Clinical impact of interlaboratory variation in International Normalized Ratio determinations

Variation among laboratories and point-of-care (POC) devices in Internation al Normalized Ratio (INR) determinations for individual patients in the sam e community was studied. For each patient, blood samples obtained from a single venipuncture followe d by a finger stick were tested at th ree laboratories and with two POC dev ices. A nonparametric analysis of variance and regression analysis were use d to compare INRs across sources. The potential clinical impact of differen ces between pairs of INR readings (i.e.. whether a difference was likely to result in a change in therapy) was evaluated. Complete data were obtained and analyzed for 41 patients. Of the five sourc es of INRs, results from two laboratories and one POC device were significa ntly similar. Results from one laboratory and one POC device were significa ntly different from the other results. Variation between INR readings from two different sources might have resulted in a change in therapy 10-30% of the time. Variations occurred in INR readings among three laboratories and two POC de vices and could have led to a substantial number of inappropriate changes i n therapy.