Longitudinal validity and reliability of the Myeloma Patient Outcome Scale (MyPOS) was established using traditional, generalizability and Rasch psychometric methods

PURPOSE: The Myeloma Patient Outcome Scale (MyPOS) was developed to measure quality of life in routine clinical care. The aim of this study was to determine its longitudinal validity, reliability, responsiveness to change and its acceptability. METHODS: This 14-centre study recruited patients with multiple myeloma. At baseline and then every two months for 5 assessments, patients completed the MyPOS. Psychometric properties evaluated were as follows: (a) confirmatory factor analysis and scaling assumptions (b) reliability: Generalizability theory and Rasch analysis, (c) responsiveness and minimally important difference (MID) relating changes in scores between baseline and subsequent assessments to an external criterion, (d) determining the acceptability of self-monitoring. RESULTS: 238 patients with multiple myeloma were recruited. Confirmatory factor analysis found three subscales; criteria for scaling assumptions were satisfied except for gastrointestinal items and the Healthcare support scale. Rasch analysis identified limitations of suboptimal scale-to-sample targeting, resulting in floor effects. Test–retest reliability indices were good (R = > 0.97). Responsiveness analysis yielded an MID of +2.5 for improvement and -4.5 for deterioration. CONCLUSIONS: The MyPOS demonstrated good longitudinal measurement properties, with potential areas for revision being the Healthcare Support subscale and the rating scale. The new psychometric approaches should be used for testing validity of monitoring in clinical settings. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11136-017-1660-z) contains supplementary material, which is available to authorized users.