The correlation between plasma anti-factor Xa activity and haemostatic tests in healthy dogs, following the administration of a low molecular weight heparin

The aim of the study was to examine how activated partial thromboplastin ti me (ap-rr, two different reagents), thrombin time (rr, thrombin activity in the reagent: 3 or 6 IU ml(-1)) and reaction time of the resonance thrombog ram (RTG-r) in healthy dogs are influenced by low molecular weight heparin (LMWH). Three different LMWH doses were given subcutaneously or intravenous ly to groups, each of five healthy dogs. Mean plasma anti-FXa activities of 0.43, 0.88 and 1.86 anti-FXa IU ml(-1) were measured 2 min after intraveno us injection of 25, 50 or 100 anti-FXa IU kg-1. At this time, a dose-depend ent increase of the coagulation times, above the baseline values (P < 0.05) , was observed for all haemostatic tests. The significant prolongation of c oagulation time lasted 10 minutes to 3 hours, and it was dependent on the t est employed and LMWH dose. After subcutaneous LMWH injection of 50, 100 an d 200 anti-FXa IU kg(-1), significant changes of the coagulation time above initial values were limited to the period around the time when maximum ant i-FXa activities (0.23, 0.43 or 0.90 anti-FXa IU ml(-1)) were observed. For the tests which were less affected by the LMWH (APTT, TT[6 IU ml-1]) only small increases (< 4 seconds) were observed even after the highest subcutan eous LMWH dose. The correlation between plasma heparin activity and the rel ative alteration compared to the initial value (ratio), of the different co agulation tests was only moderate and considerably lower for RTG-r (r(s) = 0.526) than for the TT (r(s) = 0.711([6 IU ml-1])) r(s) = 0.780([3 IU ml-1] )) and APTT (r(s) = 0.667([reagent 1]), r(s) = 0.727([reagent 2])) The low degree of prolongation, which was found particularly for the group tests AP TT and TT[6 IU ml-1], reflects the low anti-thrombin activity of LMWH. The results indicate that measurement of anti-FXa activity with chromogenic sub strates is the method of choice to control LMWH therapy in dogs, as is the case in humans. (C) 2000 Harcourt Publishers Ltd.