EPO and polycythemia: Low EPO (<3.8 mU/mL) had a Sn of 0.77 (95% CI, 0.62–0.87), Sp of 0.98 (95% CI, 0.94–0.99), −LR of 0.23 (95% CI, 0.13–0.41), and +LR of 33 (95% CI, 12.5–89.6) for the diagnosis of JAK2 mutant erythrocytosis (Table 2, and Supplementary Materials Table S1).