There is currently no specific clinical test or procedure that can definitively
diagnose Alzheimer's disease (AD). AD is therefore considered a diagnosis
of exclusion - that is, the many other causes of dementia must be ruled
out by thorough evaluation before the clinical diagnosis of AD can be made.
In order to exclude other causes the doctor will:
- obtain a detailed history
- conduct a physical examination (including neurological examination)
- test intellectual functioning (often referred to as a cognitive assessment
or mental status exam)
- order various laboratory tests and usually some form of diagnostic neuroimaging
procedure
The reason a thorough evaluation is so important is that sometimes other treatable
causes of dementia are detected. For example, although AD can not be definitively
diagnosed on the basis of a specific laboratory test, laboratory testing may
reveal that someone's confusion is caused by a thyroid disorder.
The evaluation process for the detection of treatable dementia follows these steps:
- patient history
- physical examination
- cognitive assessment
- laboratory results
- diagnostic neuroimaging
- clinical diagnosis
None of the 4 major diagnostic neuroimaging techniques can be utilized to definitively
diagnose AD. These tests are:
- computerized tomography (CT)
- magnetic resonance imaging (MRI)
- dingle photon emission computerized tomography (SPECT)
- positron emission tomography (PET)
Although each technique can yield images consistent with AD, the greater diagnostic
value of neuroimaging currently resides in its ability to diagnose a number
of other causes of dementia that can mimic AD. For example, an MRI scan may
establish that someone's confusion is due to a blood clot.
Recently, the potential use of genetic tests to diagnose Alzheimer's disease
has emerged. This is limited to a number of extremely rare families where
virtually one out of two individuals is affected with AD. Genetic testing in
these circumstances should be confined to research centers. There is also now
evidence that testing for the various forms of a gene called APOE (apolipoprotein
E) may help support the diagnosis of AD. Like APOE, many other potential
diagnostic tests have been proposed but the role of these tests in clinical
practice is still being clarified.
Doctors sharing diagnostic impressions with patients and their families should emphasize that the clinical diagnosis of AD is a probable diagnosis rather than a definitive diagnosis. Ultimately, a definitive diagnosis of AD requires confirmation of the diagnosis by brain biopsy or autopsy.