False-positive detection of epoetin-β in urine. (A) Urine samples collected from the athlete immediately after a 4 × 1000 m sprint (0 hours) and 1 hour later (1 hour) contained 1.2 g and 0.3 g proteins/L, respectively. The samples were concentrated 200-fold and 800-fold, respectively, and processed for the detection of Epo by double immunoblotting after isoelectric focusing, as detailed in “Study design.” Also shown are the reference samples darbepoetin-α (1 ng) and epoetin-β (0.6 ng) (B) Immunoblotting of a postexercise urine sample (0 hours, 10-fold concentrated) after SDS-PAGE with anti-Epo antibodies. Also illustrated are epoetin-β (0.9 ng) and a mixture of the urine sample and 0.9 ng epoetin-β. (C) Immunoblotting of the same samples before and after a treatment with N-glycosidase F, as indicated.