Dimensions of psychological flexibility and their significance in people with somatic symptoms: The 18-item Flexibility Index Test (FIT-18).

Psychological flexibility has been hypothesized to preserve health in bad times. We examined whether psychological flexibility as assessed with an abbreviated questionnaire, was indicated to preserve mental and physical health when having somatic symptoms. Principal axis factoring indicated that two dimensions best represented the 60-item Flexibility Index Test (FIT-60) questionnaire: "mindfulness and acceptance" (M&A) and "commitment and behavior change" (C&BC). We selected 18 items that best denoted these dimensions (FIT-18 questionnaire). Regression analyses in 2060 Dutch people with and without persistent somatic symptoms, indicated that the M&A dimension (β = 0.33, p  < 0.001) and C&BC dimension (β = 0.09, p  < 0.001) were additively associated with mental well-being, but not with physical functioning. Moreover, the M&A dimension was indicated to protect mental well-being when having more severe somatic symptoms (β = 0.11, p  < 0.001). The observed differential associations with health suggest the significance for health of the two dimensions of psychological flexibility as assessed with the FIT-18 questionnaire.