Clinical laboratories are often on the front lines when new pharmacologic therapies are implemented, and utilization of already available specific assays can help to improve patient outcomes. The landscape is changing for patient anticoagulation.
Therapeutic alternatives for VTE
Patients with acute or ongoing risk of venous thromboembolism (VTE), also known as thrombosis, are prescribed anticoagulant blood-thinning therapies. Therapeutic options for outpatients at risk of VTE have greatly expanded in recent years with the introduction of direct oral anticoagulants (DOACs). Of the DOACs, dabigatran was the first introduced to the market for nonvalvular atrial fibrillation (NVAF) patients at risk of stroke. Rivaroxaban and apixaban closely followed dabigatran to the market; they were first indicated for NVAF patients, and then were approved to use in patients at risk of thrombosis following orthopedic surgery, and for thrombosis prevention and treatment in other patient types. Edoxaban is another recent entry to the market, intended for NVAF and orthopedic patients. It was followed by betrixaban, which is intended only for inpatients with thrombotic risk.
With the exception of betrixaban, the DOACs were approved for NVAF patients after demonstration of noninferiority to vitamin K antagonist (VKA) therapy, or warfarin, in large-scale clinical trials. Similar noninferiority trials comparing DOACs to other mainstays of anticoagulant therapy were conducted to gain approval for the other indications. Unlike warfarin, which acts against many factors within the coagulation cascade, dabigatran is a specific, small molecule inhibitor targeting thrombin, just one component in the coagulation cascade. Similarly, rivaroxaban, apixaban, edoxaban, and betrixaban target a different key enzyme in the cascade, factor Xa. For those confused by the generic DOAC names: for the DOACs directed against factor Xa, “xa” is included in the name of the drug, which is “banning” the activity of factor Xa.
Benefits and limitations of DOACs
Compared to VKAs, DOACs have the perceived benefits of no routine monitoring required, faster onset and offset of action, fewer interactions with food and other medications, and less complex dosing requirements. On the other hand, DOACs utilize the renal system for clearance from the body, and patients with kidney failure or borderline function will require maintenance with VKA therapy. Mechanical heart valve patients also require VKA therapy, as DOACs do not exert enough of a multifactor anticoagulant effect compared to VKAs. We can expect to see additional indications for DOACs in the future for patients with acute coronary syndrome (ACS) and cancer-associated thrombosis and for pediatric patients.
Often, newly diagnosed thrombotic risk patients receive DOAC prescriptions, whereas patients with historical successful maintenance on VKA probably will not be transitioned to DOACs. A lack of specific antidotes and laboratory measurement assays cleared by regulatory bodies in the United States has produced concerns for clinicians. Regardless of those concerns, however, prescription trends have shown strong growth since the introduction of DOACs. Of the DOACs, apixaban has received more attention, due to superior outcomes in large-scale clinical trials with respect to prevention of bleeding and clotting events. Specific antidotes for several DOACs are now available, with idarucizumab available to reverse the action of dabigatran, along with andexanet alfa for reversal of rivaroxaban and apixaban.
Impact on the clinical lab
Clinical laboratories have been on the front lines of the changing anticoagulation landscape, often observing unpredictable effects on routine and specialized laboratory coagulation assays from DOAC presence, and they may have trouble implementing specific DOAC measurement assays if requested by clinicians. In the literature, along with clinical practice outside the U.S., DOAC tests such as the ecarin-based chromogenic assay (ECA) or dilute thrombin time (dTT) are useful for measurement of dabigatran levels. Similarly, the chromogenic anti-Xa assay is useful for measurement of rivaroxaban, apixaban, and edoxaban levels.1
Unlike traditional anticoagulants such as VKAs and unfractionated heparin (UFH), which require monitoring, DOACs do not require routine monitoring. Instead, on-therapy levels may be useful to rule out their presence before surgery or other invasive procedures, especially if the patient has taken the drug in the previous 24 hours (or longer if creatinine clearance is < 50 mL / min). In addition, DOAC tests could be useful for identification of sub- and supratherapeutic levels in cases of patients currently taking other drugs known to affect pharmacokinetics; if patients are underweight, are obese, have deteriorating renal function, or require assessment of compliance if bleeding or clotting happens while on therapy; or if overdose is suspected. DOAC tests could also be used to monitor anticoagulation reversal, especially with the recent approval and implementation of andexanet alfa.2
Laboratory diagnostics manufacturers are in the process of developing DOAC measurement assays, but no assay has yet been cleared by regulatory authorities in the U.S. market. Of the DOAC assays in development, the chromogenic anti-Xa assay is of strong interest for accurate and precise measurement of all “xabans.”
The anti-Xa assay
The anti-Xa assay is an automated ready-to-use, calibrated chromogenic assay, which works by adding a citrated patient plasma sample to a mixture of buffer, chromogenic substrate, and excess factor Xa. With the competition between the chromogenic substrate and the anticoagulant of interest with anti-factor Xa activity (that is, any of the xabans, UFH, LMWH, or fondaparinux), an inverse relationship is established between the optical density readout from the instrument and the exact anticoagulant level. Traditional screening coagulation assays such as the prothrombin time (PT) and activated partial thromboplastin time (aPTT) are not suitable to rule out DOAC presence due to interassay variability in sensitivity and potential patient interferences from changes in factor levels and lupus anticoagulants.
To be clear, the anti-Xa assay is not a newcomer to the clinical laboratory, with its use widely implemented to measure the parenteral, antithrombin-dependent anticoagulants UFH, low molecular weight heparin (LMWH), and fondaparinux, all of which possess inhibitory activity towards factor Xa. The anti-Xa assay has shown utility as an alternative to aPTT for measuring UFH, with strong evidence demonstrating the capability of anti-Xa-based dosing nomograms to establish therapeutic anticoagulation faster and maintain therapeutic levels better than aPTT-based nomograms. In addition, aPTT-based heparin therapeutic ranges (HTR) are technically demanding to validate. When anti-Xa-based nomograms are used, fewer dosing changes and fewer tests are run on inpatients receiving UFH, freeing up nursing, phlebotomy, and laboratory time to focus on critically ill patients. Potential length-of-stay benefits have also been observed, delivering economic benefit to enable hospitals to optimize reimbursement and incrementally improve quality of patient care.3
In this way, the chromogenic anti-Xa assay is a versatile anticoagulation monitoring and measurement assay with automation capability and ease of use to support widespread routine use. Hospitals where anti-Xa has already been implemented as the standard of care in pharmacy UFH dosing nomograms and the laboratory are well positioned for the future, as DOAC measurement assays and new DOAC reversal agents move through the stages of regulatory approval.
REFERENCES
- Cuker A, Siegal DM, Crowther MA, Garcia DA. Laboratory measurement of the anticoagulant activity of the non-vitamin K oral anticoagulants. J Am Coll Cardiol. 2014;64(11):1128-1139.
- Baglin T, Hillarp A, Tripodi A, Elalamy I, Buller H, Ageno W. Measuring oral direct inhibitors of thrombin and factor Xa: a recommendation from the Subcommittee on Control of Anticoagulation of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis. J Thromb Haemost. 013;11:756-760.
- Vandiver JW, Vondracek TG. Antifactor Xa levels versus activated partial thromboplastin time for monitoring unfractionated heparin. Pharmacotherapy. 2012;32(6):546-558.
