Point of care international normalized ratio devices

Answering Your Questions

Point-of-care INR devices

Question
 

We have numerous patients who are on Coumadin therapy. We perform many protimes each week for outpatients, as well as for patient specimens brought in from the home-health agency that is a part of our hospital.  The home-health agency wants to buy several point-of-care (POC) instruments and perform the tests onsite. Are the results from POC units comparable to what the lab’s analyzer will give? Do we need to verify POC instrument results?

Answer

A recent study examined the accuracy of three newer-generation POC international normalized ratio (INR) monitors vs. a lab reference method.¹ In general, performance of the devices has improved to the extent that large systematic bias does not exist between point-of-care and laboratory methods. Limitations still exist regarding precision, likely the result of decreased precision in capillary collection (vs. venipuncture) and limitations of the devices themselves.

With newer devices, users can expect minimal systematic bias and at least 90% of results to match the laboratory reference method within 0.5 INR units.¹ This level of accuracy generally allows Warfarin-dosing decisions to be made correctly or, at least, dosing discrepancies to be minimal. Users should be aware that there will be outliers with all point-of-care methods (i.e., some small percentage of results will be more than one INR unit off with point-of-care measurement).

In addition, a few studies have observed that the bias between POC and laboratory INR results varies over time and by strip lot for POC testing.^1-3 This means a rigorous quality-assurance program must be in place to monitor the performance of POC devices, especially when changing strip lots.

Another limitation of the POC devices is that they are designed and validated to monitor INR in patients stably anticoagulated on Warfarin. Performance during times of rapid change in INR has not been evaluated for most POC devices. Most of these devices have limited hematocrit ranges in which they operate optimally, and some have interference from heparin preparations if patients are on Heparin and Warfarin, simultaneously.

With those limitations understood, however, some groups have shown improved patient outcomes (both reduced bleeding and thrombosis) using POC management, real-time education, and dosage adjustment. Be aware that the real-time measurement will not lead to improved outcome without real-time patient education and dosage adjustment.

—Brad S. Karon, MD, PhD

References

1. Karon BS, et al. Accuracy of capillary whole blood international normalized ratio on the CoaguChek S, CoaguChek XS, and i-STAT point-of-care analyzers. Am J Clin Pathol. 2008;130:88-92.
2. Kitchen S, et al. Point-of-care International Normalised Ratios: UK NEQAS experience demonstrates necessity for proficiency testing of three different monitors. Thromb Haemost. 2006;96:590-596.
3. Meijer P, et al. A national field study of quality assessment of CoaguChek point-of-care testing prothrombin monitors. Am J Clin Pathol. 2006;126:756-761.

Brad S. Karon, MD, PhD, is associate professor of laboratory medicine and pathology, and director of the Hospital Clinical Laboratories, point-of-care testing, and phlebotomy services at Mayo Clinic in Rochester, MN.