In vitro effect of hirudin on recalcification time

Hirudin, in its recombinant form (r-hirudin), is emerging as an ideal anticoagulant. As with any anticoagulant, the true anticoagulant property of hirudin should be contingent not on clinical criteria nor on anticoagulant levels, but on a functional assay that monitors changes in the rate at which blood clots. The purpose of this study was to determine the recalcification time (RT) of whole blood and of plasma on incubation with hirudin in vitro. Citrated human venous blood (n = 6) and plasma (n = 9) were incubated at 37 degrees C with either saline or 0.2 U/mL final concentration hirudin. The RT was measured instrumentally after neutralization of the citrate with CaCl2. Compared with saline, mean RT +/- SD values (minutes) with hirudin in blood increased from 6.9 +/- 0.8 to 7.6 +/- 0.6 minutes (P = .04) and in plasma from 9.0 +/- 1.4 to 11.2 +/- 1.4 (P = .001). We conclude that hirudin increases the RT values in both blood and plasma. The increased RT value in plasma is greater compared with that in blood. Whether this method can be used to titrate the dosage of hirudin warrants in vivo assessment.