Point of Care Hemostasis Testing: What's New?
ACVIM 2008
Marjory Brooks, DVM, DACVIM
Ithaca, NY, USA

Introduction

Point-of-care testing (POCT), also known as near-patient or "bedside" testing, refers to tests that are performed outside of a central laboratory to facilitate rapid results. Point-of-care hemostasis tests are typically performed to evaluate patients with active signs of hemorrhage, those likely to undergo surgery, or receiving anticoagulant drugs. The cost and complexity of POCT varies widely, from simple examination of a blood smear to computer-generated real-time tracings depicting kinetics of fibrin formation. All POCT are subject to errors of sample collection, instrument operation, and invalid reference ranges. The clinical utility of POCT depends not only on test performance, but the clinician's ability to select the appropriate test to answer a defined question.

The POC assays are broadly categorized as tests of primary hemostasis (platelet plug formation) or secondary hemostasis (fibrin clot formation). Most tests are designed to evaluate a single, specific aspect of these multi-step pathways. Recently developed POCT use technically sophisticated instrumentation to simulate complex in vivo processes with the goal of providing a more global assessment of hemostasis. The clinical utility of these POCT to improve disease diagnosis and treatment, however, remains to be established. The appropriate use of novel POCT and their integration with traditional laboratory tests are currently under active investigation in human and veterinary medicine.

Diagnostic Approach

Non-selective hemostasis testing has not proven cost-effective, and is not routinely recommended in medical practice.1,2 A directed diagnostic approach is reserved for patients with clinical signs of hemorrhage (or thrombosis), or underlying disease conditions or drug regimens likely to impair hemostasis. The results of simple POCT (Figure 1) will often rule out common hemostatic defects or indicate the laboratory assays likely to provide a definitive diagnosis. Nevertheless, careful attention to clinical history is warranted because neither POCT nor laboratory screening tests are sensitive to all hemostatic defects. Specific testing may be indicated regardless of initial screening test results.

Clinical Evaluation

Physical exam findings suggestive of primary hemostatic defects include mucosal hemorrhage, retinal hemorrhage, and ecchymoses. Petechiae are almost always an indicator of thrombocytopenia, however this sign is rarely seen in patients with von Willebrand disease (vWD). Hematoma formation and hemorrhage into body cavities (i.e., hemarthrosis, hemoperitoneum, hemothorax) are suggestive of secondary hemostatic defects (coagulopathies), but clinical signs alone do not reliably differentiate specific disorders. Severe bleeding diatheses often cause spontaneous hemorrhage, whereas mild to moderate forms become apparent only after surgery or trauma. A brief questionnaire with specific questions regarding hemostasis should be considered an integral part of the POC evaluation. Pertinent questions include:

 Has your pet ever had spontaneous bleeding from nose, mouth, urinary tract, or noticeable bleeding when teething?

 Has your pet ever had a surgical procedure, dentistry, or a traumatic injury-were there any bleeding complications?

 Are you aware of any bleeding problems in relatives, or in this breed?

 Is your pet receiving any medication or dietary supplements?

Figure 1. Diagnostic algorithm for initial evaluation of suspect hemostatic defects.
Figure 1. Diagnostic algorithm for initial evaluation of suspect hemostatic defects.

 

Point of Care Tests

Further refinements in traditional POCT include new instrumentation to standardize methodology and integrated data management systems for automatic analysis and transfer of test results to medical records. Trends in new POCT include both specific applications, such as monitoring intensity of antiplatelet drugs, and global monitors that aim to portray an overall balance of procoagulant and anticoagulant forces. A summary of POCT (Table 1) and their application is listed below.

Table 1. Point of care hemostasis tests.

Screening tests

Indication/ruleouts

Primary Hemostasis (Platelet/VWF)

Platelet Count: smear, manual, instrument

Thrombocytopenia, thrombocytosis

Template bleeding time

Platelet adhesion & aggregation defects, vWD

PFA-100

Platelet adhesion & aggregation defects, vWD

Whole blood clot retraction

Platelet aggregation (GpIIbIIIa) defects

Verify Now (Rapid Platelet Function Analyzer)

Drug monitoring (aspirin, GpIIbIIIa & ADP receptor blockers)

Secondary Hemostasis (Coagulation Cascade)

Activated clotting time (ACT)

*Intrinsic & common pathway factor deficiency/inhibition

Activated partial thromboplastin time (aPTT)

*Intrinsic & common pathway factor deficiency/inhibition

Prothrombin time (PT)

**Extrinsic & common pathway factor deficiency/inhibition

Fibrin Formation and Lysis Times

Thromboelastograph (TEG)

Abnormal rate of fibrin formation/lysis, abnormal clot strength

Sonoclot Clot Signature

Abnormal rate of fibrin formation/lysis, abnormal clot strength

* Intrinsic and common factors: Prekallikrein, kininogen, Factors XII, XI, X, IX, VIII, V, II, fibrinogen
** Extrinsic and common factors: Factors VII, X, V, II, fibrinogen

Platelet Tests

Platelet Count

Thrombocytopenia is the most common hemostatic defect, and platelet counts < 50,000/uL are readily detected by examination of a stained blood film. The finding of at least 10-12 platelets per oil immersion field (OIF) quickly rules out thrombocytopenia. Examination of the feathered edge of a smear (to detect platelet clumping) should also be performed as a confirmatory check of instrument or manual counts.3 Spurious thrombocytopenia due to platelet activation can be avoided by careful venipuncture technique with collection of blood directly into anticoagulant, maintenance of samples at room temperature, and preparation of the smear as soon as possible after collection. Because of the high prevalence of thrombocytopenia, determination of platelet count is always performed as an initial assessment of hemostasis.

