If the same procedure was carried out with stepwise variable selection, the final model included serpin-E1 (PAI-1), NOx, IL-1β, IL-6, and FIS-40 (Wilks’ lambda = 0.068; χ2 = 215; p < 0.001) and 85.9% of participants were correctly classified (67.7% of ME/CFS patients, 91.3% of those with long COVID, and 100% of matched sedentary healthy controls). Here, SERPINE1 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.