(A) To assess endogenous platelet activation, whole blood was drawn from patients with heart failure (n=32) or matched control donors without HF (Ctrl, n=20). Baseline (e.g. unstimulated) whole blood flow cytometry was performed to measure levels of circulating platelet-monocyte aggregate (PMA) formation, a sensitive index of endogenous platelet activation. (B-C) From the same whole blood samples, plasma was harvested as described in the methods. Levels of (B) platelet factor 4 (PF4) and (C) RANTES (regulated upon activation, normal T cell expressed and secreted) were measured by commercially available ELISA. Numbers on the top refer to the p-value.