A vitamin K antagonist such as coumadin will result in reduced level of coagulation factor activity in the prothrombin complex (prothrombin, Factor 9, Factor 10 and Factor 7). This will result in a prolongation of the prothrombin time (PT), which in turn will be associated with an increase in the INR (International Normalized Ratio).
Percent Prothrombin Complex Activity
Corresponding INR
Clinical Status
100%
1.0
normal
40%
1.5 (1.4 to 1.6)
subtherapeutic
30%
1.75 (1.7 to 1.8)
just below therapeutic
25%
2.0 (1.9 to 2.1)
therapeutic
20%
2.35 (2.2 to 2.5)
therapeutic
15%
2.9 (2.6 to 3.2)
therapeutic
10%
4.45 (4.0 to 4.9(
overly anticoagulated
< 5%
> 5.0
seriously anticoagulated
from Figure 1, page 680
If the values for the INR are plotted vs percent prothrombin activity:
