Platelet-bound C4d (PC4d) is a patented blood test capable of uncovering significant association with thrombosis in patients with SLE, independent of antiphospholipid antibodies. Compared to those with low levels, elevation in PC4d is associated with history of thrombosis (OR 8.4 [95% CI 2.8-24.8]),¹ and a 7.5 fold increase in all-cause mortality (HR 7.5).² Persistent positive PC4d levels in SLE patients is significantly association with ischemic stroke (OR 5.5 [95% CI 1.7-17.6]).