The most likely underlying cause of this patient's symptoms is mild cognitive impairment (MCI). MCI is a cognitive state between normal aging and dementia characterized by a decline in cognitive functioning that is greater than what is expected with normal aging but has not resulted in significant functional disability. For most patients, the onset is insidious, and for some, the course may be progressive; 10% to 15% of patients with MCI transition to dementia per year, compared with 1% to 2% per year of the general population. The Montreal Cognitive Assessment is a screening tool that is more sensitive than the Mini–Mental State Examination in the detection of MCI because it has more cognitively challenging tests of memory/recall and executive function. A score lower than 26/30 generally suggests cognitive impairment, especially in patients with 16 years of formal education. In clinical practice, a careful history and results of a standard mental examination are often sufficient to make a diagnosis of MCI, and extensive cognitive testing is not routinely required. Occasionally, a formal battery of neuropsychological testing beyond the standard mental examination is needed to distinguish particularly mild cases of cognitive impairment from normal aging.

In order to meet criteria for dementia, a patient's cognitive deficits must interfere with daily functioning and result in some loss of independence. A detailed history of the patient's abilities to perform activities of daily living, such as paying bills, managing financial records, assembling tax records, shopping alone, working on hobbies, taking medications, driving, and remembering recent holidays or family events, should be obtained to elicit any change in function. This patient does not meet the criteria for dementia.

The diagnosis of clinical depression is based on patient history and exclusion of alternative diagnoses; no additional tests can confirm the diagnosis. The evaluation must establish whether the patient meets established criteria for major depression, dysthymia, or a different psychiatric condition and also assess for substance abuse. Depressed mood and anhedonia are cardinal symptoms, and the presence of either is highly sensitive but not specific for major depression. Using a two-item questionnaire that assesses for the presence of depressed mood or anhedonia is a quick way to screen for depression. If either depressed mood or anhedonia is present, further inquiry or employing a second tool to diagnose depression should be pursued. This patient, who describes her mood as upbeat and says she enjoys her life, has neither depressed mood nor anhedonia. Therefore, depression is unlikely to be the cause of her symptoms.

Patients with memory problems due to normal aging have symptoms, most notably memory loss, that are commonly associated with cognitive impairment, but cognitive testing shows functioning within the normal range. This patient's memory difficulties are greater than what is expected with normal aging, and her score on the Montreal Cognitive Assessment is not in the normal range.