JAK2 and polycythemia: Elevated EPO (>16.9 mU/mL) had a Sn of 0.16 (95% CI, 0.11–0.22), Sp of 1.0 (95% CI, 0.91–1.0), −LR of 0.84, and an infinite +LR for the diagnosis of JAK2 negative (secondary) erythrocytosis (Table S4).