The sickling pressure index predicts hemoglobin S target achievement during chronic red cell exchange in sickle cell disease

BACKGROUND: Chronic automated red cell exchange (RCE) reduces hemoglobin S (HbS) in patients with sickle cell disease (SCD), but procedural success varies across patients in ways standard exchange parameters do not explain. We introduce the Sickling Pressure Index (SPI), derived from each patient's daily rate of HbS rise (ROR) and daily rate of hematocrit fall (ROF). STUDY DESIGN AND METHODS: This retrospective single-center cohort study included 19 patients with SCD undergoing chronic automated RCE and contributing 115 procedures over 40 months. ROR was calculated as the inter-procedure rise in HbS% per day, ROF as the inter-procedure hematocrit decline per day, and SPI as ROR/ROF. Patients were classified as Low SPI (n = 8) or High SPI (n = 11). Group differences were assessed by Mann-Whitney U test, and procedure-level target achievement by Fisher's exact test and generalized estimating equations (GEE). RESULTS: High-SPI patients had a significantly higher ROR (median 0.635 vs. 0.511 %HbS/day; p = .026) and lower ROF (0.068 vs. 0.129 %Hct/day; p = .002). The pre-procedure HbS target was met at 70.8% (34/48) of Low-SPI encounters versus 47.6% (30/63) of High-SPI encounters (OR 2.67; p = .020; GEE-adjusted OR 2.65, 95% CI 1.07-6.59; p = .036). Groups did not differ in exchange interval, fraction of cells remaining, or units transfused per day, though High-SPI patients trended toward higher absolute RBC use per procedure (p = .090). CONCLUSION: The SPI identifies patients with SCD on chronic RCE whose intrinsic HbS accumulation kinetics, rather than procedural factors, drive target failure. Calculable from routine apheresis data, the SPI may support individualized scheduling.