AD is classified, as with all other disorders, by DSM-IV and by ICD-10. In addition, it also has a specialized classification system resulting from the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association ( NINCDS-ADRDA).(25) This clinical diagnostic system is internationally accepted and widely observed. There are other classification systems for neuropathological diagnosis, most notably the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) criteria.(26)

DSM-IV stipulates that a dementia syndrome is characterized by a decline in multiple cognitive deficits, including amnesia, resulting in impairment. A gradual onset and decline in the absence of other conditions sufficient to cause dementia indicates AD. ICD-10 shares with DSM-IV the definition of a dementia syndrome as a deterioration in more than one area of cognition, but including memory that is sufficient to impair function. Again an emphasis on insidious onset and slow decline in the absence of other disorders sufficient to cause dementia indicates AD. The NINCDS-ADRDA criteria defines possible, probable, and definite categories; the latter being restricted to neuropathological confirmation of a clinical diagnosis. (25) It is important to note that both clinical and neuropathological data are required—no single neuropathological lesion is pathognomonic of AD, and it is still uncertain how often or to what extent the neuropathological lesions of AD also occur in normal ageing. Probable AD, according to NINCDS-ADRDA, requires a dementia with progressive decline in memory and other cognitive areas, cognitive impairment established by formal testing, no disturbance of consciousness, and absence of other disorders sufficient to cause dementia. Supporting features include decline in function, change in behaviour, positive family history, and decline in specific cognitive areas including aphasia, apraxia, and agnosia. Non-specific change on electroencephalography (EEG) and progressive changes on CT are supporting, but not necessary, features. Possible AD should be diagnosed if there are variations in the clinical presentation, another disorder sufficient to cause a dementia (even if it is not thought to do so in this case), or a restricted cognitive decline.

A number of studies have attempted to determine the accuracy of diagnostic criteria against postmortem diagnosis. One of the difficulties in these studies is that because AD is the most common dementia (by some way), such studies are very likely to find a high-positive predicative value. Kukull et al.(27) found the specificity of DSM-III to be higher than NINCDS-ADRDA (0.8 versus 0.65), but NINCDS-ADRDA had a higher sensitivity (0.92 versus 0.76); others find an even lower specificity.(28)

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