[Assessment of functional capacity in fibromyalgia. Comparative analysis of construct validity of three functional scales].

OBJECTIVE To compare the construct validity of three functional capacity questionnaires in patients with fibromyalgia. PATIENTS AND METHODS Transversal multicentric study of 301 patients from fifteen rheumatology outpatient clinics in Spain. Scores of Health Assessment Questionnaire (HAQ), Fibromyalgia Health Assessment Questionnaire (FHAQ) and the physical function scale of the Fibromyalgia Impact Questionnaire (PF-FIQ) were compared with extreme groups of patients defined by four external indirect measures: 6Min Walk Test, a modified Borg Fatigue Scale, Lumbar Spine Flexion Test and Patient Global Passive Mobility Assessment. Standardized differences were determined and correlation coefficients were calculated between the three questionnaires scores. RESULTS All three questionnaires showed good construct validity, but the results obtained with the PF-FIQ were poorer. Correlations between HAQ and FHAQ were very high (0.92), but correlations between these two questionnaires and PF-FIQ were only moderate (0.59). CONCLUSIONS HAQ and FHAQ are more valid measures of functional capacity than the PF-FIQ. HAQ could be substituted by FHAQ in some settings because of its shorter format (only 8 items).