Influence of haematocrit on international normalised ratio (INR) differences between a whole blood point-of-care coagulation monitor and reference prothrombin time in plasma

Summary

The CoaguChek S system is designed for use in patient self-testing. In this system a whole blood sample without sodium citrate is applied to a test strip containing thromboplastin and iron oxide particles. The detection principle is based on fibrin formation which inhibits and finally stops the movement of the iron oxide particles.In the classic prothrombin time (PT) test,ci-trate plasma is mixed with thromboplastin and an excess of calcium ions. In the monitoring of vitamin K-antagonist (VKA) treatment, all results are expressed on a common scale, i.e. international normalised ratio (INR). In patients on long-term VKA treatment, INRs were determined by the CoaguChek S system and reference methods for the classic PT. Four different CoaguChek S strip lots were evaluated.The difference in INR between the reference PT and the CoaguChek S system was negatively correlated to the haematocrit of the patients. We conclude that INR differences between CoaguChek S and plasma PT may be explained in part by the haematocrit.The magnitude of the effect of haematocrit,within the reference interval of 0.37 - 0.51,on the INR difference was not greater than approximately 10% for the combined data of the four strip lots.A bias of less than 10% seems to be acceptable clinically.

Keywords

Oral anticoagulant - prothrombin time - International Normalized Ratio - point-of-care testing - haematocrit

• References

• 1 Braun S, Watzke H, Hasenkam JM. et al. Performance evaluation of the new CoaguChek XS system compared with the established CoaguChek system by patientsexperienced in INR-self management. Thromb Haemost 2007; 97: 310-314.
• 2 Van den Besselaar AMHP, Breddin K, Lutze G. et al. Multicenter evaluation of a new capillary blood prothrombin time monitoring system. Blood Coag Fibrinol 1995; 06: 726-732.
• 3 WHO Expert Committee on Biological Standardization. Guidelines for thromboplastins and plasma used to control oral anticoagulant therapy. WHO Technical Report Series. Forty-eighth Report. WHO; Geneva: 1999
• 4 Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol 1997; 107: 105-110.
• 5 Marlar RA, Potts RM, Marlar AA. Effect on routine and special coagulation testing values of citrate anticoagulant adjustment in patients with high hematocrit values. Am J Clin Pathol 2006; 126: 400-405.
• 6 Van den Besselaar AMHP. A comparison of INRs determined with a whole blood prothrombin time device and two international reference preparations for thromboplastin. Thromb Haemost 2000; 84: 410-412.
• 7 Van den Besselaar AMHP. Accuracy, precision, and quality control for point-of-care testing of oral anti-coagulation. JThrombThrombolysis 2001; 12: 35-40.
• 8 Van den Besselaar AMHP. The response of Quick’s prothrombin time test to oral anticoagulation Influence of thromboplastin source and calcium chloride concentration. Arch Pathol Lab Med 1994; 118: 145-149.
• 9 Meijer P, Kluft C, Poller L. et al. A national field study of quality assessment of CoaguChek point-of-care testing prothrombin time monitors. Am J Clin Pathol 2006; 126: 756-761.
• 10 Poller L, Keown M, Chauhan N. et al. Reliability of international normalised ratios from two point of care test systems: comparison with conventional methods. Br Med J 2003; 327: 30.
• 11 Clinical and Laboratory Standards Institute. One Stage Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT) Test. H47-A2. Wayne, PA : Clinical and Laboratory Standards Institute. 2001