Effect of 3.2 vs. 3.8% sodium citrate concentration on anti-Xa levels for patients on therapeutic low molecular weight heparin

Dabigatran Acylglucuronide, the Major Metabolite of Dabigatran, Shows a Weaker Anticoagulant Effect than Dabigatran

Dabigatran (DAB) is an orally administered thrombin inhibitor. Both DAB and its main metabolite dabigatran acylglucuronide (DABG) have established anticoagulant effects. Here, we aimed to compare the relative anticoagulant effects of DABG and DAB in humans. Anticoagulant effects of DAB and DABG were measured in vitro using a thrombin generation assay. Additionally, their effects on other coagulation assays including PT, aPTT, TT, and fibrinogen were compared. Both DAB and DABG showed inhibitory effects on thrombin generation in a dose-dependent manner, but DABG exhibited a weaker inhibitory effect than that of DAB. The IC₅₀ values of DAB and DABG on thrombin generation AUC were 134.1 ng/mL and 281.9 ng/mL, respectively. DABG also exhibited weaker anticoagulant effects than DAB on PT, aPTT, and TT. The results of the present study indicate that the anticoagulant effect of DABG, a main active DAB metabolite, is weaker than that of DAB.

From Activated Partial Thromboplastin Time to Antifactor Xa and Back Again

OBJECTIVES

Monitoring is essential to safe anticoagulation prescribing and requires close collaboration among pathologists, clinicians, and pharmacists.

METHODS

We describe our experience in the evolving strategy for laboratory testing of unfractionated heparin (UFH).

RESULTS

An intrainstitutional investigation revealed significant discordance between activated partial thromboplastin time (aPTT) and antifactor Xa (anti-Xa) assays, prompting a transition from the former to the latter in 2013. With the increasing use of oral factor Xa inhibitors (eg, apixaban, rivaroxaban, edoxaban, betrixaban), which interfere with the anti-Xa assay, we adapted our protocol again to incorporate aPTT in patients admitted on oral Xa inhibitors who require transition to UFH.

CONCLUSIONS

Our experience demonstrates key challenges in anticoagulation and highlights the importance of clinical pathologists in helping health systems adapt to the changing anticoagulation landscape.

Clot activators and anticoagulant additives for blood collection. A critical review on behalf of COLABIOCLI WG-PRE-LATAM

In the clinical laboratory, knowledge of and the correct use of clot activators and anticoagulant additives are critical to preserve and maintain samples in optimal conditions prior to analysis. In 2017, the Latin America Confederation of Clinical Biochemistry (COLABIOCLI) commissioned the Latin American Working Group for Preanalytical Phase (WG-PRE-LATAM) to study preanalytical variability and establish guidelines for preanalytical procedures to be applied by clinical laboratories and health care professionals. The aim of this critical review, on behalf of COLABIOCLI WG-PRE-LATAM, is to provide information to understand the mechanisms of the interactions and reactions that occur between blood and clot activators and anticoagulant additives inside evacuated tubes used for laboratory testing. Clot activators - glass, silica, kaolin, bentonite, and diatomaceous earth - work by surface dependent mechanism whereas extrinsic biomolecules - thrombin, snake venoms, ellagic acid, and thromboplastin - start in vitro coagulation when added to blood. Few manufacturers of evacuated tubes state the type and concentration of clot activators used in their products. With respect to anticoagulant additives, sodium citrate and oxalate complex free calcium and ethylenediaminetetraacetic acid chelates calcium. Heparin potentiates antithrombin and hirudin binds to active thrombin, inactivating the thrombin irreversibly. Blood collection tubes have improved continually over the years, from the glass tubes containing clot activators or anticoagulant additives that were prepared by laboratory personnel to the current standardized evacuated systems that permit more precise blood/additive ratios. Each clot activator and anticoagulant additive demonstrates specific functionality, and both manufacturers of tubes and laboratory professional strive to provide suitable interference-free sample matrices for laboratory testing. Both manufacturers of in vitro diagnostic devices and laboratory professionals need to understand all aspects of venous blood sampling so that they do not underestimate the impact of tube additives on laboratory testing.

The Effect of 3.2% and 3.8% Sodium Citrate on Specialized Coagulation Tests

Context.—

Coagulation testing is challenging and depends on preanalytic factors, including the citrate buffer concentration used.

Objective.—

To better estimate preanalytic effects of the citrate buffer concentration in use, the difference between results obtained by samples with 3.2% and 3.8% citrate was evaluated.

Design.—

In a prospective observational study with 76 volunteers, differences related to the citrate concentration were evaluated. For both buffer concentrations, reference range intervals were established according to the recommendations of the C28-A3 guideline published by the Clinical and Laboratory Standards Institute.

Results.—

In our reagent-analyzer settings, most parameters evaluated presented good comparability between citrated samples taken with 3.2% and 3.8% trisodium buffer. The ellagic acid containing activated partial thromboplastin time reagent (aPTT-FS) indicated a systemic and proportional difference between both buffer concentrations, leading to an alteration in its reference ranges. Further, a confirmation test for lupus anticoagulant assessment (Staclot LA) showed only a moderate correlation (rρ = 0.511) with a proportional deviation between both citrate concentrations. Further, a statistically significant difference was found in the diluted Russell viper venom time confirmation testing, coagulation factors V and VIII, and the protein C activity, which was found to be of minor clinical relevance.

