Fibromyalgia (FM) and Chronic Widespread Pain (CWP) are common diseases in primary care and, in addition to the pain they cause, fatigue is a major problem. Fatigue is regarded as a multidimensional concept and instruments assessing fatigue should therefore cover several different dimensions. The Multidimensional Fatigue Inventory (MFI-20) measures five different dimensions of fatigue. The aim of the study was to investigate the test-retest reliability and convergent construct validity of the Swedish version of the MFI-20 in female patients with FM or CWP.
To investigate the convergent construct validity, 166 female patients with FM or CWP completed the Swedish version of the MFI-20 and a Visual Analogue Scale (VAS) measuring global fatigue. Thirty-six of the 166 patients completed the Swedish version of the MFI-20 on two occasions, one day apart to evaluate the test-retest reliability.
Spearman's correlation coefficient revealed a significant association between each of the five subscales of the MFI-20 and the VAS for global fatigue. General Fatigue had the highest correlation with the VAS (rs = 0.62, p < 0.001), while Reduced Motivation had the lowest (rs = 0.32, p <0.001). The intra-class correlation coefficients (ICC) were satisfactory for all the five subscales of the MFI-20.
The results of this study indicate that the Swedish version of the MFI-20 is a reliable tool for assessing the degree of fatigue in patients with FM or CWP. This study also supports the theory that fatigue is a multidimensional concept and different aspects of fatigue should be measured separately.