in five minutes.

The tests are not in-depth. Ranganath describes the MMSE as a 鈥渂lunt instrument鈥: Unless someone has established language problems, he said, 鈥渂asically, you have to be seriously cognitively impaired鈥 to do poorly, 鈥渆specially for somebody who is functioning at a high level, like a high-level politician.鈥

The MoCA, he said, is good at detecting when someone has mild cognitive impairment. It鈥檚 a test for 鈥渨hen people can really tell there鈥檚 a memory problem, and that鈥檚 often when they鈥檒l be talking to a doctor about it,鈥 Ranganath said 鈥 and perhaps seeking more detailed results.

More detailed cognitive tests

If a doctor or family member suspects more serious cognitive problems, a person may be referred to specialists who can do further tests that may take one or two days to complete.

Usually, the person will be referred to a behavioral neurologist who specializes in aging and dementia and non-dementia diagnosis, as well as a neuropsychologist and possibly a social worker who will meet with them and their family, according to Dr. Emily Rogalski, a professor of neurology who runs the University of Chicago鈥檚 Health Aging & Alzheimer鈥檚 Research Care Center.

鈥淧art of those evaluations will look at the overall health of the individual, from neurologic health to also cognitive health,鈥 she said.

Cognitive health includes how a person can learn, think and remember. Neurological health involves the nervous system. Neurological disorders like Parkinson鈥檚 or Alzheimer鈥檚, or even a headache, can affect cognitive functions. But other conditions can have an impact too, such as an infection or tumour.

鈥淭he neuropsychologist is able to do an in-depth evaluation of the strengths and challenges that an individual may be experiencing and start to understand the interactions between those strengths and challenges,鈥 Rogalski said.

Testing could also include blood tests or imaging that may tell doctors what鈥檚 going on in the brain itself.

There will also be a physical examination. If a person has a history of congestive heart failure, a doctor may do a cardiac exam to check for deterioration that could contribute to cognitive issues, said Dr. Brad Dickerson, a professor of neurology at Harvard Medical School and an associate neurologist who sees patients at Massachusetts General Hospital.

鈥淵ou have to take all that into consideration when you鈥檙e trying to figure out why their cognitive functioning may be different than it was,鈥 he said.

At this level of testing, part of the evaluation will also include a report from people close to the patient.

鈥淭hat sometimes is a spouse or a close family member who can comment on whether the individual is experiencing significant change from their previous level of function and whether that change is interfering with what we call activities of daily living,鈥 Rogalski said.

Who should get tested 鈥 and when

Dickerson said patients will often ask him when they should get tested. His answer: It depends.

If, for example, an 80-year-old tells him that they keep forgetting the name of an actor in a movie they鈥檝e just seen or that they go into another part of the house to get three things and remember only two, and if the problem has gotten more annoying but over time isn鈥檛 too disruptive, they could get a baseline cognitive and neurological assessment in the office. He said he would not be worried that this was a sign of a progressive disorder that needs to be diagnosed and treated.

鈥淭here are elements of the history that would raise a doctor鈥檚 concerns about a significant problem or would reduce the doctor鈥檚 concerns about a significant problem,鈥 Dickerson said. 鈥淪ometimes, you can鈥檛 be sure of that until you sit down in the office and interview people.鈥

Rowe said he tells concerned family members and patients that the time to get tested is when they start asking whether they should.

鈥淭here鈥檚 no harm in getting tested,鈥 Rowe said. 鈥淚t鈥檚 always good to have a baseline so that you can get tested again in a year or two if you think things are getting worse.鈥

Testing, he said, should be able to tell them if they have issues that impair their function. But people shouldn鈥檛 worry if they can鈥檛 remember someone鈥檚 name.

Rowe, who served as chairman and CEO of the insurance company Aetna and was president and CEO of Mount Sinai NYU Health, believes that testing should be the default for people in several roles: presidential candidates, regardless of age; US Supreme Court justices; airline pilots.

He says the public shouldn鈥檛 get 鈥渃arried away鈥 with scores: If one candidate鈥檚 score is higher than the others, that doesn鈥檛 mean that one is more qualified to be president, he said.

鈥淏ut if somebody fails a test,鈥 Rowe said, 鈥淚 think that it鈥檚 something that people should know.鈥

Testing doesn鈥檛 always lead to a dementia diagnosis. Addressing other health problems, such as a sleep disorder or medications interacting with each other, may lead to improvement. Patients may see symptoms reduced through lifestyle changes. If someone is found to have early-stage Alzheimer鈥檚, new treatments can slow the progression of disease.

Distinguishing between cognitive impairment and typical age-related cognitive decline 鈥渁re not trivial questions,鈥 and it鈥檚 not something a family doctor can typically do, Ranganath said. Even in-depth testing may not give a clear indication of what a person is capable of now or in the near future.

鈥淭here is also a bit of an art in figuring out how to translate someone鈥檚 test results into a statement about real-world functioning (i.e., whether a person is fit to drive, live independently, do a demanding job, etc.),鈥 Ranganath wrote in an email. 鈥淢any times, the recommendation is obvious, but on occasion, it can be hard to tell, and there are no clear-cut answers. And if someone seems relatively competent at the moment, that doesn鈥檛 necessarily mean that they will be doing so well two to four years later.鈥

CNN鈥檚 Nadia Kounang and John Bonifield contributed to this report.