A Simplified Procedure for Thromboplastin Calibration -The Usefulness of Lyophilized Plasmas Assessed in a Collaborative Study^[*]

Summary

The International Sensitivity Index (ISI) of thromboplastins is determined by calibration using fresh plasmas from 60 patients stabilized on oral anticoagulants and 20 healthy subjects. This procedure is demanding, particularly for those who have no easy access to patients. The alternative use of a smaller number of lyophilized plasmas has already been considered, but one important issue, the number of repeated measurements to be carried out, has never been addressed. Two commercial rabbit thromboplastins, A and B, were calibrated in 3 laboratories against CRM 149R. On each of 10 working days, prothrombin times were measured for a different set of 8 fresh plasmas and for the same set of 8 lyophilized plasmas. The ISI values for both thromboplastins were estimated by orthogonal regression on fresh and lyophilized plasmas. The between- and within-laboratory CV values of the estimated ISI were taken as measures of precision of the calibration. In addition, ISI and CV were calculated daily on cumulative results obtained with lyophilized plasmas from day 1 to day 10. The ISI values for both thromboplastins calculated with lyophilized plasmas were not significantly different from those with fresh plasmas (mean of 3 laboratories: 1.42 vs 1.48 for A and 1.22 vs 1.20 for B). The between-laboratory precision of the calibration with lyophilized plasmas was not considerably different from that with fresh plasmas (CV for 3 labs: 5.2% vs 6.8% for A and 0.9% vs 2.2% for B). The ISI estimated with lyophilized plasmas on results of day 1 were not different from those of days 2 through 10. Good within-laboratory precision of the calibration (CV around 2%) was already achieved on day 3. In conclusion, this study shows that lyophilized plasmas pooled from normals and patients on oral anticoagulants can be used as substitutes for individual fresh plasmas to simplify the existing procedure for thromboplastin calibration.

• References

• 1 WHO Expert Committee on Biological Standardization. 33rd Report. Tech Rep Ser 687 81-105 Geneva: WHO 1983
• 2 Kirkwood TBL. Calibration of reference thromboplastins and standardization of the prothrombin time ratio. Thromb Haemost 1983; 49: 238-244
• 3 Thomson JM, Darby KV, Poller L. Calibration of BCT/441, the ICSH Reference Preparation for thromboplastin. Thromb Haemost 1986; 55: 379-382
• 4 van den Besselaar AMHP, Bertina RM. Multicenter calibration of the second reference material for thromboplastin, rabbit, plain, coded CRM 149R. Thromb Haemost 1991; 65: 263-267
• 5 BMPD Statistical software manual. Berkeley, CA: University of California Press 1990
• 6 Loeliger EA, van den Besselaar AMHP, Lewis SM. Reliability and clinical impact of the normalization of the prothrombin times in oral anticoagulant control. Thromb Haemost 1985; 53: 148-154
• 7 Poller L, Thomson JM, Tabemer DA, Clarke DK. The correction of the co-agulometer effects on international normalized ratios: a multicenter evaluation. Br J Haematol 1994; 86: 112-117
• 8 Poller L, Triplett DA, Hirsh J, Carroll J, Clarke K. The value of plasma cali-brants in correcting coagulometer effects on international normalized ratios. An international multicenter study. Am J Clin Path 1995; 103: 358-365