Measuring fatigue in cancer patients: a common metric for six fatigue instruments

Fatigue is one of the most disabling symptoms in cancer patients. Many instruments exist to measure fatigue. This variety impedes the comparison of data across studies or to the general population. We aimed to estimate a common metric based on six different fatigue instruments (EORTC QLQ-C30 subscale fatigue, EORTC QLQ-FA12, MFI subscale General Fatigue, BFI, Fatigue Scale, and Fatigue Diagnostic Interview Guide) to convert the patients' scores from one of the instruments to another. Additionally, we linked the common metric to the general population.

For n = 1225 cancer patients, the common metric was estimated using the Item Response Theory framework. The linking between the common metric of the patients and the general population was estimated using linear regression.

The common metric was based on a model with acceptable fit (CFI = 0.94, SRMR = 0.06). Based on the standard error of measurement the reliability coefficients of the questionnaires ranged from 0.80 to 0.95. The common metric of the six questionnaires, also linked to the general population, is reported graphically and in supplementary crosswalk tables.

Our study enables researchers and clinicians to directly compare results across studies using different fatigue questionnaires and to assess the degree of fatigue with respect to the general population.