Conclusions.—

With caution regarding the potential impact of the reagent-analyzer combination, our findings demonstrate the comparability of data assessed with 3.2% and 3.8% buffered citrated plasma. As an exception, the aPTT-FS and the Staclot LA assay were considerably affected by the citrate concentration used. Further studies are required to confirm our finding using different reagent-analyzer combinations.

Evaluation of the activated clotting time and activated partial thromboplastin time for the monitoring of heparin in adult extracorporeal membrane oxygenation patients

Introduction:

Historically, the activated clotting time (ACT) has been the preferred monitoring test of the heparin effect in extracorporeal membrane oxygenation (ECMO) patients. However, few adult studies have evaluated its correlation to the heparin dose or other monitoring tests, such as the activated partial thromboplastin time (aPTT). This retrospective study sought to evaluate the correlation between the heparin dose and these monitoring tests.

Methods:

Patients administered a heparin drip during ECMO were included in this study. The primary endpoints were the correlation between heparin dose and ACT or aPTT and the relationship between paired ACT and aPTT samples.

Results:

Forty-six patients met the criteria for study inclusion. A better correlation was observed for heparin dose and aPTT (Pearson product-moment correlation coefficient ( r) = 0.43 – 0.54) versus ACT ( r = 0.11 – 0.14). Among the paired sample data, ACT values did not differ significantly between Groups two (aPTT 60 – 75 seconds) and three (aPTT >75 seconds).

Conclusion:

The heparin dose correlated better with aPTT relative to ACT and, thus, may be considered a more effective tool for the dosing of heparin in adult ECMO patients. Paired ACT and aPTT sample data suggested a poor relationship between these two anticoagulant monitoring tests.

Antifactor Xa levels versus activated partial thromboplastin time for monitoring unfractionated heparin

Intravenous unfractionated heparin (UFH) remains an important therapeutic agent, particularly in the inpatient setting, for anticoagulation. Historically, the activated partial thromboplastin time (aPTT) has been the primary laboratory test used to monitor and adjust UFH. The aPTT test has evolved since the 1950s, and the historical goal range of 1.5-2.5 times the control aPTT, which first gained favor in the 1970s, has fallen out of favor due to a high degree of variability in aPTT readings from one laboratory to another, and even from one reagent to another. As a result, it is now recommended that the aPTT goal range be based on a corresponding heparin concentration of 0.2-0.4 unit/ml by protamine titration or 0.3-0.7 unit/ml by antifactor Xa assay. Given that several biologic factors can influence the aPTT independent of the effects of UFH, many institutions have transitioned to monitoring heparin with antifactor Xa levels, rather than the aPTT. Clinical data from the last 10-20 years have begun to show that a conversion from aPTT to antifactor Xa monitoring may offer a smoother dose-response curve, such that levels remain more stable, requiring fewer blood samples and dosage adjustments. Given the minimal increased acquisition cost of the antifactor Xa reagents, it can be argued that the antifactor Xa is a cost-effective method for monitoring UFH. In this review, we discuss the relative advantages and disadvantages of the aPTT, antifactor Xa, and protamine titration tests, and provide a clinical framework to guide practitioners who are seeking to optimize UFH monitoring within their own institutions.

Challenges in Variation and Responsiveness of Unfractionated Heparin

Unfractionated heparin (UFH) has been in clinical use for more than half a century. Despite its undoubted contribution to the treatment and prevention of thrombosis, heparin is significantly limited by its variable biochemical composition and unpredictable pharmacokinetics. The situation is compounded by the fact that methods for monitoring heparin do not necessarily reflect its therapeutic effect. The activated partial thromboplastin time (aPTT) is a method for monitoring heparin therapy that is simple, cheap, and readily available. However, it is also poorly standardized and is affected by numerous factors-both analytic and preanalytic-that are unrelated to the heparin effect. Establishing an appropriate therapeutic range for the aPTT is challenging for smaller clinical laboratories, and the antifactor Xa method of measuring heparin levels is not widely available. The College of American Pathologists published consensus guidelines in an effort to improve the laboratory monitoring of UFH therapy. However, it seems unlikely that the laboratory problems associated with monitoring UFH will be resolved. Unfractionated heparin is highly antigenic and carries a significant risk of heparin-induced thrombocytopenia (HIT). Even in the absence of thrombocytopenia or thrombosis, the presence of heparin-associated antibodies may predict adverse clinical outcomes and strengthen the rationale for the ultimate replacement of UFH. Fortunately, alternatives to UFH, such as low-molecular-weight heparins, direct thrombin inhibitors, and more specific factor Xa inhibitors, are becoming available for clinical use. The pharmacokinetics of these agents are more predictable and rely much less on laboratory monitoring. Nonheparin agents also eliminate the risk of HIT. The emergence of these newer anticoagulants makes the continued use of UFH increasingly difficult to justify.