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

Objective: The 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. Methods: Representatives 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 (Choker 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). Results: The original four-factor structure of the CIS was replicated. Internal consistency (a = 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. Conclusion: The 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.