Effect of citrate concentration in specimen collection tubes on the International Normalized Ratio.

PROBLEM

Does citrate concentration in specimen collection tubes affect the International Normalized Ratio?

METHODS

The International Normalized Ratio was determined on quadruplicate plasma specimens from 32 patients treated long term with oral anticoagulants-two from tubes with 3.2% citrate and two with 3.8% citrate. Two laboratories, using two different coagulometers, tested the specimens.

RESULTS

International Normalized Ratios of plasma from tubes with 3.8% citrate were significantly higher than those from tubes with 3.2% citrate when tested with either coagulometer. Patients given adequate anticoagulation on the basis of the International Normalized Ratio at one concentration of citrate appeared either overanticoagulated and at risk of bleeding or underanticoagulated and at risk of thromboembolism at the other concentration of citrate.

CONCLUSION

Results emphasize the need for using a single concentration of citrate for prothrombin time testing. We recommend 3.2% citrate.