Impact of Erythrocytapheresis on Endogenous Anticoagulants in Sickle Cell Disease
Authors: Sharma Ruchika , Woods Gary , Creary Susan E , Hor Kan , Young Cody , Gallagher Christina , McLain Emily , Mongol Trevorline , Cohen Brooke , Kumar Riten , Dunn Amy L. 
Published: October 2019

Abstract
Introduction
The rate of venous thrombo-embolism (VTE) in patients with sickle cell disease (SCD) is estimated to be 4 - 8%; the incidence of catheter-associated VTE in this cohort is 0.16-0.99 per 1000 catheter days. Possible mechanisms for VTE in patients with SCD include hyperviscosity, ischemia reperfusion injury and chronic vascular inflammation. Additionally, patients with SCD exhibit decreased plasma levels of natural anticoagulants such as proteins C and S compared to healthy adults. A retrospective review at our institution found that the incidence of central venous line (CVL)-associated VTE in children with SCD was 0.12/1000 catheter days. On multivariate analysis, CVLs were identified as the only independent risk factor for VTE. Among patients with CVL, we found an association between bilateral CVLs and VTE (Odds Ratio [95% CI] = 10.3 [1.1-92.2]). Children with SCD frequently require CVLs for intravenous access or chronic erythrocytapharesis to reduce SCD-related complications. It is unknown, however, if erythrocytapharesis may also disrupt their natural anticoagulants levels and further increase their risk for subsequent VTE. We hypothesized that erythrocytapheresis may contribute to the pro-coagulant phenotype by altering patients' natural anticoagulants levels.
Methods
As part of a quality improvement initiative we measured antithrombin (AT), protein C antigen and activity, protein S antigen (total and free) and activity prior to being placed on the apheresis circuit and after completion of exchange in patients with SCD (0-21 years of age) undergoing chronic erythrocytapheresis who had a CVL (between January 1, 2009 and January 31, 2015). D-Dimer levels were obtained pre exchange only. Protein C and S levels were measured using ELISA based assays; protein C and S activities were measured using a clotting based assay and AT was measured using a chromogenic assay. The resulting levels, pre and post-erythrocytapheresis were analyzed using a paired nonparametric Wilcoxon rank test.
Results
Eleven patients were eligible for this study (8 females, 3 males). Median age at the time of erythrocytapheresis was 14 years (±3.65 SD) years. Indications for erythrocytapheresis included primary/ secondary stroke prevention or recurrent acute chest syndrome. All patients had bilateral CVLs in place. Four of the included patients had a history of VTE; 2 patients were on anticoagulation with low molecular weight heparin at the time of the study. The mean value for total protein S antigen (65u/dL; normal >72u/dL), free protein S antigen (53u/dL; normal >70u/dL), and protein S activity (51IU/mL; normal 65-138IU/ml) were all abnormal pre- erythrocytapheresis and decreased significantly following erythrocytapheresis (52u/dL, 44u/dL, and 44IU/mL respectively). Mean protein C antigen (71u/dL; normal >55u/dL), protein C activity (71%; normal 55-111%), and AT activity (104%; normal 77-132%) were within the normal range pre-erythrocytapheresis and decreased significantly post-erythrocytapheresis. We demonstrated significantly lower levels of protein C antigen (p=0.01), protein C activity (p=0.01), total protein S antigen (p=0.005), free protein S antigen (p=0.036), protein S activity (p=.04), and AT activity (p=.004) following erythrocytapheresis (Figure). D-Dimer levels were elevated in 6/11 patients (54.5%).
Conclusions
Pre- and post-erythrocytapheresis levels of natural endogenous anticoagulants investigated in a cohort of patients with SCD undergoing chronic erythrocytapheresis demonstrated that levels of all investigated proteins were significantly lower in patients following erythrocytapheresis. D-dimer levels were elevated in the majority of patients pre exchange but the importance of this finding is unclear. Given the relatively short half-life of the natural anticoagulants (2-3 hours for protein C and 36-60 hours for AT and protein S), it is unclear if this acute decrease of natural anticoagulants is clinically relevant. Further investigation is needed to determine if this acute decrease may contribute to an increased VTE risk in children with SCD undergoing apheresis.
Figure 1. Figure 1.

Disclosures
Woods: Biogen: Other: Educational Grant. Dunn:Novo Nordisk: Consultancy, Other: Educational Grant; CSL Behring: Consultancy, Other: Data Safety Monitoring Board; Bayer: Consultancy, Other: educational grant; Biogen: Consultancy, Other: Educational grant, Research Funding; Baxalta: Consultancy, Other: educational grant, Research Funding; Ohio State University: Employment; Pfizer: Other: Grant Review Board.