Serum LDH as a marker of intravascular hemolysis and NO resistance in 26 patients with sickle cell disease. Serum LDH levels correlate with plasma heme levels (A) and the amount of plasma NO consumption (B) as measured by an in vitro assay (Spearman correlation). (C) Patients with higher LDH levels (340-589 IU/L, n = 13) have NO resistance compared to patients with lower LDH levels (190-337 IU/L, n = 13) as indicated by the blunted degree of increased forearm blood flow induced by brachial artery infusion of the NO donor sodium nitroprusside at 0.8, 1.6, and 3.2 μg/min, measured by venous occlusion plethysmography (P = .016, ANOVA with repeated measures). Results are depicted as mean ± SEM.