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Preanalytical Variables and Off-Site Blood Collection: Influences on the Results of the Prothrombin Time/International Normalized Ratio Test and Implications for Monitoring of Oral Anticoagulant Therapy

¹ Thrombosis Service, Department of Laboratory of Clinical Chemistry and Haematology, Jeroen Bosch Hospital, ‘s-Hertogenbosch, The Netherlands.
² Thrombosis Service and Contract Research, Departments of Stichting Huisartsen Laboratorium, Etten-Leur, The Netherlands.
³ Haemostasis and Thrombosis Research Center, Department of Haematology, Leiden University Medical Center, Leiden, The Netherlands.

^aAddress correspondence to this author at: Trombosedienst ‘s-Hertogenbosch, Jeroen Bosch Ziekenhuis, Deutersestraat 2, 5223 GV ’s-Hertogenbosch, The Netherlands. Fax 31-73-6998257; e-mail ankees.vangeest{at}planet.nl.

Background: The quality of oral anticoagulant therapy management with coumarin derivatives requires reliable results for the prothrombin time/International Normalized Ratio (PT/INR). We assessed the effect on PT/INR of preanalytical variables, including ones related to off-site blood collection and transportation to a laboratory.

Methods: Four laboratories with different combinations of blood collection systems, thromboplastin reagents, and coagulation meters participated. The simulated preanalytical variables included time between blood collection and PT/INR determinations on samples stored at room temperature, at 4–6 °C, and at 37 °C; mechanical agitation at room temperature, at 4–6 °C, and at 37 °C; time between centrifugation and PT/INR determination; and times and temperatures of centrifugation. For variables that affected results, the effect of the variable was classified as moderate when <25% of samples showed a change >10% or as large if >25% of samples showed such a change.

Results: During the first 6 h after blood collection, INR changed by >10% in <25% of samples (moderate effect) when blood samples were stored at room temperature, 4–6 °C, or 37 °C with or without mechanical agitation and independent of the time of centrifugation after blood collection. With one combination of materials and preanalytical conditions, a 24-h delay at room temperature or 4–6 °C had a large effect, i.e., changes >10% in >25% of samples. In all laboratories, a 24-h delay at 37 °C or with mechanical agitation had a large effect. We observed no clinically or statistically relevant INR differences among studied centrifugation conditions (centrifugation temperature, 20 °C or no temperature control; centrifugation time, 5 or 10 min).

Conclusions: We recommend a maximum of 6 h between blood collection and PT/INR determination. The impact of a 24-h delay should be investigated for each combination of materials and conditions.

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