To the Editor:

The accuracy of the devices discussed in “Improving Anticoagulation Management at the Point of Care” [February 2002, page 35] is of prime importance. How can I assure myself of the accuracy of these tools?

Authors’ response:

Questions of diagnostic and laboratory accuracy often assume a widely accepted, reliable, reference test or “gold standard.” Unfortunately, many standards contain more tin than gold. Such is the case with International Normalized Ratio (INR) anticoagulation assessment, as the thromboplastin reagents used to derive prothrombin times (PTs) vary across laboratories. This variance should be ameliorated by use of the International Sensitivity Index (ISI) to calculate the INR. However, even when using reagents with similar ISIs, resultant inter-laboratory INR values may still vary (Hirsh J, Dalen JE, Anderson DR, et al. Chest. 2001;119:8S–21S). Such variance may result in clinically relevant INR differences (Finck KM, Doetkott C, Miller DR. Am J of Health Syst Pharm. 2001;58:684–688).

Though several studies, including the study by Finck et al, have demonstrated that whole blood point-of-care anticoagulation monitors correlate highly with laboratory based serum INR assessments (CoaguChek to laboratory correlation coefficients of 0.92–0.95 and ProTime to laboratory coefficients of 0.87–0.90), it is unclear how much inter-laboratory correlation affects outcomes that matter to patients – the safe reduction of thrombotic events.

More important than choosing which device to employ is the method of employing that device. Patients who receive regularly scheduled provider-present anticoagulation assessments maintain their INRs in target ranges more often than those who use other methods (Jones RM, Pedersen CA. Hosp Pharm. 1998; 33:66–69). The clinical advantage of point-of-care devices is that they replace reliance on a single laboratory value with complete patient assessments.