Psychometric Evaluation of the Cognitive State Test (COST) in a Sample of Iranian Elderly People

An absolute prerequisite to the effective management of dementia is its early diagnosis. Successful dementia screening requires precise and sensitive instruments that can be completed even by illiterate elderly people. The aim of this study was to evaluate the psychometric properties of the Persian version of the cognitive state test (COST). This methodologic study was conducted in Kashan, Iran, during 2013 - 2014. A purposeful sample of 150 healthy elderly people and 50 elderly patients with dementia was recruited. After translating the instrument by using the standard forward-backward technique, we assessed its qualitative and quantitative face and content validity. The validity of the test was assessed by using the concurrent validity and the exploratory factor analysis. We also calculated Cronbach’s alpha and employed the test-retest method for evaluating the internal consistency and the stability of the test, respectively. Study data were analyzed by using the SPSS v16.0, the Spearman-Brown, and the intraclass correlation coefficient tests and the principal components factor analysis with varimax rotation. The Persian COST consists of nineteen items. The impact scores, the content validity ratios and the content validity indices of all test items were greater than 4.5, 0.69, and 0.84, respectively. The COST had a significant correlation with the clinical dementia rating (rS = -0.76, P value < 0.001), indicating an acceptable concurrent validity for the test. The exploratory factor analysis revealed a five-factor structure that explained 60.59% of the total variance of the total cognitive state score. The Cronbach’s alpha, Spearman-Brown, and interclass correlation coefficients were 0.82, 0.95, and 0.88, respectively (P value < 0.001). The Persian version of the COST can be used as a valid and reliable instrument for assessing cognitive state and screening dementia in literate and illiterate elderly people.