Construct validity of instruments measuring impairments in body structures and function in rheumatic disorders: which constructs are selected for validation? A systematic review.

PURPOSE This paper focuses on the construct validity of instruments measuring impairments in body structures and function in rheumatic disorders. The objective is: 1) to make an inventory of constructs, based on the domains of the International Classification of Functioning, Disabilities and Health problems (ICF), against which instruments measuring impairments in body structures and function were validated; 2) to analyse whether validation against a similar construct resulted in higher correlation coefficients than validation against a dissimilar construct. METHODS In a systematic review papers were identified in which instruments measuring impairments in body structures and function for patients with rheumatic disorders were validated. The instruments identified were assessed on their methodological properties and the constructs against which they were validated. Subsequently, pooled (interclass) correlations of similar constructs and dissimilar constructs against which was validated were compared. An instrument was decided to have good construct validity, if the correlation coefficient was 0.50 or higher, and the measurement instrument in question is validated against similar constructs. RESULTS In total 216 papers were identified analysing the validity of 42 different instruments. Only 16% of these instruments were validated against instruments that represent the most similar construct. In general, estimates of construct validity were lower when validated against dissimilar constructs, except for instruments measuring impairments in mental functions. CONCLUSION There is a trend that validation against a similar construct yields higher correlation coefficients than validation against a dissimilar construct. If an instrument measuring impairments is validated against the most similar construct, and a criterion of r > 0.50 is applied, only 10 out of the 42 identified instruments turned out to be valid.