The agreement between capillary and venous sampling for plasma PT/INR results

Abstract

Regular measurement of prothrombin time (PT) as an international normalized ratio (INR) is of great importance in ensuring a safe Warfarin treatment due to the narrow therapeutic range. Hence, accurate results of the PT/INR analysis are crucial. Due to the high sampling load that patients on Warfarin experience, some prefer capillary sampling (from the tip of the finger) as an alternative to standard venous sampling (from the cubital fossa). The present study evaluated the method of capillary sampling compared to the standard venous method for plasma PT/INR measurement. Both venous and capillary samples were collected simultaneously for plasma PT/INR testing from 100 patients of which the majority undergoing active Warfarin treatment. The samples were analyzed on a Sysmex 2100i instrument (Siemens) using the Owrens method with PT reagents from MediRox. The capillary sampling method showed statistically significant lower values (p < .001) for plasma PT/INR than the standard venous sampling method with a mean difference (bias) of –0.14. It is improbable that the difference (bias) between capillary and venous sampling for plasma PT/INR analysis has clinical relevance. However, since the capillary blood collection and handling for plasma PT/INR is more technically demanding than the standard venous method we recommended that only specially trained personnel be entrusted to draw capillary blood for plasma PT/INR analysis.

Keywords:

• Prothrombin time
• international normalized ratio
• quality control
• anticoagulant
• warfarin

Acknowledgements

The authors thank Erling Englund for statistical advice and Nicolena Stephensen Nyberg for helpful linguistic comments.

Disclosure statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.