The assessment of fatigue: Psychometric qualities and norms for the Checklist individual strength

•The Checklist Individual Strength (CIS), an instrument used to assess fatigue, is a reliable and valid measure of fatigue.•Cut-off scores for severe fatigue are presented that can be used in clinical practice and research.•Norms of CIS scores for the general population and eight different medical condition are presented.

ObjectiveThe Checklist Individual Strength (CIS) measures four dimensions of fatigue: Fatigue severity, concentration problems, reduced motivation and activity. On the fatigue severity subscale, a cut-off score of 35 is used. This study 1) investigated the psychometric qualities of the CIS; 2) validated the cut-off score for severe fatigue and 3) provided norms.MethodsRepresentatives of the Dutch general population (n = 2288) completed the CIS. The factor structure was investigated using an exploratory factor analysis. Internal consistency and test-retest reliability were determined. Concurrent validity was assessed in two additional samples by correlating the CIS with other fatigue scales (Chalder Fatigue Questionnaire, MOS Short form-36 Vitality subscale, EORTC QLQ-C30 fatigue subscale).To validate the fatigue severity cut-off score, a Receiver Operating Characteristics analysis was performed with patients referred to a chronic fatigue treatment centre (n = 5243) and a healthy group (n = 1906). Norm scores for CIS subscales were calculated for the general population, patients with chronic fatigue syndrome (CFS; n = 1407) and eight groups with other medical conditions (n = 1411).ResultsThe original four-factor structure of the CIS was replicated. Internal consistency (α = 0.84-0.95) and test-retest reliability (r = 0.74-0.86) of the subscales were high. Correlations with other fatigue scales were moderate to high. The 35 points cut-off score for severe fatigue is appropriate, but, given the 17% false positive rate, should be adjusted to 40 for research in CFS.ConclusionThe CIS is a valid and reliable tool for the assessment of fatigue, with a validated cut-off score for severe fatigue that can be used in clinical practice.