Background: The Cognitive Abilities Screening Instrument (CASI) consists of items which are designed to measure the nine domains of cognitive ability. Its total score is used clinically to represent the overall underlying cognitive ability of the patient. This study aimed to evaluate the justification of the uses of the all-domain-total-score as an overall cognitive measure and the domain scores as measures of designated individual cognitive ability.

Methods: To justify the use of total score of all the items as an overall measure of cognitive ability, a second-order confirmatory factor analysis was performed to examine whether the items in CASI contributed significantly to a common underlying construct. The uses of domain scores were also examined by inspecting the loadings of the items on their designated domains. The CASI data from 608 patients, 68 normal and 540 with Alzheimer's disease, were analyzed.

Results The goodness-of-fit indices for the second-order factor model were as follows: CFI was 0.912 for WLSMV and 0.977 for WLSM; TLI and RMSEA values were 0.975 and 0.090 respectively. The loadings of the items on the common underlying construct are all salient (>0.3). The loadings of all but the long-term memory items on their respective subdomains were also salient.

Conclusions: The items of the CASI C-2.0 were useful not only in profiling the correlated cognitive domain scores, but also in forming an overall measure of the underlying cognitive ability of the patients.