The first step in diagnosing Alzheimer's disease is to establish that dementia is present. Then, the type of dementia should be clarified. A health care provider will take a history, do a physical exam (including a neurological exam), and perform a mental status examination. Tests may be ordered to help determine if there is a treatable condition that could be causing dementia or contributing to the confusion of AD. These conditions include thyroid disease, vitamin deficiency, brain tumor, drug and medication intoxication, chronic infection, anemia, and severe depression. AD usually has a characteristic pattern of symptoms and can be diagnosed by history and physical exam by an experienced clinician. Tests that are often done to evaluate or exclude other causes of dementia include computed tomography (CT), magnetic resonance imaging (MRI), and blood tests. In the early stages of dementia, brain image scans may be normal. In later stages, an MRI may show a decrease in the size of the cortex of the brain or of the area of the brain responsible for memory (the hippocampus). While the scans do not confirm the diagnosis of AD, they do exclude other causes of dementia (such as stroke and tumor).

Using special MRI methods, researchers have identified a pattern of regional brain atrophy in patients with mild cognitive impairment (MCI) that indicates a greater likelihood of progression to Alzheimer's disease. The findings are published in the online edition of Radiology. "Previously, this pattern has been observed only after a diagnosis of probable Alzheimer's disease," said the study's lead author, Linda K. McEvoy, Ph.D., assistant project scientist in the Department of Radiology at the University of California San Diego School of Medicine in La Jolla. "Our results show that some individuals with MCI have the atrophy pattern characteristic of mild Alzheimer's disease, and these people are at higher risk of experiencing a faster rate of brain degeneration and a faster decline to dementia than individuals with MCI who do not show that atrophy pattern." According to the Alzheimer's Association, more than five million Americans currently have Alzheimer's disease. One of the goals of modern neuroimaging is to help in early and accurate diagnosis, which can be challenging. There is no cure for Alzheimer's disease, but when it is diagnosed early, drug treatment may help improve or stabilize patient symptoms. In Alzheimer's disease, nerve cell death and tissue loss cause areas of the brain to atrophy. Structural MRI allows radiologists to visualize subtle anatomic changes in the brain that signal atrophy. MCI is associated with an increased risk of progression to Alzheimer's disease. Rates of progression vary. Some patients progress rapidly, while others remain stable for relatively long periods of time. For the study, Dr. McEvoy and colleagues set out to determine if they could identify a pattern of regional atrophy characteristic of mild Alzheimer's disease in order to aid in the prediction of cognitive decline in patients with MCI. In the study, the researchers analyzed brain MR images from 84 patients with mild Alzheimer's disease, 175 patients with MCI and 139 healthy controls, using with semi-automated, individually specific quantitative MRI methods. The results showed widespread cortical atrophy in some patients with MCI, involving all cortical areas except those involved with processing of primary motor and sensory information. However, most indicative of future cognitive decline were atrophy in parts of the medial and lateral temporal lobes and in the frontal lobes. This pattern was also present in the patients with mild Alzheimer's disease. Brain scans. A brain scan — either computed tomography (CT) or magnetic resonance imaging (MRI) — is generally included in the standard evaluation for Alzheimer's disease and other forms of dementia. CT and MRI scans, which reveal the anatomic structure of the brain, are used to rule out such problems as tumor, hemorrhage, stroke, and hydrocephalus, which can masquerade as Alzheimer's disease. These scans can also show the loss of brain mass associated with Alzheimer's disease and other dementias. In Alzheimer's disease, the region of the brain known as the hippocampus may be disproportionately atrophied. Other brain scans may be performed if CT and MRI scans are inconclusive. Positron emission tomography (PET) and single positron emission computed tomography provide images of brain activity based on blood flow, oxygen consumption, or glucose use. These techniques can help narrow down a diagnosis, but even they do not reveal the microscopic changes in brain tissue that characterize Alzheimer's disease and thus can't identify the disease with certainty. But the diagnostic capability of brain scans is improving. Especially promising is a kind of PET scan that uses a chemical tracer called Pittsburgh Compound-B (PIB). This chemical binds specifically to amyloid deposits in the brain, allowing them to show up clearly on the brain scans. The scans were detailed enough to identify people with early Alzheimer's disease in a 2004 pilot study in Annals of Neurology . Experts expect PET scans with PIB to be in general use within the next few years. Researchers also hope to perfect MRI techniques that can enhance physicians' ability to measure brain atrophy and diagnose Alzheimer's with greater accuracy.

nzguy is correct....I would only add that you can never truly confirm a diagnosis of Alzheimer's till post mortem, unfortunately. To some degree, it is a diagnosis of exclusion.