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Does Thromboelastography Predict Postoperative Thromboembolic Events? A Systematic Review of the Literature

Dai, Yue, MB, MSc*; Lee, Anna, PhD*; Critchley, Lester A. H., MD*; White, Paul F., PhD, MD

doi: 10.1213/ane.0b013e31818f8907
Cardiovascular Anesthesiology: Research Reports
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BACKGROUND: Since thromboelastography (TEG) can detect hypercoagulable states, it is a potentially useful test for predicting postoperative thromboembolic complications. Therefore, we performed a systematic review of the literature to evaluate the accuracy of TEG in predicting postoperative thromboembolic events.

METHODS: PUBMED and EMBASE electronic databases were searched by two independent investigators to identify prospective studies involving adult patients undergoing operative procedures in which a TEG test was performed perioperatively and outcomes were measured by reference standards. The quality of included studies was assessed and measures of diagnostic test accuracy were estimated for each included study.

RESULTS: Ten studies (with a total of 1056 patients) were included in this analysis; however, only five reported measures of TEG test accuracy. The overall quality of the studies and level of diagnostic evaluation of the studies were highly variable, from poor to good. As there were variations in the definition of hypercoagulability, TEG methodology and patient characteristics, reference standards used and outcomes measured, a meta-analysis was not undertaken. The sensitivity and specificity ranged from 0% to 100% and 62% to 92%, respectively. The diagnostic odds ratio ranged from 1.5 to 27.7; area under the curve ranged from 0.57 to 0.91. Of the TEG variables, maximum amplitude seems to be the best parameter to identify hypercoagulable states and to predict thromboembolic events.

CONCLUSIONS: The predictive accuracy of TEG for postoperative thromboembolic events is highly variable. To determine if the TEG is a clinically useful screening test in high-risk surgical populations, more prospective studies are needed.

IMPLICATIONS: Based on the available data in the peer-reviewed literature, the predictive accuracy of thromboelastography (TEG) for postoperative thromboembolic events is highly variable. Therefore, more data are needed to determine if TEG is a clinically useful screening test for this purpose.

From the *Department of Anaesthesia and Intensive Care, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, NT, Hong Kong, China; and †Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, Dallas, Texas.

Accepted for publication September 18, 2008.

Supported by Department and Institutional Funding.

Reprints will not be available from the author.

Address correspondence to Anna Lee, PhD, Department of Anaesthesia and Intensive Care, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, NT, Hong Kong, China. Address e-mail to annalee@cuhk.edu.hk.

Thromboembolic (TE) complications, such as deep venous thrombosis (DVT), pulmonary embolism, and myocardial infarction, are not uncommon after major surgery. The occurrence of a TE event is associated with increased postoperative morbidity and higher health care costs.1,2 Surgery involving the lower extremities, trauma, prolonged immobilization, cancer, obesity, and the elderly are the most common perioperative risk factors for DVT.3 The incidence of postoperative venous TE event in patients not receiving thromboprophylaxis varies in different surgical populations, ranging from 20% to 25% (open meniscectomy) to 75%–80% (knee arthroplasty and spinal cord injury).4 The incidence of major perioperative cardiac events is 3.9% (95% CI 3.3%–4.6%) in noncardiac surgical patients undergoing major surgical procedures who had or were at risk of cardiac disease.5 Although the incidence of perioperative stroke ranges from 1% to 2.5%,6,7 up to 5% has been reported in patients undergoing coronary artery bypass graft surgery.8 Routine use of prophylaxis against TE events is recommended in high-risk patients i.e., those who have undergone a recent surgical procedure, advanced cancer, history of TE complications, advanced age, and prolonged immobilization. However, TE events may develop even in patients receiving standard prophylaxis.9

Clinical prediction rules have been developed because standard laboratory tests often have poor predictive values.10 The rules are based on high risk factors and physical findings to estimate the probability of DVT. Since Virchow initially identified a triad of factors associated with TE complications in 1856,11 hypercoagulability has been implicated in the pathogenesis of TE events. Although there is no routine coagulation test that can detect hypercoagulability, thromboelastography (TEG) is a technique that allows rapid global assessment of hemostatic function using whole blood.12 It has been used in clinical settings to detect and quantify hypo- and hyper-coagulability, fibrinolysis, clot strength, and anticoagulant drug effects. Several studies have shown that the TEG is able to detect hypercoagulable states in the clinical setting.13–18 Therefore, a systematic review was performed to evaluate the diagnostic accuracy of TEG for predicting postoperative TE complications in “at risk” surgical patients.

