Psychometric evaluation of the shortened version of the Functional Difficulties Questionnaire to assess thoracic physical function

Abstract

Objective: The aim of this study was to investigate the psychometric properties of the shortened version of the Functional Difficulties Questionnaire (FDQ).

Design: This is a multisite observational study.

Setting: The study was conducted in four tertiary care hospitals in Australia.

Subjects: A total of 225 participants, following cardiac surgery, were involved in the study.

Intervention: Participants completed the original 13-item FDQ and other measures of physical function, pain and health-related quality of life.

Method: Item reduction was utilized to develop the shortened version. Reliability was evaluated using intraclass correlation coefficients (ICCs), the smallest detectable change and Bland-Altman plots. The validity and responsiveness were evaluated using correlation. Anchor and distribution-based calculation was used to calculate the minimal clinical important difference (MCID).

Results: Item reduction resulted in the creation of a 10-item shortened version of the questionnaire (FDQ-s). Within the cohort of cardiac surgery patient, the mean (SD) for the FDQ-s was 38.7 (19.61) at baseline; 15.5 (14.01) at four weeks and 7.9 (12.01) at three months. Validity: excellent internal consistency (Cronbach's α > 0.90) and fair-to-excellent construct validity (>0.4). Reliability: internal consistency was excellent (Cronbach's α > 0.8). The FDQ-s had excellent test-retest reliability (ICC = 0.89-0.92). Strong responsiveness overtime was demonstrated with large effect sizes (Cohen's d > 1.0). The MCID of the FDQ-s was calculated between 4 and 10 out of 100 (in cm).

Conclusion: The FDQ-s demonstrated robust psychometric properties as a measurement tool of physical function of the thoracic region following cardiac surgery.

Keywords: Functional Difficulties Questionnaire; cardiac surgery; outcome measurement; physical function; psychometric.