There is no definitive diagnostic test for Alzheimer's disease and diagnosis can often be delayed because the early symptoms (e.g., forgetfulness, mild confusion) may be similar to some of the signs of normal aging. It may be especially difficult to obtain a diagnosis if the patient is under 65 and experiencing early-onset Alzheimer's disease because early symptoms may be similar to those experienced by people under extreme stress or people who are depressed. An important aspect for physicians is to assess whether the symptoms the patient is experiencing could be due to another cause, such as vitamin deficiency, dehydration or a side effect of medication the patient is taking. Patients may be referred to a neurologist, psychiatrist or psychologist following the presentation of symptoms.

If a physician suspects Alzheimer's disease, diagnosis will begin with a physical examination and a medical history. Patients or their caregivers are encouraged to keep a log of symptoms to report to the physician. A physician may also ask family members or close friends about the nature of the patient’s symptoms. During the medical history, the physician may ask questions related to the patient’s dietary habits and use of alcohol and/or drugs in order to rule out other potential causes of dementia. During the physical examination, the physician may assess neurological function by testing the patient’s reflexes, balance and coordination.

A physician may also use an electroencephalogram (EEG). An EEG is a test in which electrodes are placed on a patient’s head and information about the brain’s electrical activity is recorded as a series of brain waves. This type of test can be used to rule out other causes of dementia, such as infections or metabolic problems.

A physician may conduct a mental status examination to assess the stage of dementia that a patient may have reached. During the exam, the physician may ask the patient a variety of questions aimed at testing the patient’s awareness of surroundings, problem solving skills and memory skills. Examples of these questions may include:

• Situational questions such as “What year is it?” or “What is the address of this office?”
  
  
• Remembering and recalling a short list of items (e.g., a ball, a pencil, a dog)
  
  
• Counting backward or spelling a word backward
  
  
• Naming familiar objects in the room as the physician points to them
  
  
• Following simple instructions or writing a simple sentence

This test can also be used after diagnosis to evaluate the progression of Alzheimer's disease in the patient.

In diagnosing Alzheimer's disease, a physician may also recommend imaging tests, such as magnetic resonance imaging (MRI), computed tomography (CT) and positron emission tomography (PET) scans. Imaging tests can be used to identify other potential causes of dementia such as tumors, evidence of strokes or damage from head trauma. Imaging tests can also be used to measure the brain, which shrinks over time in people with Alzheimer's disease as parts of the brain atrophy. However, imaging tests cannot identify the microscopic “plaques” and “tangles” in the brain characteristic of Alzheimer's disease because of their small size.

In some cases, blood tests, urine tests and spinal taps may be performed to rule out other conditions that may have symptoms similar to Alzheimer’s disease.