Relationship between FMD and cell free hemoglobin. (A) The nonlinear relationship between FMD of the brachial artery and cell free hemoglobin for all subjects; at higher cell free hemoglobin levels, FMD is lower across all 3 groups. (B) A higher proportion of patients with cell free hemoglobin levels >10 mg/dL (elevated) have an FMD <6%, the lower limit of our control population. (B inset) A 2 × 2 table showing relative risk assessment of plasma free hemoglobin >10 mg/dL for FMD<6%.