the need for an adapted initiation nomogram during fiix prothrombin time monitoring of warfarin

The new fiix prothrombin time (fiix-pt) and its derived fiix-normalized ratio (fiix-nr) is affected only by reductions in coagulation factors (f) ii and x, the two factors responsible for the antithrombotic effect of vitamin k antagonists (vka). due to insensitivity to reductions in the short half-life fvii, the fiix-nr rises later than standard pt-inr during warfarin initiation. to describe a warfarin initiation nomogram adapted for monitoring with fiix-nr, anticoagulation development was assessed during use of standard pt-inr based initiation nomogram and after adapting the initiation nomogram for fiix-nr monitoring. normalized ratios were retrospectively assessed in consecutive warfarin naïve patients during their first 60 days of warfarin intake for one year prior to (pt-inr period) and for one year after replacing the pt-inr with the fiix-nr (fiix-nr period). the inr target was nr 2.0–3.0. we evaluated 160 patients monitored with pt-inr and dosed with the pt-nomogram, 57 monitored with fiix-inr but dosed with pt-nomogram, and 163 fiix-nr monitored patients dosed using a new fiix nomogram. mean pt-inr over 2.0 was reached on day 7 during the pt-period and remained around 2.5 thereafter. when the pt-nomogram continued in use during fiix-monitoring significantly more patients became overanticoagulated during days 11–29. after the nomogram was modified to respond to rising fiix-nr with larger initial dose reduction, the mean fiix-nr reached over 2 on day 8–9 and remained around 2.5 thereafter. when warfarin is monitored with fiix-nr, an adjusted dosing nomogram should be used during initiation to prevent early overanticoagulation.