Tired of blunt tools? Sharpening the clinical assessment of fatigue and sleepiness.

Fatigue and sleepiness are ubiquitous symptoms in various conditions and are frequently associated to impaired sleep quality. While separate fatigue and sleepiness scales exist, both constructs are often confused. Unraveling this issue requires estimating the instruments' measurement properties, potential scale recalibration and re-evaluation of symptom intensities on a comparable basis. This study aims at improving the assessment of these symptoms and quantifying their degree of overlap using common-person-equating (CPE). One hundred fifty-nine patients, either with complaints of fatigue, sleepiness and/or non-restorative sleep, addressed to an academic sleep unit for a full-night polysomnography (PSG), enrolled in the study. Symptom levels were measured with the Fatigue Severity (FSS) and Epworth Sleepiness (ESS) scales. Sleep quality was assessed by the Pittsburgh Sleep Quality Index, defining 'good' and 'poor' sleeper groups. Good and poor sleepers did not differ statistically regarding demographics and PSG parameters. Rasch analysis revealed that, considering proper calibration, the ESS and FSS generate reliable and valid, unidimensional linear measures and to be invariant to perceived sleep quality. CPE showed predominantly fatigued, rather than sleepy patients, being more likely to present as poor sleepers. A concordance diagram based on scale scores is provided, in order to improve the differentiation of both symptoms.