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METHODS

Research Question

We sought to determine the accuracy of perioperative TEG testing for the prediction of postoperative TE events (venous or arterial) in adult surgical patients.

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Search Techniques

Two of the authors (Y.D. and A.L.) independently searched the electronic databases PUBMED and EMBASE for clinical studies assessing the utility of parameters of TEG to predict postoperative TE complications in adult surgical patients. The electronic search used the following subject headings: thrombelastography and postoperative complications (myocardial infarction, or thrombosis, or pulmonary embolism, or intracranial embolism and thrombosis, or graft occlusion, vascular). The search dates were limited to 1980 to February 2008, without language restriction. The reference lists of included studies and previous reviews were screened to identify further studies for possible inclusion.

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Inclusion Criteria

All studies that met the following criteria were included in this systematic review: namely, a prospective clinical study involving adult patients undergoing surgery who had TEG tests performed perioperatively with TE complications (as defined by the authors) which was confirmed by an appropriate reference standard.

We initially retrieved 18 articles using the search strategies outlined above. After the search, full-texts of the studies were retrieved and assessed independently by the two reviewers. One study was excluded because the full text was not available in the Hong Kong library system or by contacting the principal author.19 When there was disagreement, consensus was reached by discussion among the investigators. Seven studies were excluded because no clinically relevant outcomes were identified.20–26 Two reviewers independently extracted data from 10 eligible studies27–36 using a standard data collection form. The primary authors were contacted by e-mail or fax for further data that were not presented in the original publication (one author replied by e-mail but was unable to provide additional information).

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Reference Standards and Outcome Measures

The “gold standard” for the diagnosis of DVT is venography, but we considered compression ultrasound and Doppler imaging or 125I-labeled fibrinogen uptake test as acceptable alternatives. Diagnosis for pulmonary embolism was confirmed by autopsy, ventilation/perfusion scanning or pulmonary computed tomography angiography.

We included the following secondary outcomes: myocardial infarction (confirmed by electrocardiography and biochemical markers of myocardial necrosis), ischemic stroke (confirmed by head computerized tomography), and vascular graft occlusion (confirmed by computerized tomography angiography or venography).

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Quality Assessment

The quality of the studies was scored using the criteria of Lijmer et al.37 The appropriate spectrum of cases included consecutive series of patients undergoing relevant surgical procedures. If TEG was evaluated in separate groups of patients already known to have or not to have experienced TE events, this was referred to as a case-control study and was not included in this systematic review. Verification was considered complete if both TEG and at least one reference test were performed on all the patients. Partial verification occurred if more than 10% of the study group was not subjected to the reference tests. Differential reference standard occurred if different reference tests were used for one type of TE outcome. The details of a test were graded sufficient if there were clear definitions of positive and negative test results, details about the type of TEG methodology used, information about blood sampling, and a definition for normal or hypercoagulability states. The description of the study population was graded sufficient if the patient characteristics (i.e., age of participants, male-to-female ratio, and type of surgery) were described.

To examine the impact of the test on health outcomes and society, we evaluated the studies of TEG by categorizing them into 6 levels using the criteria of Fryback and Thornbury (Table 1).38 This framework is hierarchal with studies graded from level 1 (technical feasibility) to level 6 (societal impact).

Table 1

Table 1

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Statistical Analysis

The sensitivity, specificity, positive likelihood ratio and negative likelihood ratio, and a diagnostic odds ratio were calculated for each included study. The potential problems associated with sensitivities and specificities of 0% were solved by adding 0.5 to all cells of the diagnostic 2 × 2 table.39 The diagnostic odds ratio and 95% confidence intervals (95% CI) were converted to an equivalent area under the curve40 to facilitate a qualitative assessment of TEG accuracy.41 The area under the curve can also be thought of as the average sensitivity (true positive rate) over the entire range of false positive rate (1–specificity) values.40 As there were variations in the definition of hypercoagulability, TEG methodology, patient characteristics, reference standard used, and clinical outcome measures, a summary receiver operator characteristic curve42 was not created to pool the results.

