Association between high neutrophil-platelet aggregates and low ectonucleotidase activity in patients with aPL⁺. (A) Role of ectonucleotidase activity in the step-by-step phosphohydrolysis of extracellular ATP to adenosine. (B-C) Flow cytometric determination of neutrophil-platelet aggregates (CD41+ events within the CD15+ CD16+ population) in fresh blood of healthy controls (n = 38), patients who are aPL⁺ (n = 70), and patients with aPL-negative venous thromboembolic disease [VTE (aPL⁻), n = 11]. (D) Levels of circulating neutrophil-platelet aggregates in aPL⁺ patients who had a history of thrombosis, as compared with aPL⁺ patients without a history of thrombosis or patients with VTE (aPL⁻). (E-H) Estimation of ectonucleotidase activity by measuring free phosphates (μM) using the malachite green assay kit, after the addition of ATP (100 μM) to neutrophils (E) or platelets (F), or the addition of AMP (100 μM) to neutrophils (G) or platelets (H). (I-L) Spearman correlation of neutrophil-platelet aggregates with ectonucleotidase activity for aPL⁺ patients as indicated. ∗P < .05, ∗∗∗P < .001, ∗∗∗∗P < .0001, and ns: nonsignificant by a one-way ANOVA with the Tukey multiple comparisons test.