There are some brief tests (5–15 minutes) that have reasonable reliability to screen for dementia.

While many tests have been studied, presently the mini mental state examination (MMSE) is the best studied and most commonly used. The MMSE is a useful tool for helping to diagnose dementia if the results are interpreted along with an assessment of a person's personality, their ability to perform activities of daily living, and their behaviour. Other cognitive tests include the abbreviated mental test score (AMTS), the, "Modified Mini-Mental State Examination" (3MS), the "Cognitive Abilities Screening Instrument" (CASI), the Trail-making test, and the clock drawing test. The MOCA (Montreal Cognitive Assessment) is a very reliable screening test and is available online for free in 35 different languages. The MOCA has also been shown somewhat better at detecting mild cognitive impairment than the MMSE.

Another approach to screening for dementia is to ask an informant (relative or other supporter) to fill out a questionnaire about the person's everyday cognitive functioning. Informant questionnaires provide complementary information to brief cognitive tests. Probably the best known questionnaire of this sort is the "Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE)". There is not sufficient evidence to determine how accurate the IQCODE is for diagnosing or predicting dementia. The Alzheimer's Disease Caregiver Questionnaire is another tool. It is about 90% accurate for Alzheimer's and can be completed online or in the office by a caregiver. On the other hand, the "General Practitioner Assessment Of Cognition" combines both, a patient assessment and an informant interview. It was specifically designed for the use in the primary care setting.

Clinical neuropsychologists provide diagnostic consultation following administration of a full battery of cognitive testing, often lasting several hours, to determine functional patterns of decline associated with varying types of dementia. Tests of memory, executive function, processing speed, attention, and language skills are relevant, as well as tests of emotional and psychological adjustment. These tests assist with ruling out other etiologies and determining relative cognitive decline over time or from estimates of prior cognitive abilities.

Routine blood tests are also usually performed to rule out treatable causes. These tests include vitamin B, folic acid, thyroid-stimulating hormone (TSH), C-reactive protein, full blood count, electrolytes, calcium, renal function, and liver enzymes. Abnormalities may suggest vitamin deficiency, infection, or other problems that commonly cause confusion or disorientation in the elderly.

Since the term of "reversible" has been used, it simply implies a high possibility of recovery from the disease. Cunha (1990) examined the recovery process of 26 patients with reversible dementia. Unfortunately, only 2 patients have found as return to normal function indicated by the MMSE scores. Poor results have also been reported in Copeland et al. (1992)'s studies, as 1 out of 21 DD patients had fully recovered. Thus, attention should be arisen that the diagnosis of reversible or pseudodementia needs to be given with extreme care, and the recovery pattern for individual patient remains uncertain.

Pseudodementia is a phenotype approximated by a wide variety of underlying disorders (1). Data indicate that some of the disorders that can convert to a pseudodementia-like presentation include depression (mood), schizophrenia, mania, dissociative disorders, Ganser syndrome, conversion reaction, and psychoactive drugs (2). Although the frequency distribution of disorders presenting as pseudodementia remains unclear, what is clear is that depressive pseudodementia, synonymously referred to as depressive dementia(3) or major depression with depressive dementia (4), represents a major subclass of the overarching category of pseudodementia (4).

It has long been observed that in the differential diagnosis between dementia and pseudodementia, depressive pseudodementia appears to be the single most difficult disorder to distinguish from nosologically established "organic" categories of dementia(5), especially degenerative dementia of the Alzheimer type (6).

Depressive Pseudodementia is a syndrome seen in older people in which they exhibit symptoms consistent with dementia but the cause is actually depression.

Older people with predominant cognitive symptoms such as loss of memory, and vagueness, as well as prominent slowing of movement and reduced or slowed speech, were sometimes misdiagnosed as having dementia when further investigation showed they were suffering from a major depressive episode. This was an important distinction as the former was untreatable and progressive and the latter treatable with antidepressant therapy or electroconvulsive therapy or both. In contrast to major depression, dementia is a progressive neurodegenerative syndrome involving a pervasive impairment of higher cortical functions resulting from widespread brain pathology.

As certain of pseudodementia remains potentially treatable, it is essential that they are distinguished from primarily dementia of the Alzheimer's type (DAT), and multi-infarct dementia (MID). For instance, pseudodementia associated with depression (DD) has been found as the most frequently appearing, while as many as 10% to 20% patients are misdiagnosed as primary degenerative dementia (PDD) or vice versa. A significant overlapping in cognitive and neuropsychological dysfunction in DD and PDD patients seemed to increase the difficulty in diagnosis. However, differences in the severity of impairment and quality of patients' responses could be observed, and DD patients exhibited a greater depressive symptomatology. Additionally, a test of antisaccadic movements may be used to differentiate DD from PDD patients. as PDD patients significantly display poorer performance on this test. A general comparison between aspects of DD and PDD is shown below.

In general, pseudodementia patients present a considerable cognitive deficits, including disorders in learning, memory and psychomotor performance. Substantial evidences from brain imaging such as CT scanning and positron emission tomography (PET) have also revealed abnormalities in brain structure and function.

The history of disturbance in pseudodementia is often short and abrupt onset, while dementia is more often insidious. Clinically, people with pseudodementia differ from those with true dementia when their memory is tested. They will often answer that they don't know the answer to a question, and their attention and concentration are often intact, and they may appear upset or distressed. Those with true dementia will often give wrong answers, have poor attention and concentration, and appear indifferent or unconcerned.

Investigations such as SPECT imaging of the brain show reduced blood flow in areas of the brain in people with Alzheimer's disease, compared with a more normal blood flow in those with pseudodementia.

Hospitalization may be necessary during the acute phase of symptoms, and psychiatric care if the patient is a danger to self or others. A neurological consult is advised to rule out any organic cause.

According to the DSM-IV-TR, which classifies Ganser syndrome as a dissociative disorder, it is "the giving of approximate answers to questions (e.g. '2 plus 2 equals 5' when not associated with dissociative amnesia or dissociative fugue)."

Diagnosing Ganser syndrome is very challenging, because some measure of dishonesty is involved and because it is very rare.

Usually when giving wrong answers, they are only slightly off, showing that the individual understood the question. For instance, when asked how many legs a horse has they might say, "five." Although subjects appear confused in their answers, in other respects they appear to understand their surroundings.