Laboratory Testing of Hemostasis
Hemostasis is the result of a complex interaction between vessels, platelets, and coagulation proteins that stops bleeding (primary hemostasis, coagulation) while maintaining blood flow in the vessel (fibrinolysis).
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Laboratory Testing of Hemostasis Fanny Bonhomme and Pierre Fontana
2.1
Introduction
Hemostasis laboratories can carry out large numbers of assays either to obtain accurate and comprehensive diagnoses of hemostatic abnormalities or to monitor antithrombotic treatment. Activated partial thromboplastin time (aPTT) and prothrombin time (PT) are the most prescribed routine tests. The evaluation of a patient’s history, symptoms and clinical signs are essential to assess their bleeding tendency and to determine which laboratory test to use. Moreover, in order to interpret laboratory test results, it is important to understand how the assays are performed and to be aware of their limitations. A normal range (reference range) is defined as the interval into which 95 % of the values of a reference population fall; thus, 2.5 % of values are inferior to the lower limit, and 2.5 % are superior to the upper limit (Fig. 2.1). Applied to hemostatic testing, this means, for example, that 2.5 % of healthy individuals have a prolonged aPTT (longer than the upper limit).
2.2
Importance of Sample Collection, Processing, and Storage
Pre-analytical variables strongly influence hemostasis test results, and particular attention should be paid to blood collection, sample transportation, and storage. The accuracy of hemostasis tests depends on the sample quality (Lippi et al. 2012). F. Bonhomme (*) Division of Anesthesiology, Geneva University Hospitals, Geneva, Switzerland e-mail: [email protected] P. Fontana Division of Angiology and Hemostasis, Geneva University Hospital and Faculty of Medicine, Geneva, Switzerland C.E. Marcucci, P. Schoettker (eds.), Perioperative Hemostasis, DOI 10.1007/978-3-642-55004-1_2, © Springer-Verlag Berlin Heidelberg 2015
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F. Bonhomme and P. Fontana
Fig. 2.1 Normal range definition. When values of the reference population are normally distributed, the reference range is defined as the interval containing 95 % of the values
Normal range
95 % 2.5 %
2.5 %
Blood collection must be as thorough as possible in order to obtain reliable results: samples should be obtained from a peripheral vein using an atraumatic puncture, away from any intravenous perfusion line. Tubes containing 3.2 % (0.109 M) sodium citrate are recommended as other anticoagulants may yield invalid results. These tubes must be carefully filled to predetermined levels in order to respect the blood-to-anticoagulant ratio and then gently inverted five times to mix them together. The first few milliliters of blood collected after the puncture should be discarded.
2.3
Coagulation Testing
2.3.1
Coagulation Cascade
The cell-based coagulation model focuses on the successive steps of thrombin generation. These are initiation, propagation, and a burst of thrombin generation that occurs in the close vicinity of cell membranes (i.e., platelets) that provide the phospholipids required for the coagulation reaction (see Chap. 1). Depending on how coagulation is initiated, the classic coagulation cascade describes two somewhat artific
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