Local INR calibration of the Owren type prothrombin assay greatly improves the intra- and interlaboratory variation. A three-year follow-up from the Swedish national external quality assessment scheme - Hillarp,A.; Egberg,N.; Nordin,G.; Stigendal,L.; Fagerberg,I.; Lindahl,T.L. (Thromb Haemost, 2004)