From Dave McGlasson: George, Here is a potential answer for Kelly Townsend’s February 13 post, Factor II Assay in place of INR.
Kelly is right. We did look at the chromogenic factor X and published that work a few years ago McGlasson DL, Romick BG, Rubal BL. Comparison of a chromogenic factor X assay with international normalized ratio for monitoring oral anticoagulation therapy. Blood Coagulation and Fibrinolysis 2008; 19:513–17. We found that when the INR was above 3.0, big differences occurred in correlations. The INR thusly was invalid. We had instaances when a subject had an INR of 12.8 who had almost the same level of FX as many people in the therapeutic range.
We did look at clottable FII, FVII, FX and chromogenic FVII and FX when monitoring coumadin patients with unstable INRs, published in McGlasson DL. Unexpected levels of FVII when monitoring coumadin patients with unstable INRs. Clin Hemost Rev. October 2000:8–9. In that study we found that INRs on 163 specimens on subjects with a mean INR of 2.26 using the Stago reagent/instrument system had a FII mean of 29.5%; clottable FVII of 52.9% and a clottable FX of 22.0%. The chromogenic FVII mean however was 62.7% with 83.2% of the values over our lower reference interval limit of 40%. The chromogenic FX level was 33.2%. The study showed that subjects in our population with unstable INR values may fall within the INR therapeutic range while still demonstrating FVII levels higher than desired in OAT subjects. Those normal FVII levels may provide an explanation for unstable INRs.