There is no single indicator that can be tested for which would confirm a diagnosis of Alzheimer's Disease. The progression of is characterized by a series of gradual changes which some people wrongly think is just part of growing old. Many of its symptoms are similar to other dementing illnesses. It is not uncommon for patients to have a combination of dementing illnesses. This makes Alzheimer's Disease difficult to diagnose.

Experts often observe that the only completely certain diagnosis is through post-mortem examination when the brain tissue can be examined under a microscope for the presence of the neurofibrillary tangles and neuritic plaques that cause the brain damage associated with this particular disease. A confirmed diagnosis of this sort after the patient is dead may have some scientific value but is of little use to the patient or to their family and friends. So it is important to realise that doctors can in fact accurately diagnose 90 percent of dementia cases.

In the absence of a single test, doctors usually diagnose Alzheimer's Disease based on combination of information gathered from both the individual and family members and a series of tests. Doctors attempt to identify other causes of thinking and behavior change before they conclude it is Alzheimer's Disease or another dementia.

The process of diagnosis will involve

• Medical history - Both the individual and family members or friends will be asked questions regarding the person's symptoms now and in the past. There will be questions about past illnesses and about family medical and psychiatric history.
• Mental status examination - The individual will be asked a series of questions to determine sense of time and place as well as the ability to remember, express him/herself and do simple calculations. It may involve exercises such as recalling words and objects, drawing and spelling, and questions such as about past events and/or important people.
• Physical examination - To help rule out other causes, a physical examination will be done. The doctor will look for heart, lung, liver, kidney or thyroid problems that may be causing the symptoms. To evaluate whether other nervous system disorders are causing the symptoms, the doctor will test muscle tone and strength, co-ordination, eye movement, speech and sensation.
• Blood tests - A number of blood tests will be done to help detect problems such as anaemia, diabetes, thyroid problems or infection that might be causing the symptoms.
• Neuroimaging - Others tests such as X-rays and EEG's (electroencephalogram) may be used to determine the source of the problem. In some cases, specialised scans may be used to rule out strokes or brain tumors, which potentially could account for the changes in memory and behavior. These may include
  • CT scan (computerized tomography) and MRI (magnetic resonance imaging), both of which take images of the brain;
  • SPECT (single proton emission computed tomography), which shows how blood is circulating to the brain;
  • PET (positive electron tomography) and and magnetic resonance spectroscopy imaging (MRSI), which allow doctors to see interactions of brain chemicals, such as neurotransmitters and their receptors, at the molecular level. Tests may show how the different areas of the brain respond during certain activities such as reading and talking.
   

  Some of these tests may be recommended, but are not always necessary for a diagnosis. Those interested in more detail about the use of this sort of testing should visit the Harvard Brain Atlas, where samples are shown of scans and how the doctors interpret them.

• Psychiatric and psychological evaluations - A psychiatric evaluation may be helpful in ruling out other illnesses such as depression which can cause memory loss similar to Alzheimer's Disease. Neuro-psychological testing can evaluate memory, reasoning, writing etc.