In an attempt to identify the optimal panel that could serve as markers to differentiate ME/CFS patients from the long COVID and healthy control groups, we performed a discriminant function analysis including only those biomarkers that varied significantly among study groups (ET-1, TSP-1, VCAM-1, serpin E1 (PAI-1), E-selectin, NOx, IL-1β, IL-4, IL-6, IL-10, and TNF-α), as well as scores on the patient-reported outcome measures (FIS-40, HADS, COMPASS-31, PSQI, OGS, and SF-36). Here, SERPINE1 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.