There are no biological markers to identify AD, therefore a presumptive clinical diagnosis is made. All criteria for AD share two basic requirements: (1) gradual, progressive onset of global memory and intellectual dysfunction in an alert adult, and (2) other causes for progressive cognitive decline are ruled out.

These criteria are intended to serve as guidelines for the diagnosis. Ultimately, it is clinical judgment that makes the determination. There are no criteria which specify how the earliest stages of the disease are to be identified. It is relatively easy to recognize moderate or severe disease, particularly when the disease follows a typical course and if other diseases are appropriately ruled out. It is very difficult to make the diagnosis in the early stage of the disease, since memory complaints or other age-associated memory deficits are common.

Neuropsychological tests also have limitations. For example, these tests may not be able to distinguish between people of low intelligence without dementia and people with high intelligence who score in the normal range but have mild dementia. Sociocultural and linguistic confounders also affect test performance.

Since the diagnosis of dementia requires a decline from a prior level of function, a knowledgeable source, such as a spouse or child, is needed to enable the clinician to establish the diagnosis.