The effect of different diagnostic criteria on the prevalence of dementia.

There are several widely used sets of criteria for the diagnosis of dementia, but little is known about their degree of agreement and their effects on estimates of the prevalence of dementia. We examined 1879 men and women 65 years of age or older who were enrolled in the Canadian Study of Health and Aging and calculated the proportion given a diagnosis of dementia according to six commonly used classification systems: the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM), third edition (DSM-III), the third edition, revised of the DSM (DSM-III-R), the fourth edition of the DSM (DSM-IV), the World Health Organization's International Classification of Diseases (ICD), 9th revision (ICD-9) and 10th revision (ICD-10), and the Cambridge Examination for Mental Disorders of the Elderly (CAMDEX). The degree of concordance among classification schemes and the importance of various factors in determining diagnostic agreement or disagreement were examined. The proportion of subjects with dementia varied from 3.1 percent when we used the criteria of the ICD-10 to 29.1 percent when the DSM-III criteria were used. The six classification systems identified different groups of subjects as having dementia; only 20 subjects were given a diagnosis of dementia according to all six systems. The classifications based on the various systems differed little according to the patients' age, sex, educational level, or status with respect to institutionalization. The factors that most often caused disagreement in diagnosis between DSM-III and ICD-10 were long-term memory, executive function, social activities, and duration of symptoms. The commonly used criteria for diagnosis can differ by a factor of 10 in the number of subjects classified as having dementia. Such disagreement has serious implications for research and treatment, as well as for the right of many older persons to drive, make a will, and handle financial affairs.