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:
percent prothrombin complex activity =
= (113.2 / ((INR)^3)) - (69.9238 / ((INR)^2)) + (57.484 / (INR)) - 0.765
The INR required to achieve a given percent prothrombin complex activity (from plot of SQRT(PT) vs 1/INR):
INR to achieve percent prothrombin complex activity =
= 1 / ((-0.0073 * (percent)) + (0.2093 * SQRT(percent)) - 0.361)
Specialty: Hematology Oncology, Clinical Laboratory
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