Abstract: A thromboplastin reagent comprises: (i) activated sTF, (ii) a metal-chelating lipid, (iii) a metal ion, and (iv) a phospholipid. Activated sTF preferably includes the extracellular domain of TF and an oligohistidine moiety having at least 2 histidine residues, more preferably 2-10 histidine residues. Preferably, the histidine residues are consecutive. Attaching a metal binding domain, such as an oligohistidine tag, to the C-terminus of sTF allows the protein to bind to phospholipid vesicles that contain metal-chelating lipid. Metal complexes of this activated sTF and metal-chelating lipids have all of the desirable expression, handling, and solubility characteristics of sTF, and exhibit procoagulant activities in plasma clotting tests that are comparable to relipidated rTF. In addition, it was discovered that, under some circumstances, Ni-lipids are themselves procoagulant, even in the absence of activated sTF.