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RESULTS

Ten studies published in the peer-reviewed literature from 1981 through 2005 were identified (Table 2). In the study by Wen et al.,27 11 patients were excluded because they were part of a retrospective thrombosis group. Overall, 1056 patients were included in prospective cohort or randomized, controlled trials in this systematic review. The characteristics of the included studies are summarized in Table 2. The mean age of the patients studied ranged from 36 to 82 yr, and the male-to-female ratio varied across all studies. Most patients underwent surgery with general anesthesia. The majority of studies reported favorable results supporting the use of TEG for prediction of TE events (Table 2).

Table 2

Table 2

The characteristics of the TE events are presented in Table 3. The prevalence of TE events varied from 1% to 45%. The TE events included arterial, venous and vascular graft thrombosis, with 8 of the 10 studies evaluating DVT. Most of the studies followed the patients for 1 wk, and all TE events were confirmed by at least 1 reference test. Of the 8 studies evaluating DVT, 4 confirmed DVT using ultrasound,31–34 2 studies confirmed using the fibrinogen uptake test28,29 and 2 studies using venography.27,30 As part of standard clinical practice, most of these patients were given mechanical or pharmacological prophylaxis against DVT.

Table 3

Table 3

The TEG characteristics of the included studies are displayed in Table 4. Six studies used the TEG device manufactured by one company (Hemoscope). There was no consensus on the definition of hypercoagulability, with 6 of the 10 studies including maximum amplitude (MA)27,28,30,32,35,36 as a criteria for hypercoagulability. Four studies28,30,32,35 found MA changed significantly in patients with TE disorders compared to the normal subjects, or after surgery versus before the operation. However, only two of the studies27,32 provided actual “cutoff” values for hypercoagulability (Hemoscope MA values of 68 and 70 mm, respectively). In the other four studies, summated values (TEG® index derived from parameters including MA) were used. The best sampling time for detecting TE events was in the early postoperative period, with eight studies obtaining blood samples during this period.

Table 4

Table 4

The results of our assessment of the study quality and diagnostic test evaluation are summarized in Tables 1 and 5, respectively. The quality of the eligible studies varied widely, with 1 study scoring 3 and 2 studies scoring 7. Investigators were not blinded in their interpretation of test results in 7 studies. In most studies, there were insufficient details of the TEG test sampling and the reference standard used. Only one study tried to evaluate the impact of TEG, on patient outcome, but the extent of the clinical impact could not be determined.31 Four studies assessed the technical feasibility (level 1) of TEG testing (Table 1), and 5 studies27,32–34,38 had sufficient data to determine the test accuracy measures. The remaining five studies compared the mean values of TEG parameters of diseased versus normal subjects and, therefore, the sensitivity and specificity could not be determined. A summary of the accuracy of TEG testing is shown in Table 6. The sensitivity and specificity of the 5 studies ranged from 0% to 100% and 62% to 92%, respectively. The positive and negative likelihood ratios ranged from 1.43 to 7.67 and 0.28 to 0.96, respectively. The ratio of odds for a hypercoagulable state to be detected by TEG in a patient with a TE event, compared to a patient without a TE complication, ranged from 1.5 to 27.7, representing the equivalent area under the curve of 0.57 to 0.91 (Table 6).

Table 5

Table 5

Table 6

Table 6

Using data from the study with the largest sample size of the five studies provided the most precise measure of test accuracy.32 If the pretest probability of a TE event in a postoperative patient was 4% (the prevalence of this study), a positive TEG test would generate a new posttest probability of 8% and a negative TEG test would generate a posttest probability of 1%. In patients at higher risk, if the pretest probability was 45% (which is the highest prevalence reported in the 10 studies included in this analysis),35 a positive TEG test result increased the posttest probability to 63%, and a negative TEG test decreased the probability to 21%.

