Plasma free hemoglobin (PFH) is a direct biomarker for hemolysis that has been associated with clinical complications such as pulmonary hypertension and death in patients with sickle cell disease (SCD). We sought to characterize the relationship between PFH and more clinically available hemolytic markers including lactate dehydrogenase (LDH), aspartate aminotransferase (AST), bilirubin, reticulocyte percentage and to derive a composite hemolysis score derived from principal component analysis (PCA) of these biomarkers. In 68 adult patients (median age 31 years old, IQR 25-39) with HbSS or HbSβ0-thalassemia enrolled in the IMPROVE II study, median PFH was elevated at 21.9 mg/dL (IQR 9.9-44.9 mg/dL). Using Pearson correlation analysis, PFH had a stronger relationship to LDH (R=0.699), AST (R=0.587), and total bilirubin (R=0.475), compared to reticulocyte count (R=0.316). The hemolysis score was significantly associated with PFH (R=0.677). When compared with other laboratory measures, PFH correlated with hemoglobin (R= -0.275) and HbS (R=0.277),but did not correlate with white blood cell count (WBC) or HbF. The hemolysis score was significantly associated with WBC (R=0.307), hemoglobin (R = -0.393), HbF (R=- 0.424), and HbS (R=0.423). This study confirms that the conventional hemolytic biomarkers LDH, AST, bilirubin, and reticulocyte percentage correlate with PFH. Additionally, the hemolysis score is a valid tool to measure hemolysis and that it may be a marker of global hemolysis as opposed to PFH, which quantifies intravascular hemolysis. Further studies will be needed to elucidate the role of PFH and intravascular hemolysis in the development of clinical complications of sickle cell disease.
Keywords: Sickle cell disease; hemolysis; hemolysis score; plasma free hemoglobin.