Despite the onslaught of direct oral anticoagulants, Coumadin still occupies an important place in anticoagulation. Most facilities employ POC instruments that rapidly generate the PT/INR , and most choose to confirm all INRs that exceed 4.0 by collecting venous blood and performing a plasma-based PT/INR.
A 2016 report describes the R-T estimation formula designed to correlate POC and plasma INR results in the 4.0–6.0 range, thereby reducing the confirmation requirement, thus reducing the time to treatment modification. The formula, 0.621 X POC INR + 0.639 = Plasma INR . is derived from a regression equation. The derivation is described in Richter C, Taylor J, Shuster J. Correction of point of care INR results in warfarin patients. Point of Care 2016; 15:1–3. The authors then applied the formula in a double-blinded trial reported in Richter C, Taylor J, Wright J, Fletcher B. Clinical validation of R-T estimation for CoaguChek XS INR results. Annals of Pharmacology 2016; 50: 645–8.
George reviewed the Richter papers with for Jake Fray, MLS of Denver Health Medical Center. Here is George's discussion:
Despite poor precision, I advocate for point of care PT/INR instruments like the CoagUChek XS for their convenience and TAT provided operators are aware of their limitations. POC results are reasonably reliable and reproducible when the INR is within the therapeutic range of 2.0–3.0 or 2.5–3.5 when there is a mechanical heart valve. Generally, an INR of 5.0 or higher could indicate a medical emergency and requires communication with the managing physician. Some use an INR of 4.0 as their "call-back" number.
As the Richter article describes, the CoagUChek XS POC PT/INR reproduces the plasma-based PT/INR within +/– 30%, but the correlation is poor above 4.5, a critical value. Most laboratory directors require a plasma-based assay when the POC result exceeds 4.0. Richter develops a formula that uses a variable and a constant to normalize POC results to the POC INR when the INR exceeds 4.5. In the article, Richter correlates the mathematically-derived INR to the plasma-based INR in 30 patients whose POC INR is 4.0 to 5.9.
I would recommend that the investigators extend the study to incorporate a minimum of 120 patients and to include INRs in the 2–4 range to validate the formula over the entire therapeutic and supertherapeutic range. Also, because the formula is reagent and coagulometer dependent, the study should be extended to optical endpoint instruments and a variety of reagents. Given that it is somewhat risky to apply a fixed formula to all clinical situations, a local anticoagulation clinic should confirm the study for their own reagent and instrument combination.