Co-calibration of self-reported measures of Pain Interference: the Patient Reported Outcomes Measurement Information System and the Brief Pain Inventory

Abstract

Introduction: To help researchers in MS take advantage of the measurement properties of the PROMIS Pain Interference instruments while maintaining continuity with previous research, a co-calibration table was developed that transforms scores from the Brief Pain Inventory (BPI-PI) to scores from the PROMIS-PI short form (SF). Methods: Data for the development of the score co-calibration were collected as part of a longitudinal study of pain and fatigue in adults (n=369) with Multiple Sclerosis (MS). Data for the co-calibration validation were collected as part of a separate longitudinal study of Aging in individuals with physical disabilities (n=360). The development of the co-calibration was a multistage procedure that included dimensionality assessment, fitting an item response theory based two-parameter logistic model, estimating difficulty and discrimination parameters for items of the BPI-PI, deriving trait scores of pain interference, and matching instrument level summary scores. Results: The co-calibration table functioned well with a mean difference between the actual SF scores and SF scores predicted by the table of 0.51 (sd=3.9). The largest differences were observed at the low-end of pain interference where variance in measurement is typically higher and where high precision is not necessarily as important as it is at higher levels of pain interference. The co-calibration performed nearly as well in terms of predictive accuracy in the validation dataset. With respect to residual errors from the equating procedure, approximately 80% of the predicted scores in the developmental sample were within 4 points difference (<1/2 sd) of their respective PROMIS-PI SF scores, and 70% were within 4 points in the validation sample. Conclusions: Two measures of pain interference were co-calibrated and a reference table of corresponding scores is presented. MS researchers and clinicians can use this table to facilitate comparisons between studies and to maintain continuity with previous research.