Psychometric properties of the Fibromyalgia Assessment Status (FAS) index: a national web-based study of fibromyalgia

Fibromyalgia (FM) is a generalised chronic pain condition that is often accompanied by symptoms such as fatigue, sleep disturbances, psychological and cognitive alterations, headache, migraine, variable bowel habits, diffuse abdominal pain, and urinary frequency. Its key assessment domains include pain, fatigue, disturbed sleep, physical and emotional functioning, and patient global satisfaction and health-related quality of life (HRQL). A number of evaluation measures have been adapted from the fields of rheumatoid arthritis, psoriatic arthritis and ankylosing spondylitis, and others such as the Fibromyalgia Assessment Status (FAS) index and the Fibromyalgia Impact Questionnaire (FIQ) have been specifically developed. The aim of this study was to assess the impact of FM on HRQL by comparing the performance of the FAS index, the FIQ and the Health Assessment Questionnaire [HAQ] in 541 female and 31 male FM patients (mean age 50 years; mean disease duration 7.7 years) entered in the database of a web-based survey registry developed by the Italian Fibromyalgia Network (IFINET). Tests of convergent validity showed that the FAS index and FIQ significantly correlated with each other (rho=0.608, p<0.0001), but there were also significant correlations between the FAS index and other clinical measures of disability, including the HAQ (rho=0.423, p<0.0001), anxiety (rho=0.138, p=0.0009), depression (rho=0.174, p<0.0001) and, especially, the number of comorbidities (rho=0.147, p=0.0004). The FAS index revealed a statistically significant difference between males and females (p=0.048), analysed using the Mann- Whitney U-test for all pair wise comparisons. The FAS index is a valid three-item instrument (pain, fatigue and sleep disturbances) that performs at least as well as the FIQ in FM patients, and is simpler to administer and score. Both questionnaires may be useful when screening FM patients, with the choice of the most appropriate instrument depending on the setting.