A Rasch analysis of the pain catastrophizing scale supports its use as an interval-level measure.

OBJECTIVES To evaluate the properties of the Pain Catastrophizing Scale (PCS) from a Rasch paradigm. METHODS A secondary analysis of 235 patients with work-related pain conditions was performed using the Rasch methodology. Unidimensionality, item fit, location independence, differential item functioning, response option structure, and linearity were evaluated for the 13-item PCS score. RESULTS Two items (8 and 12) required rescoring to address disordered response thresholds. Significant misfit to the Rasch model was corrected through the use of testlets based on the original 3 factors of the PCS (rumination, magnification, and helplessness). After rescoring and creation of testlets, the scale showed good fit to the Rasch model (χ(2)=6.93, P=0.91) and could be logically considered an interval-level scale. No evidence of differential item functioning was found for sex or location of pain. The items in the scale covered the spectrum of catastrophizing levels reported by the sample. A transformation matrix is presented that allows simple conversion of ordinal to interval-level scores. DISCUSSION The results of this secondary analysis suggest that the PCS can be appropriately evaluated as an interval-level scale when the composite 13-item score is considered, as has been standard practice to date. Implications for clinical and research use are discussed.