Estimation of premorbid IQ in individuals with Alzheimer’s disease using Japanese ideographic script (Kanji) compound words: Japanese version of National Adult Reading Test

Abstract  The National Adult Reading Test (NART) is widely used as a measure of premorbid IQ of the English‐speaking patients with dementia. The purpose of the present study was to develop a Japanese version of the NART (JART), using 50 Japanese irregular words, all of which are Kanji (ideographic script) compound words. Reading performance based on JART and IQ as measured by the Wechsler Adult Intelligence Scale–Revised (WAIS‐R) was examined in a sample of 100 normal elderly (NE) persons and in 70 age‐, sex‐, and education‐matched patients with Alzheimer’s disease (AD). The NE group was randomly divided into the NE calculation group (n = 50) and the NE validation group (n = 50). Using the NE calculation group, a linear regression equation was obtained in which the observed full‐scale IQ (FSIQ) was regressed on the reading errors of the JART. When the regressed equation computed from the NE calculation group was applied to the NE validation group, the predicted FSIQ adequately fit the observed FSIQ (R2 = 0.78). Further, independent t‐tests showed that the JART‐predicted IQs were not significantly different between the NE and AD groups, whereas the AD group performed worse in the observed IQs. The reading ability of Kanji compound words is well‐preserved in Japanese patients with AD. The JART is a valid scale for evaluating premorbid IQ in patients with AD.