Utility of Thromboelastography for the Diagnosis of Von Willebrand Disease

Background: Von Willebrand Disease (VWD) is the most common bleeding disorder. Current gold standard diagnostic testing includes: VWF Activity (VWF:RCo), VWF Antigen (VWF:Ag) and Factor VIII Activity (FVIII). There are many difficulties associated with the current diagnostic methods. Thromboelastography (TEG) is a viscoelastic method of measuring coagulation function. The standard TEG assay has not been thought to be of use in VWD because of the lack of shear stress which is essential for the activation of VWF. Modified TEG using Ristocetin activation has been found to be useful in the diagnosis of VWD. The aims of this study were to evaluate the different parameters of Tissue factor (TF) initiated TEG in patients with VWD, to determine if this assay is sensitive to dysfunctional/low levels of VWF, as this does not require any significant change in procedure except for the use of TF as the activator instead of Kaolin.

Methods: A retrospective chart review of patients who presented for a bleeding disorder workup that had TF initiated TEG analysis and Von Willebrand laboratory tests completed between January 2007 and December 2015 was performed. IRB approval was obtained, and current diagnostic tests for Von Willebrand Disease (CBC with platelet count, VWF:RCo and VWF:Ag, FVIII, ABO blood type; PT, PTT, Fibrinogen) and TF initiated TEG parameters, specifically K-Time and MRTG (Maximum rate of thrombin generation), were compared. To perform the TEG analysis, the citrated whole blood samples were activated using 20mL of 1:10,000 dilution of recombinant human tissue factor (Innovin, Dade Behring) and CaCl₂.

Results: A total sample size of 160 patients (ages ranging 2 weeks to 18 years) who had a workup for a bleeding disorder that included Von Willebrand studies and TEG were reviewed. Of these 160 patients, 75 patients had a VWF:RCo <50 IU/dL. These patients were categorized into two categories for the purposes of this study: Low VWF:RCo (30-50 IU/dL) and Abnormal VWF:RCo (<30 IU/dL). The clinical and lab characteristics of patients studied are presented in Table 1, including FVIII, platelet count, and fibrinogen levels. Fibrinogen levels on 15 patients with VWF:RCo <50 IU/dL were not reported, and are not included in the means as noted in Table 1. The TEG parameter, K-Time, (time for increase in amplitude from 2mm to 20mm representing the dynamics of clot formation, normal <2min) was determined abnormal for values greater than 2.2 (>10% above the normal). Of the patients with VWF:RCo <30 IU/dL, 23/30 (77%) had an abnormal K-Time of >2.2, with a p-value of ≤0.001 (Table 2); whereas patients with low VWF:RCo 30-50 IU/dL, only 13/45 (29%) had an abnormal K-Time of >2.2, which was not statistically significant. A ROC curve for patients with VWF:RCo <30 IU/dL and abnormal K-Time showed an area under the curve of 0.675, with a p-value of 0.003 (Fig 1). An analysis of the MRTG showed a mean of 9.45 in patients with VWF:RCo <30 IU/dL and a mean of 11.26 in patients with normal VWF:RCo, which was statistically significant (p=0.05). There was a moderate correlation (0.33) seen between patients with abnormal K-Time and FVIII, however, when analyzing regression data the correlation only accounts for 11% of the explained variance in abnormal K-Time values. Patients with abnormal K-Time and abnormal fibrinogen levels and platelet counts were also compared, and showed no significant correlation, indicating that these were not the determinants that influenced the K time.

Conclusions: The TF activated TEG reflects impaired clot formation in patients with VWF <30% as reflected by the prolongation in the K-Time and the low MRTG. The TEG unlike VWF:RCo can be done in real time and results are available to the clinician within an hour. This will definitely be beneficial in acute situations like evaluation of and management of acute bleeding in patients with acquired deficiencies of VWF and may play an important role in the surgical management of patients with VWD. It will also help physicians monitor response to treatment, frequency of treatment, and the need for prophylactic dosing in patients with VWD. The next step will be to evaluate if this difference is also seen with Kaolin, which is the standard TEG assay. We anticipate that Kaolin being a stronger agonist may make the test less sensitive to VWF related changes.

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