Monday, May 21, 2018

"What is the INR of FFP?"

I frequently encounter people touting that "The INR of FFP is like 1.6 or something". Indeed I learned something similar myself at some point. It turns out however that the mean INR of FFP is actually 1.1 (Transfusion 2005;45:1234-5.). So why does it seem like no matter how much FFP you tend to give, it is very difficult to get the INR much lower than 1.6 or so?
As can be seen from the image below from (Transfusion. 2012;52 Suppl 1:45S-55S.), The relationship between INR and circulating coagulation factors (I am talking about warfarin here) is non-linear. While an INR of 1.7 still considered to be within the zone of "normal hemostasis", it also corresponds to almost 70% of circulating factors being depleted. In contrast, beyond that point, the INR increases more dramatically with small reductions in factor concentrations

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FFP replaces a relatively fixed percentage of coagulation factors (vertical arrows), the difference is, at higher INR values(below 30% coagulation factors), this corresponds to a large change in INR, however when the INR is lower than 1.7 or so, the same fixed increase in factor concentrations translates to a minor/negligible change in INR. This means that to get an INR back to "normal" an unrealistic amount of FFP would be needed.

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