How can I find out why a person gets dementia? In this answer user287279 wrote that dementia may have different causes.

How can I find out what causes dementia of someone? For example, what tests (like blood or urine tests) can they take to confirm or refute hypotheses why they are getting dementia?
As there are several diseases and conditions that can mimic dementia and as there are many possible causes of dementia, in order to avoid unnecessary investigations, the diagnosis of dementia and of the cause have to be done methodologically. The best way is to meet a neurologist to do this.

First to diagnose whether someone has dementia, a clinical history of his/her symptoms, especially those related to memory, speech, cognitive functions, behavior, and functional performances, and a mental state exam, such as MMSE and MMSE in elderly perople, have to be taken. This step will tell whether or not the person has dementia or whether the person has other conditions such as benign forgetfulness, mild cognitive impairment, or depression.

Next, if the person has dementia, the cause of dementia can be diagnosed by the following steps.


Clinical history and physical examination. In some overt cases, this step alone can diagnose the cause of dementia, such as clinical pictures of brain tumor, chronic meningitis, and CJD.
Imaging, such as CT scan, MRI scan, and PET scan. This can diagnose some diseases right away, such as brain tumor, normal pressure hydrocephalus, and Alzheimers's disease.
Laboratory tests, such as thyroid function tests, blood calcium level, and vitamin B12 level. These tests can diagnose metabolic causes of dementia.
Other tests, such as lumbar puncture to examine the cerebrospinal fluid, Electroencephalography, and brain biopsy. This may have to be done in some cases. They can diagnose some difficult diseases such as neurosyphilis, CNS lymphoma, cerebral vasculitis, and subacute sclerosing panencephalitis.


Which investigations in 2.- 4. are to be done first depend on the clinical picture in 1. and subsequent investigations may be needed later.