Background: In sickle cell disease (SCD) abnormally shaped RBCs interact with white blood cells and the endothelium, leading to a vasculopathy and thrombotic/ prothrombotic complications such as stroke and pulmonary hypertension. About 10 % of patients with a thrombotic event in the general population will have the presence of an antiphospholipid (aPL) antibody (Andreoli et al. 2013). One proposed mechanism for the thrombophilic nature of aPL antibodies is the disruption of annexin A5. Annexin A5 is a potent anticoagulant protein that has an affinity to phospholipids. In the presence of aPL antibodies, annexin A5 is unable to form its crystallized anticoagulant shield (annexin A5 resistance). There is a paucity of data which assesses the association of aPL antibodies with vasculopathic complications of SCD, and there have been no studies investigating the annexin A5 resistance assay (A5R). We designed a pilot study assessing aPL antibody levels and A5R in a pediatric sickle cell population. Methods: Patients with a history of stroke, abnormal transcranial doppler (TCD) and elevated TR gradient by echocardiography (>25mm of Hg) were eligible. A5R, lupus anticoagulant- DRRVT, anti β2GP1, anti phosphatidylserine and anti cardiolipin antibody (IgG, IgA, IgM) assays were performed on samples obtained prospectively when patients were at a steady state (at least 4 weeks after an acute event). A5R measures coagulation times in the presence and absence of annexin A5. Resistance to the anticoagulant effects of annexin is expressed as a reduction in this ratio (Rand et al. 2004). Statistical analysis assessed multiple variables including age, gender, hemoglobin, reticulocyte count, LDH, hydroxyurea therapy, transfusion therapy, elevated TR gradient, stroke and silent stroke. Univariate analysis and multivariate logistic regression was performed with abnormal annexin A5 as the outcome variable. Results:There were a total of 39 patients: 12 patients with a history of stroke, 6 with an elevated TCD velocity and 15 patients with an elevated TR gradient. Of the 27 patients that did not have a stroke, 25 had a screening MRI in the prior year, and 9 of these patients had silent infarcts. Only 1 of 39 patients had elevated anticardiolipin IgG antibodies and 1 had an abnormal lupus coagulant (DRVVT). In contrast, 5/39 patients (12.8%) had low or abnormal annexin A5 resistance, 7/39 (18%) were in the borderline range and 27/39 (69%) were normal. This frequency of abnormality was unexpected in the antiphospolipid antibody and lupus anticoagulant negative population. None of the patients, except the one with the positive lupus anticoagulant, developed any thrombotic events in 3.5 years of follow up. None of the patients with overt stroke or abnormal TCDs had an abnormal A5R. Multivariate logistic regression analyses showed statistically significant association of hemoglobin (p= 0.037, OR 0.25, CI 0.07-0.92)), age (p =0.047, OR 1.43, CI 1.01-2.04) and silent infarct (p =0.015, OR 28.5, CI 1.9-420.5) with abnormal annexin A5 resistance. A multivariate analysis using linear regression with annexin A5 resistance as a continuous outcome variable (Table 1), showed persistence of the significant association of silent infarcts (p =0.037). Conclusion: We report an association between annexin A5 resistance and low hemoglobin, older age and presence of silent infarct in a subgroup of SCD patients. Prevalence of abnormal aPL antibody assays and lupus anticoagulant was strikingly low in this cohort. A potential role for perturbed annexin A5 resistance in the pathophysiology of silent infarction in SCD will need to be evaluated further in carefully designed prospective studies and may be a novel therapeutic target. Table 1. Hemoglobin, Age and Silent Stroke are Associated with Abnormal Annexin A5 Resistance Characteristic Odds Ratio Odds Ratio Confidence Interval p-value Hemoglobin* 0.25 0.07-0.92 0.037 Reticulocyte count 0.77 0.54-1.08 0.13 LDH 1.00 0.99-1.00 0.51 Age* 1.43 1.01-2.04 0.047 Monocyte count 1.00 1.00-1.00 0.34 Silent infarct* 28.5 1.9-420.5 0.015 Stroke 0.47 0.05-4.88 0.53 Elevated TR gradient 0.34 0.03-3.49 0.36 Gender 0.47 0.05-4.88 0.53 Allosensitization 0.56 0.05-5.86 0.63 HU vs no treatment 0.46 0.03-6.93 0.58 Transfusion vs no treatment 0.18 0.01-4.26 0.29 Disclosures No relevant conflicts of interest to declare.