[Less variation in laboratory results after the introduction of INR. Differences between hospital laboratories and laboratories within primary health care are levelled out].

In 1999 a new and simplified procedure for calibration of the Owren prothrombin time (Owren PT) assay was introduced in Sweden by the national external quality assessment scheme (Equalis). The new protocol allowed local calibration by means of lyophilised national plasma calibrators and expression of results as an international normalised ratio (INR). A two-year follow-up involving analysis of data from all laboratories that have returned results to Equalis is reported. There was a significant reduction in both between-laboratory and within-laboratory variation after the introduction of the new calibration procedure. For the larger hospital laboratories analysing external controls with INR > 2, the mean coefficient of variation (CV) was reduced from 9.1% to 5.6% (P < 0.0001). The corresponding results from smaller laboratories in the primary health care units showed a similar decrease in CV from 8.8% to 6.3% (P < 0.0001). This study shows that the Owren PT assay is well suited for INR calibration employing calibrant plasmas.