Accuracy, precision, and quality control for point-of-care testing of oral anticoagulation.
Oral anticoagulant (OAC) therapy is usually monitored by noting changes in a tissue factor-induced coagulation time ("prothrombin time") test on whole blood or plasma and expressed as an International Normalized Ratio (INR). Current point-of-care (POC) instruments for monitoring OAC therapy display both the calculated prothrombin time (PT) and the INR. Although many attempts have been made to improve the accuracy and precision of INR determinations in daily practice, it is impossible to eliminate all uncertainty because the PT test is sensitive to multiple factors in the patient's blood specimen. The accuracy of the average INR determined with a POC instrument depends on its calibration against reference methods. Quality control (QC) materials for POC devices are different from patients' samples and may not exactly reflect the real clinical situation. Nevertheless, internal and external QC schemes for POC devices are valuable to investigate their performance in daily practice. Calibration can be improved by direct comparison of a POC system against an established international reference preparation method. In general, the precision of the INR measured with a POC device is slightly lower than the precision achieved with available automated laboratory instruments. The greater imprecision should be weighed against the clinical advantages of a POC testing device.