Background Low back pain is defined as “pain, muscle tension, soreness, stiffness and discomfort, localized below the costal margin and above the inferior gluteal folds” [1,2]. Although, patients are complaining of stiffness, stiffness has not received much attention. We have developed a questionnaire consisted of 6 items pain and stiffness measured at 11-point Numeric Rating Scale (NRS). NRS is an ordinal scale, which mean they can be counted and ordered, but not measured, such as calculation based on adding or subtracting. The Rasch models ensures that measurement is invariant (specific objective) and that the sum-score encompass all information (sufficiency), so the items can be added to a sum-score .
Objectives To what extent patients with low back pain (LBP) are bothered with stiffness; if the “pain-stiffness questionnaire” fulfils the requirements for partial credit Rasch model.
Methods The “pain-stiffness questionnaire” was completed by patients with chronic LBP. It consisted of 6 items, including two questions about pain and 4 questions about stiffness at 11-point Numeric Rating Scale (NRS) and was analyzed using Rasch analysis.
Results Of 218 patients with the average age of 39.2, 50.5% were women, 98% and 93% reported they were bothered with pain and stiffness, respectively. Patients rated “stiffness after inactivity” and “pain at activity” at mean (SD) of 6.4-6.0 (2.8-2.4), while “stiffness at activity”, “after activity” and “at inactivity”, ranged from 4.1 to 5.0 (2.6-2.8). The overall model fit demonstrates a Chi-square probability value of 0.60, total item chi-square 10.18 and item fit statistics chi-square probability values above 0.05.All thresholds except one functioned as intended. No DIF (differential item functioning) for gender, age or profession was revealed and all indicators of unidimensionality show a fit to the partial credit Rasch model.Investigating targeting shows normally distribution. There was no item redundancy. “Pain at activity” was the “easiest item”, and “stiffness at inactivity” the “severest item”. The Person Separation Index (PSI) was 0.88 and Chronbach's alpha 0.84, demonstrating good reliability.
Conclusions The main part of LBP patients was bothered with stiffness. The “pain-stiffness questionnaire” is valid, reliable, invariant across age, gender and profession. Pain and stiffness is nuances of the same construct and is justified to add the raw scores of the items into a sum-score.
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Disclosure of Interest : None declared