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Measurement of Prothrombin Time in EDTA Plasma with Combined Thromboplastin Reagent

2000; American Association for Clinical Chemistry; Volume: 46; Issue: 11 Linguagem: Inglês

10.1093/clinchem/46.11.1844

1530-8561

Juha Horsti,

Blood Coagulation and Thrombosis Mechanisms

Prothrombin time (PT) tests usually are performed on blood collected with citrate. Because blood samples for other common tests use EDTA as anticoagulant, the use of EDTA plasma for PT tests could be convenient, and we wished to explore this possibility. PT tests are needed at regular intervals for control of anticoagulant therapy with vitamin K antagonists (1), and they are the most commonly performed coagulation test in our laboratory. Two major reagent systems are used to measure PT: the Quick method and the Owren method (combined thromboplastin reagent). The latter is the main method in Nordic countries, Benelux, and Japan. The Owren method measures only coagulation factors II, VII, and X because fibrinogen and factor V are in the reagent. We studied only the Owren method, and our results may not apply to the Quick method, in which the sample constitutes a much larger portion of the final reaction volume. The effects of preanalytical factors on PT depend, in part, on the units in which PT results are expressed. In the Nordic countries, percentage has previously been the unit used. When percentages or seconds are used as the unit, the result depends on the reagent. Clinical laboratories are moving toward the use of a new International Normalized Ratio (INR) unit. The use of INR units, INR = (samples/normals)ISI, where “normal” is a plasma pool obtained from healthy persons, ISI is the International Sensitivity Index, is designed to eliminate dependence on sensitivity of the reagents so that the international result is the same and recommendations for care are uniform. INR units are not without problems: reagents do not behave in the same way, and in some cases, the citrate concentration in the sample affects results (2)(3)(4). In our study, we …