Association of HP 1 allele with lower cell-free hemoglobin-to-HP concentration and multiorgan failure risk in SCD. (A-C) Serum concentrations of (A) HP, (B) cell-free hemoglobin (hgb), and (C) cell-free hgb-to-HP ratio by the HP genotype in patients with SCD at steady state (n = 243), VOE (n = 42), and acute chest syndrome (n = 9). Test for linear trend P values are provided. (D-F) Proportion of patients who developed (D) acute chest syndrome, (E) rapidly progressive acute chest syndrome, and (F) acute chest syndrome complicated by multiorgan failure by the HP genotype on longitudinal follow-up. (G) The proportion of patients with acute organ injury by the HP genotype. (H) The risk for developing acute chest syndrome complicated by multiorgan failure was lower in patients with the HP 1 allele. Log-rank P value provided.