What Long COVID investigators can learn from four decades of ME/CFS research.

Four decades of research in the field of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) have yielded lessons that may be instructive for those devising criteria to better comprehend Post-Acute Sequelae of SARS CoV-2 Infection (PASC) and Long COVID. For instance, substantial effort has been devoted to defining classification systems, operationalizing methods, and developing instruments with adequate reliability and validity in the ME/CFS field. The current article provides guidelines for developing a case definition for Long COVID and discusses the significance of psychometric issues and criterion variance, including how to specify symptoms, and develop thresholds, subtypes, and exclusionary conditions. ME/CFS research could enhance our knowledge of Long COVID pathophysiology, early diagnosis, prognosis, and the identification of effective treatments.