ARG1 and endometriosis: Serum Arg-1 levels obtained in this study approach the threshold of replacement test criteria (sensitivity ≥ 94% and specificity ≥ 79%) and meet the criteria of both the SnOUT test (high SeNsitivity to rule OUT—i.e., sensitivity ≥ 95% at specificity ≥ 50%, negative result rules out the disease) and SpIN (high SPecificity to rule IN—i.e., sensitivity ≥ 50% at specificity ≥ 95%, positive result confirms the presence of the disease) [12] for the discrimination between endometriosis patients of any stage and age-matched healthy controls.