title: Correlation between rivaroxaban (Xarelto) plasma activity, patient clinical variables and outcomes in a South African centre.
abstract: BACKGROUND
  Low-molecular-weight heparin and vitamin K antagonists such as warfarin are the gold standard for prohylaxis and treatment of venous thromboembolic disease (VTED). Direct oral anticoagulants (DOACs) result in predictable anticoagulation with significantly reduced inter- and intra-patient variability. DOAC absorption is rapid, with a short half-life and relatively few drug interactions. DOACs are effective and safe at fixed doses without activity monitoring. However, specific situations may require assessment of accurate drug activity. Rivaroxaban, a DOAC targeting activated coagulation factor X (FXa), is registered for the prevention and treatment of VTED in South Africa.
  
  
  OBJECTIVE
  To establish a prophylactic rivaroxaban activity level range and determine any associations with clinical complications, viz. haemorrhage and/or thrombosis.
  
  
  METHODS
  Samples from 115 orthopaedic patients were tested 3 hours after a prophylactic oral dose of 10 mg rivaroxaban with STAGO rivaroxaban anti-FXa reagent on an automated coagulation analyser. Patient demographics and clinical outcomes were documented.
  
  
  RESULTS
  The mean rivaroxaban anti-FXa level was 105.7 ng/mL. Two patients developed adverse events on therapy. One patient had minor bleeding (menorrhagia) (drug activity level 288.7 ng/mL) and another a deep-vein thrombosis (drug activity level 34.7 ng/mL). Statistical analysis demonstrated an association between drug activity and advancing age (p=0.008), most apparent among those aged ≥65 years.
  
  
  CONCLUSIONS
  Measuring rivaroxaban activity levels reduces uncertainty if treatment failure and complications occur. Patients aged ≥65 years should be closely monitored. A local rivaroxaban activity level for patients on rivaroxaban prophylaxis has been established.
authors.affiliation: Department of Molecular Medicine and Haematology, University of the Witwatersrand, Johannesburg; National Health Laboratory Service, Johannesburg, South Africa. Sulouw123@gmail.com.
authors.forename: S
authors.initials: S
authors.lastname: Louw
authors.forename: N P
authors.initials: NP
authors.lastname: Saragas
authors.forename: P N
authors.initials: PN
authors.lastname: Ferrao
authors.forename: T F
authors.initials: TF
authors.lastname: Chirwa
authors.forename: B F
authors.initials: BF
authors.lastname: Jacobson