Structural Validity of the Boston Carpal Tunnel Questionnaire and its Short Version the 6-Item CTS Symptoms Scale: A Rasch Analysis One Year After Surgery
Abstract Background: The Boston Carpal Tunnel Questionnaire (BCTQ) and its shorter version Six-Item Carpal Tunnel Symptoms Scale (CTS-6) are widely used for assessing function and/or symptoms in patients with carpal tunnel syndrome. The present study examined the structural validity of the BCTQ and CTS-6 among patients who had undergone surgery for treatment of carpal tunnel syndrome. Methods: The data was obtained from 217 adult patients who had undergone carpal tunnel release one year earlier. The Rasch Measurement Theory was applied to investigate the unidimensionality, residual correlation, differential item functioning, scale coverage/targeting, and person separation of the CTS-6, Symptom Severity Scale (SSS) and Functional Status Scale (FSS) of the BCTQ.Results: The FSS of the BCTQ showed good scale and item fit and unidimensionality. FSS items had ordered response categories. Eight items in the FSS displayed differential item functioning favoring age or gender. The multidimensional structure of the CTS-6 was absorbed by creating a testlet for frequency of symptoms or testlets for pain and numbness. The testlets supported unidimensionality in the BCTQ SSS. Two items had differential item functioning favoring age or gender in the CTS-6 and the BCTQ SSS. Some items in the BCTQ SSS and CTS-6 had disordered response category thresholds. Merging of response categories led to ordered response category thresholds. The person separation indices were 0.73, 0.86 and 0.77 for the CTS-6 and BCTQ SSS and FSS, respectively, indicating good reliability of the measures.Conclusions: Based on the Rasch analysis, the CTS-6 has superior psychometric properties compared to the BCTQ SSS in surgically treated patients. The CTS-6 might be more accurate when separated into item sets measuring pain or numbness. The FSS of the BCTQ has acceptable construct validity, although sex differences at some ages were observed in responses.