The measurement of fibromyalgia severity: converting scores between the FIQR, the PSD and the FASmod

Objectives: The revised Fibromyalgia Impact Questionnaire (FIQR) is a widely used fibromyalgia severity assessment tool that was introduced in 2009 prior to the publication of the American College of Rheumatology (ACR) preliminary fibromyalgia criteria in 2010 and its revision in 2016. In 2020, the modified Fibromyalgia Assessment Scale (FASmod) was published. The Polysymptomatic Distress scale (PSD) of the fibromyalgia criteria and FASmod include assessments of pain location severity and can be used for diagnosis as well as in non-fibromyalgia patients. The aim of this study is to provide equations for the conversion of the FIQR scores to PSD and FASmod as an aid to understanding and sharing fibromyalgia severity information. Methods: 3089 patients with fibromyalgia, diagnosed according to the ACR 2010/2011 criteria and belonging to the Italian Fibromyalgia Registry completed FIQR, FASmod and PSD questionnaires. Pearson's correlation coefficient was used to test the correlations between indices. The least square regression approach was used to produce predictive equations for each scale based on the remaining scales. Results: FIQR was correlated with PSD (r=0.714) and FASmod (r=0.801); PSD and FASmod showed the highest correlation (r=0.897), expected since they assess the same constructs. Predictive equations showing a linear model were effective in producing mean cohort values, but individual predictions deviated substantially, precluding prediction in the individual patient. Conclusions: Conversion equations that allow for interconversion of multiple scales fibromyalgia severity assessment scales are produced. These can be useful in obtaining mean values for cohorts but are not accurate enough for use in individual patients.