Template Bleeding Time

Bleeding time tests are performed by making a superficial incision in a capillary bed and measuring the time to cessation of blood flow. A template device, triggered on the buccal mucosa of the upper lip in dogs (or cats), is used to standardize the incision. Control of hemorrhage from the bleeding time wound depends on formation of a stable platelet plug, but does not require generation of a fibrin clot. The expected bleeding time is less than 4 minutes, and marked prolongation of bleeding time (> 12 minutes) is found in patients with clinically severe defects of platelet adhesion and aggregation.4 Prolongation of bleeding time is compatible with vWD (due to a failure of platelet adhesion), hereditary thrombopathias, and acquired diseases or drugs that impair platelet aggregation. The bleeding time should only be performed in patients with normal platelet count, since thrombocytopenia prolongs bleeding time regardless of platelet function. The bleeding time endpoint is also influenced by hematocrit and blood viscosity. The technique is operator-dependent and wide daily variations have been reported.5 In human studies, bleeding time has proven to be a poor predictor of surgical hemostasis when used as a routine preoperative screening test.1

Platelet Function Analyzer (PFA-100®, Dade Behring)

The PFA-100 is a table-top instrument that measures platelet plug formation in citrate whole blood under conditions of high shear flow. The test endpoint (closure time) is the time for occlusion of an aperture coated with either collagen/ADP or collagen/epinephrine to stimulate platelet activation. Prolongation of closure time is expected in patients with defects of platelet adhesion or aggregation, similar to the template bleeding time. The instrument was developed to avoid the inherent variability and discomfort of bleeding time determinations, and has been used in human patients to aid in the diagnosis and management of vWD and platelet dysfunction. Closure times, however, are influenced by many of the nonspecific conditions that influence bleeding time including low hematocrit and changes in blood viscosity. Additional limitations for its use in animals include the failure of collagen/epinephrine to consistently induce platelet activation in dogs and the insensitivity of closure time to detect platelet antagonist treatment in cats.6,7

Dilute Whole Blood Clot Retraction

This simple test is performed by collection of whole blood directly into chilled saline and then allowing the blood to clot in glass tubes containing thrombin. The time for clot retraction from the sides of the tube depends on platelet count and the function of the platelet fibrinogen receptor (GpIIbIIIa). The test provides a rapid screen for thrombasthenia.8

VerifyNowTM(Accumetrics)

This instrument measures platelet aggregation in citrate whole blood samples and is designed to detect the antiplatelet effects of platelet inhibitory drugs (aspirin, clopidogrel, GpIIbIIIa antagonists).9 The assay is performed in test cartridges that contain fibrinogen coated beads and platelet agonists chosen to specifically test for the inhibitory action of different drugs. The VerifyNow instrument reports the intensity of platelet inhibition as platelet aggregation or drug reaction units (PAU or DRU). Although clinical trials utilizing this instrument have not been performed in animals, the VerifyNow IIbIIIa assay was recently found to be equivalent to whole blood aggregometry in monitoring the effects of abciximab in healthy cats.7

Coagulation Tests

Activated Clotting Time (ACT)

The ACT is a screening test of the intrinsic and common coagulation pathways. The test is performed by collecting whole blood directly into a vacuum tube containing particulate matter that initiates coagulation through activation of the contact pathway factors. The ACT is a functional test whose endpoint is formation of a fibrin clot. The time for clot formation can be determined visually or by photo-optical or mechanical endpoint instruments. Reference ranges for ACT should be determined in-house for the method in use, however the expected ACT for dogs is generally less than 120 seconds, and values for cats and horses less than 180 seconds.10,11 The ACT is sensitive to deficiency or inhibition of one or more factors in the intrinsic and common coagulation pathways and deficiency or inhibition of fibrinogen. The ACT is prolonged in animals with the most commonly encountered coagulopathies such as anticoagulant rodenticide toxicity, hepatic failure, hemorrhagic disseminated intravascular coagulation, and hereditary deficiencies of Factors VIII and IX (hemophilia A and B), Factor X, Factor XI, and Factor XII. The ACT is sensitive to unfractionated heparin's anticoagulant effect and the ACT remains the standard POCT for monitoring unfractionated heparin therapy in human and veterinary medicine. While ACT is a rapid and technically simple test, the assay may be influenced by nonspecific variables such as severe thrombocytopenia, anemia, changes in blood viscosity, and assay incubation temperature.

