In order to make an accurate diagnosis you must:

rule out other conditions that may have similar symptoms as dementia but may be treatable e.g. hypothyroidism, depression, vitamin deficiencies etc.
rule out other possible causes of confusion such as poor vision or hearing, side effects of drugs etc.
An accurate diagnosis of dementia is important in order to:
implement an appropriate treatment plan
access information and support from the heath care system and the community support groups
assist the patient and family to plan and make arrangements for the future
Dementia is commonly under-diagnosed in primary health care settings because of the multiple etiologies and the wide range of symptoms at the time of presentation [1].
short term memory impairment (amnesia)
and at least one of:
language disturbance (aphasia)
impaired ability to carry out motor activities or ADLs (apraxia)
failure to recognize/identify familiar objects (agnosia)
poor executive functioning (planning, organizing, abstracting, disinhibition and inappropriate behaviour) [2]
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1. Dugue M, Neugroschl J, Sewell M, Marin D. Review of dementia. Mount Sinai Journal of Medicine 2003; 70:45-53.
2. American Psychiatric Association. Diagnostic and statistical manual of mental disorders (DSM IV) 4th edition. Washington: The Association; 1994.