Reading ability as an estimator of premorbid intelligence: does it remain stable in emergent dementia?

The 6-year stability of reading performance was investigated in subjects who were normal at baseline but suspect for dementia at follow-up (MMSE score < or = 23; n = 197), and in a cognitively intact control group (n = 117). The Dutch version of the National Adult Reading Test (DART) was used. The DART-based estimate of IQ appeared to be very stable in healthy elderly. In the "suspect" group, the decline after 6 years was about 3 IQ-points in subjects who were still not demented, minimally demented, or mildly demented. Reliability remained satisfactory in these subgroups. In cases with moderate and severe dementia, the decline was considerable (> or = 15 IQ points). The decline of DART IQ was related to deterioration of semantic memory as reflected in verbal abstraction and category fluency. It is concluded that the DART remains a valid estimator of premorbid verbal intelligence in mild and questionable dementia. A formula is presented which can correct the underestimation on the basis of the MMSE score.