Activated Partial Thromboplastin Time and Prothrombin Time (aPTT and PT)

The aPTT and PT are screening tests of coagulation configured with reagents that specifically activate coagulation through the intrinsic or extrinsic pathways, respectively. Point-of-care coagulation instruments developed for use with human non-anticoagulated and citrate whole blood samples can be used to measure aPTT and PT in animals.12 Use of citrate whole blood allows a lag time between collection and assay performance and provides better test sensitivity and specificity for animal samples. Like the ACT, in-house aPTT and PT reference ranges should be generated for each assay method and reagent system. The endpoint of the coagulation screening tests is formation of a fibrin clot, with prolongation of clotting time indicative of factor deficiency or inhibition. Prolongation of aPTT, with normal PT, indicates an intrinsic pathway factor defect. Prolongation of PT, with normal aPTT, indicates a lack of Factor VII activity. Coagulopathies due to vitamin K deficiency cause prolongation of aPTT and PT, due to a deficiency of Factors II, VII, IX, and X. Severe hepatic failure and hemorrhagic DIC are often associated with prolongation of aPTT and PT, too, however in these cases fibrinogen deficiency and dysfibrinogenemia accompany multifactor deficiencies. Collection technique is important for valid aPTT and PT, whether performed as POCT or in the main laboratory. Poor venipuncture technique, and incorrect or inadequate anticoagulant cause ex vivo activation and depletion of coagulation factor activity and spurious findings of prolonged aPTT or PT. Samples should be drawn directly into 3.2% or 3.8% sodium citrate (in the ratio of 1 part citrate : 9 parts blood). Valid POC coagulation tests should identify clinically severe coagulopathies, however these assays may not identify mild to moderate factor deficiencies or more complex coagulopathies.

Viscoelastic Tests

Thromboelastograph (TEG®, Haemoscope)

The TEG is a whole blood assay that evaluates the cellular elements and plasma factors involved in clot formation, from initiation of clotting through the process of clot dissolution. The instrument provides a visual tracing that corresponds to the dynamic process of fibrin formation and subsequent fibrinolysis. The test has been evaluated in companion animals with the use of citrate whole blood and calcium or tissue factor/calcium reagents to initiate fibrin formation.13,14 In addition to the qualitative TEG tracing, derived numeric parameters are calculated: R (reaction time) is the time to initial fibrin formation; K (clotting time) and alpha angle reflect the time to develop a clot of specific tensile strength and the rate of clot formation. These parameters depend on platelets, clotting factors, and fibrinogen. The parameter MA (maximum amplitude) is a measure of absolute clot strength; an indicator of platelet/fibrin bonding and a reflection of platelet number and function. Interpretation of the TEG, therefore, is best performed in the context of the results of platelet and coagulation screening tests. The TEG holds promise as a "global" screening test to assess the effects of alterations in hemostasis caused by platelet dysfunction, procoagulant and anticoagulant imbalance, or fibrinolytic pathway defects. Applications of TEG in human, and more recently veterinary medicine, have focused on pathophysiology of hemorrhagic and thrombotic conditions and investigations of antithrombotic drug regimens.

Viscoelastic Coagulation and Platelet Function Analyzer (Sonoclot®, Sienco)

The Sonoclot, like TEG, is a whole blood assay that provides a dynamic clot "signature" depicting fibrin formation and derived numeric parameters designed to reflect the contribution of platelets and plasma factors in a global assessment of hemostasis.15 The numeric parameters generated by this assay include SonACT, a measure of time to initial fibrin formation, CR, the rate of fibrin formation, and PF a measure of platelet function in supporting clot retraction.

Current medical investigations of viscoelastic monitors have revealed some utility in optimizing transfusion protocols for cardiac bypass and liver transplant procedures, however their broader use as diagnostic tests in any species will require more extensive clinical evaluation.16-18

References

1.  Sie P, et al. Can J Anesth 2006;53:S12.

2.  Kamal AH, et al. Mayo Clin Proc 2007;82:864.

3.  Koplitz SL, et al. JAVMA 2001;217:1552.

4.  Brooks M, et al. Thromb Haemostas 1993;70:777.

5.  Sato I, et al. Res Vet Sci 2000;68:41.

6.  Mischke R, et al. Vet J 2003;165:43.

7.  Morrison TJ, et al. JVIM 2006 (abstr)242.

8.  Boudreaux. Vet Pathol 2001;38:249.

9.  Dyszkiewicz AM, Thromb Res 2007;120:485.

10. Gerber B, et al. JVIM 1999;13:433.

11. Bay JD, et al. J Vet Res 2000;61:7850.

12. Tseng LW, et al. Am J Vet Res 2001;62:1455.

13. Winberg B, et al. Vet Clin Pathol 2005;34:389.

14. Alwood A, et al. JVIM 2007;21:378.

15. Yamada T, et al. J Anesth2007;21:148.

16. Avidan MS, et al. Br J Anaesth 2004;92:178.

17. Hobson AR, et al. Platelets 2006;17:509.

18. Luddington RJ. Clin Lab Haematol 2005;27:814.

Speaker Information
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Marjory Brooks, DVM, DACVIM
Cornell University
Ithaca, NY


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