Beyond Fatigue: The Fatigue Assessment Scale as a Potential Indicator of Long COVID Severity

Patient-reported outcomes (PROs) are often regarded in clinical practice as less reliable than biomarkers because they are patient-dependent and not objectively measurable in the same way as circulating molecules or imaging findings. This view may be reductive. Biomarkers are frequently considered reliable because they are easily quantifiable, yet biological meaning does not depend solely on measurement. In precision and personalized medicine, the absolute level of a circulating hormone, cytokine, or metabolite is often insufficient unless interpreted in relation to how a given patient, at a given time, responds to that molecular signal. If biomarkers are considered less absolute and more context-dependent, the relevance of PROs becomes clearer. Standardized PROs capture the subjective impact of disease on the patient system in a reproducible and clinically interpretable manner. This is particularly relevant in Long COVID, where no validated diagnostic biomarker is currently available, and severity stratification cannot rely exclusively on objective biological measures. Among available tools, the Fatigue Assessment Scale (FAS) may be particularly useful for quantifying the fatigue burden in Long COVID, including both physical and mental components. Moreover, FAS scores have been shown to correlate with the number of symptoms reported by patients, suggesting that this scale may help stratify patients not only according to fatigue severity but also according to overall symptom burden. Greater standardization of clinical characterization through instruments such as the FAS may improve comparability across cohorts and facilitate interpretation of data from different clinical trials.