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The effect of low-molecular weight heparin on thromboelastography in prengnancy - an in vitro study
Abstract Number: S-60
Abstract Type: Original Research
Background: Low molecular weight heparin (LMWH) is often used for prophylaxis or treatment of venous thromboembolism. Anti-Xa assays are used as a surrogate for LMWH activity, but there is a lack of consistency between LMWH dose, anti-Xa activity and anticoagulant effect(1). Due to the physiologic changes of pregnancy, standard weight-based dosing may underestimate the LMWH doses required for anticoagulation(2). Thromboelastography (TEG) is a point-of-care monitor of whole blood coagulation. The aim of this study was to determine if serial doses of LMWH added in vitro to whole blood samples from term, pregnant women changed TEG parameters in a dose-dependent manner.
Methods: ASA I or II parturients presenting for elective caesarean delivery were recruited. Blood was collected before delivery and serial dilutions of dalteparin in normal saline were added to yield final concentrations of 0 (control), 0.05, 0.25, 0.5, 0.75 and 1.0 U/ml anti-Xa activity. TEG tracings were obtained for all six samples using the standard kaolin protocol, and measured parameters included r time, k time, alpha angle and maximum amplitude (MA). Group medians underwent pair-wise multiple comparisons with Dunn testing. Receiver operating characteristic (ROC) curves were created for each TEG parameter and cut-off values for each parameter that best identified anti-coagulated samples (highest negative predictive value (NPV)) were also determined.
Results: 30 parturients were recruited. Samples containing ≤0.05 U/mL anti-Xa activity were considered “normal”, while ≥ 0.25U/mL were “anticoagulated”. TEG r time showed a dose-dependent response to increasing LMWH concentrations. There was a statistically significant difference in median TEG r time, k time, alpha angle and MA between normal and samples ≥ 0.5U/ml (p<0.05). r and k time ROC curves yielded an AUC of 0.99 and 0.94, respectively (Fig1).
Conclusions: This pilot study demonstrates that TEG is able to detect the presence of LMWH in maternal whole blood. TEG r time is the most sensitive and specific parameter for detection of LMWH and an r time cut-off of 6.1 min yields the best combined sensitivity and specificity. At this value, the NPV for detecting anticoagulated samples was 95%. This finding supports a need for further study to differentiate if TEG can be used to determine real-time coagulation status, particularly for safety of neuraxial techniques.
References: 1. J Trauma. 2009;66:1509-17. 2. Obstet Gynecol. 2008;112:884-9