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DISCUSSION

This is the first systematic review to examine the diagnostic value of TEG to predict DVT and other TE events in postsurgical patients. The majority of studies reported favorable results associated with the use of TEG testing. Although the MA value seemed to be the best parameter for defining a hypercoagulable state, there was no consensus regarding the minimum (cutoff) MA value that should be used for predicting TE events. Based on the 5 studies with measures of test accuracy, the diagnostic odds ratio ranged from 1.5 to 27.7, area under the curve ranged from 0.57 to 0.91, suggesting that TEG testing had considerable variability in its accuracy for predicting postoperative TE events. Of note, 3 studies showed that the TEG had diagnostic odds ratios of 5–6 for predicting postoperative TE events.28,32,33 Therefore, when TEG is used alone, the posttest probabilities did not change significantly from the pretest probabilities with respect to their ability to alter clinical decision-making.27,28,32,33 The TEG may, however, have some incremental value when combined with the Wells’ score43 or other decision-making aids for predicting TE events.

The TEG has been used to evaluate both preexistent and acquired hypercoagulable states.13–18 Compared to healthy reference subjects, patients with hyperhomocysteinaemia and heterozygosity for the factor V Leiden polymorphism had a significantly greater hypercoagulant whole blood coagulation clot signature as defined by a shortened clotting time and an accelerated maximum velocity of clot propagation.13 TEG can also serve as a useful adjunctive test for thrombophilia screening, particularly in patients in whom the regular thrombophilia screen tests were normal.44

Surgery itself is an important contributing factor to hypercoagulable states detected by TEG testing.16,45–48 The mechanisms responsible for surgery-induced hypercoagulation include tissue factors released from damaged vessels, local tissue trauma, increases in stress, hormone levels, inflammatory cytokine network activation and compromised fibrinolysis. Schreiber et al.45 reported that hypercoagulability after traumatic injuries was most prevalent during the first 24 h, and TEG testing was more sensitive than routine coagulation assays for the detection of a hypercoagulable state. Mahla et al.14 revealed independent contributions of platelets and procoagulatory proteins to clot strength. Spiel et al.15 reported that endotoxemia significantly shortened TEG clotting time and maximal lysis, which correlated with established in vivo markers of coagulation activation (F[1 + 2] and fibrinolysis [t-PA]), respectively. Several studies have confirmed the changes in fibrinolysis detected by TEG testing,49–52 but the relationship between hyperfibrinolysis and TE events requires further study.

In the last 26 yr, many changes have occurred in TEG methodology, which helps explain the heterogeneity observed in TEG test characteristics and reference ranges. Wide variability was reported in the mean values of TEG variables between patients with and without TE events. Of the six studies with a clear definition of hypercoagulability, only two had sufficient data for patients to be classified as either normal or hypercoagulable. There was no universal definition among the 10 studies for defining the value for normal or hypercoagulable state. Obviously, the heterogeneity of the methodology was the main reason for the lack of consensus in reference ranges. There were two commonly used TEG devices, namely TEG® (Hemoscope Corporation, Niles, IL) and the RoTEM® (Pentapharm GmbH, Munich, Germany).51 These two monitors use different reaction cups and activators, which helps to differentiate coagulation, platelet function, and fibrinolysis. Although most of the parameters are comparable and measure similar changes in the tracings (e.g., both the MA of TEG® and maximum clot firmness of RoTEM® represent the ultimate strength of the fibrin clot as defined as the maximal width of the tracing), one needs to be aware that differences may be found when comparing results using TEG® and RoTEM®. For example, a previous study53 found divergent results when these two devices were directly compared, and these differences could affect clinical decision-making. Additional comparative studies are clearly needed to develop clinically useful reference ranges. One multicenter study reported that the reference ranges were comparable among centers.54 Therefore, it is likely that with the introduction of more sophisticated techniques and reliable reference ranges, TEG testing will be more useful for evaluating hypercoagulable states in the future.

Although reagents for the individual pathways provide differentiation between the coagulation system, platelet, fibrinolysis and the influence of anticoagulant drugs, TEG testing by its nature is a “global assay” for the entire hemostasis system. The ability to detect the systematic changes caused by different coagulation disorders makes TEG a suitable tool to predict postoperative TE events because there are many factors affecting the hemostasis system during the perioperative period. Differentiated assays are more useful in attempting to understand the underlying mechanism(s) involved and the pathway(s) that are altered.

