The development and validation of a Patient-Perceived Methotrexate Intolerance Scale for use in adult rheumatoid arthritis patients

Introduction/Objective To develop a scale to measure methotrexate intolerance for use in adult rheumatoid arthritis (RA) patients and to describe its psychometric properties. Methods A three-phase study was conducted. During Phase 1, we conducted individual interviews with RA patients (n = 14) to inform our item development process. During Phase 2, we asked for RA patients' (n = 10) feedback on item readability, clarity and the scale's ability to measure methotrexate intolerance. During Phase 3, we had patients with RA (n = 204) complete the scale to develop a final version and to describe the scale's internal validity (Cronbach's alpha), test-retest reliability (intra-class correlation coefficients), construct validity and discriminant validity and the ability of the scale to discriminate between past and present methotrexate users. Results The newly developed, weighted scale (Patient-Perceived Methotrexate Intolerance Scale [PPMIS]) includes four subscales: Methotrexate Benefits, Methotrexate Risks-Side Effect Considerations, RA Risks and Methotrexate Risks-Willingness to Take Methotrexate Despite Risks. Cronbach's alpha ranged from 0.79 to 0.94. Test-retest reliability at 2 weeks was 0.73-0.88. Construct validity was supported with significant logical relationships between subscale scores and the existing methotrexate intolerance scale and past/present methotrexate use. The PPMIS was able to correctly classify RA patients as a past versus present methotrexate user 77% of the time. At the cut point of 3.29, the sensitivity of the PPMIS is 74% and specificity is 72% to correctly classify patients into past/present methotrexate use. Conclusion This is the first known scale with favourable measurement properties to evaluate methotrexate intolerance using a patient-centred perspective.