Development and psychometric evaluation of the Severity of Myalgic Encephalomyelitis Scale (SoMES) and the Myalgic Encephalomyelitis Occupational Status Scale (MEOSS)

Objective: To develop and psychometrically evaluate two new brief assessments measuring disease severity (the Severity of ME Scale, SoMES) and occupational status (the ME Occupational Status Scale, MEOSS) in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) Methods: Draft SoMES and MEOSS were co-produced from relevant literature and lived experience. An online survey completed by adults with ME/CFS and was repeated two weeks later to assess reliability. The SF-36 (physical function) assessed criterion-related validity (Spearmann correlations) and discriminant validity (Kruskal-Wallis tests). Weighted kappa co-efficients assessed test-retest reliability and measurement error calculated minimal detectable difference (MDD). Results: 995 adults with ME/CFS participated and 579 completed the retest survey. Construct validity was confirmed from feedback. Criterion related validity was moderate-good (SoMES r = 0.663, MEOSS r=0.472, both p<0.001). Discriminant validity was demonstrated by significant differences in SF36-PF scores in different levels of SoMES and MEOSS (p=0.003 - 0.0001). SoMES test-retest reliability was moderate (k =0.53, P<0.001) with a MDD of 1.6, and excellent for the MEOSS (k = 0.76 p<.001) and the MDD was 1.4 Conclusions: The Severity of ME Scale (SoMES) and the ME Occupational Status Scale (MEOSS) are valid, reliable, sensitive assessments of ME/CFS severity and occupational status respectively.