VALIDATION DATA

A three‐phase mixed‐methods study was conducted. Qualitative methodologies and psychometric evaluation were used. During Phase 1, a literature review and individual interviews with patients with RA (n = 14) were performed. Individual interviews were analysed using qualitative descriptive design and related analysis (complete results reported elsewhere) to fully inform our item development process (Salt et al., 2021). Expert feedback was obtained while developing the scale structure and as scale items were developed and edited. During Phase 2, feedback from RA patients (n = 10) on the items developed in Phase 1 was retrieved. During Phase 3,  patients with RA (n = 204) completed the scale to develop the final version and estimate scale psychometric properties. The three‐phase process was used to ensure that scale derivation was truly centred in the patient experience. 

Internal consistency estimates for the subscales Methotrexate Benefits, Methotrexate Risks‐Side Effect Considerations, RA Risks and Methotrexate Risks‐Willingness to Take Methotrexate Despite Risks were all high with Cronbach’s alphas of 0.94, 0.84, 0.85 and 0.81, respectively.

To estimate the test–retest reliability or stability of the total scale and its subscales, participants were asked to complete the questionnaire a second time— 2 weeks after completing the initial survey. The test–retest reliabilities measured with intra‐class correlation coefficients (N = 131; ICC) at 2 weeks for each subscale (Methotrexate Benefits, RA Risks, Methotrexate Risks‐Side Effect Considerations and Methotrexate Risks‐Willingness to Take Methotrexate Despite Risks) were 0.88 (lower bound [LB]: 0.83, upper bound [UB]: 0.92), 0.80 (UB: 0.72, UB: 0.86), 0.79 (LB: 0.71, UB: 0.85) and 0.73 (LB: 0.63, UB: 0.81), respectively.

Iterative changes to scale items were made based on participant feedback as it related to item readability, ease of comprehension and the scale’s ability to measure methotrexate intolerance. All participants agreed that the scale measured the construct of methotrexate intolerance. Based on participant feedback regarding readability and word connotations, alternative wording was selected for some item stems

. Initially, using EFA with Promax rotation and the Scree test, the 18 scale items loaded cleanly on four factors. Thus, the PPMIS (mean 3.26 +- 0.54; range: 1.88–4.48) includes four subscales: Methotrexate Benefits (weight: 0.41; mean: 3.26 +- 1.05; range: 1.00–5.00), RA Risks (weight: 0.21; mean: 3.963 +-0.825; range: 1.00–5.00), Methotrexate Risks‐Side Effect Considerations (weight: 0.20; mean: 3.53 +-0.84; range: 1.00– 4.83) and Methotrexate Risks‐Willingness to Take Methotrexate Despite Risks (weight: 0.18; mean: 2.19 +- 0.86; range: 1.0–4.17). These four subscales support the scale structure developed from Phase 1 analysis.

Present versus past use of methotrexate was an additional metric used to establish the construct validity of the total PPMIS and subscales. Present users of methotrexate had significantly higher scores on the total PPMIS, and the Methotrexate Benefits and Methotrexate Risks‐Willingness to Take Methotrexate Despite Risks subscales (total and subscales: p < 0.001). Past users of methotrexate had higher scores on the Methotrexate Risks‐Side Effect Considerations subscale (p < 0.001). There was not a significant difference between the RA Risk subscale score and past versus present methotrexate use (p = 0.84).