P-Selectin represents a cell surface glycoprotein that is constitutively present in the Weibel-Palade bodies of endothelial cells and in the α-granules of platelets. In inflam mation and thrombogenic conditions, plasmatic P-selectin lev els are markedly elevated, indicating the leakage of this marker from these sites. In this study, a quantitative sandwich enzyme- linked immunosorbent assay (ELISA) utilizing a monoclonal soluble P-selectin (sP-selectin) antibody was employed to as sess this marker in blood samples collected in various antico agulants such as heparin, hirudin, sodium citrate (3.2% and 3.8%), and ethylenediaminetetraacetic acid (EDTA). The soluble P-selectin levels ranged from 26 ng/mL to 44 ng/mL. Sodium citrate (3.8%) was used to collect platelet-poor plasma (PPP) from patients with heparin-induced thrombocytopenia (HIT), coronary angioplasty (CA), or coronary atherectomy (CAT). In comparison with the control group (∼ 30 ng/mL), all of these patient groups showed a marked elevation of sP- selectin levels (HIT = 96 ng/mL [n = 18], CA = 46 ng/mL [n = 6] and CAT = 60 ng/mL [n = 10]). In platelet-rich plasma (PRP) preparations using various anticoagulants, the sP-selectin levels were markedly higher, ranging from 87 ng/ mL to 117 ng/mL (n = 10). In patients recruited into a clinical trial (the argatroban [ARG] 911 Study), in which argatroban was used as an alternate anticoagulant in patients with HIT, a 25% to 35% decrease in sP-selectin levels was observed after 72 hours of argatroban treatment. In addition, the relative ratio between levels in PRP and PPP in these patients differed, sug gesting that the anticoagulant matrix influences the sP-selectin levels. These data clearly suggest that the anticoagulant matrix and blood collection procedures may significantly influence the plasmatic P-selectin levels. Furthermore, in different clinical conditions, elevation of this marker may reflect endogenous platelet activation; however, optimal anticoagulant for blood collection is important for proper diagnostic valida tion.