Theoretically, it would be optimal if preoperative TEG measurements could predict postoperative TE events. Earlier detection of hypercoaguable states (i.e., before the operation or at early stage after the operation) would facilitate intervention with appropriate anti-TE therapies. The results of this review suggest that evoked hypercoagulability in the early postoperative period is the most important finding for predicting TE complications. A more robust therapeutic response to early changes in a patient’s coagulation status as assessed by TEG might prove to be effective in preventing the development of postoperative TE events. Therefore, the early postoperative period may be the optimal sampling time, and 8 of the 10 studies obtained blood samples during this time period. It may be helpful in future studies to evaluate the diagnostic accuracy of preoperative versus early postoperative TEG values. Importantly, the clinical outcome variables should be standardized when evaluating the performance of the TEG as a diagnostic test.

Among the 10 studies, there was a high degree of heterogeneity in patient characteristics and reference standards, contributing to the wide variability observed in patient outcome measurements. The specific TE events included arterial, venous and vascular graft thrombosis, and the reference standards for these events also varied across all studies. The incidence of DVT varied regardless of the surgical population and included both symptomatic and asymptomatic patients. Given the clinical heterogeneity among studies, statistical pooling of test accuracy measures as part of a meta-analysis of these data would not have been appropriate.

The optimal design for assessing the accuracy of TEG testing as a predictor of TE complications is a prospective, blinded comparison of TEG and the reference test in a consecutive series of patients from a relevant (at risk) surgical patient population.55 The results of this systematic review demonstrated that the quality of the published studies is highly variable, ranging from poor (scored as 3) to good (scored as 7), and none of the studies completely fulfilled all the requirements for a high-quality diagnostic test study.37,38 Moreover, only a few studies described the study methodology in sufficient detail for replicating the findings. A majority of the studies lacked adequate details for interpretation of TEG and reference test results. The use of a blinded study design is crucial for preventing investigator bias in the clinical diagnostic evaluations, as unblinded assessments can inappropriately inflate the accuracy of a diagnostic test.56 The reference tests were not performed in all patients in 4 of the 10 studies.30,32,34,35 Since most of the patients are asymptomatic, verification bias might complicate the interpretation of these data and lead to a biased estimate of the diagnostic test’s accuracy. None of the studies performed an a priori sample size calculation and, therefore, there is uncertainty about the power of the negative studies. Of the 5 studies which provided information regarding sensitivity and specificity, 3 had relatively small sample sizes (10–65 patients).

The results of the level of diagnostic test evaluation suggested that most of the studies were poor-to-moderate quality. Four of the studies were focused on the technical feasibility (level 1), which is considered a poor diagnostic level.38 To be included in a meta-analysis of a diagnostic test, the studies should be homogeneous and ideally a minimum of a level 2 diagnostic test evaluation. Only one study31 evaluated the patient outcome impact of TEG, but it failed to reach statistical significance. Another study33 evaluated the impact of the therapeutic choice, which is considered a moderate level of diagnostic test evaluation.

In this literature review, there are some important deficiencies which must be acknowledged. First, the heterogeneity of study population and TEG characteristics was the greatest obstacle to performing statistical pooling of these data. There were insufficient data available concerning a single type of surgical procedure, patient characteristic or TEG device. Second, most of these studies used different definitions of hypercoagulability. Considering the heterogeneity of the TEG methodology, we used the definitions provided by the authors to calculate specificity and sensitivity, and then compared the results across the studies meeting the prespecified inclusion criteria. Another important confounding factor was that patients manifesting symptoms of hypercoagulability, as determined by an abnormal TEG variable, were promptly treated with standard TE prophylaxis regimens. This clinical practice would compromise the predictive value of TEG testing. Therefore, it is not surprising that sensitivity and specificity values of 50% or less were obtained in this analysis. Future prospective studies should be well-designed and more rigorously controlled cohort studies using the same treatment regimens for all patients.

In conclusion, the predictive accuracy of TEG measurements for postoperative TE events in adult postsurgical patients is highly variable; an increased MA seems to be a useful risk factor for postoperative TE events. Due to the lack of adequate data, it was not possible to perform a meta-analysis. The overall quality of the studies in this systematic literature review varied widely, and the diagnostic test evaluation was generally of a low level. High-quality diagnostic studies examining the performance of TEG as a predictor of TE events are needed in order to determine whether TEG testing is a clinically useful screening test for high risk surgical patients.

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