title: Monitoring low molecular weight heparins at therapeutic levels: dose-responses of, and correlations and differences between aPTT, anti-factor Xa and thrombin generation assays.
abstract: BACKGROUND
  Low molecular weight heparins (LMWH's) are used to prevent and treat thrombosis. Tests for monitoring LMWH's include anti-factor Xa (anti-FXa), activated partial thromboplastin time (aPTT) and thrombin generation. Anti-FXa is the current gold standard despite LMWH's varying affinities for FXa and thrombin.
  
  
  OBJECTIVE
  To examine the effects of two different LMWH's on the results of 4 different aPTT-tests, anti-FXa activity and thrombin generation and to assess the tests' concordance.
  
  
  METHODS
  Enoxaparin and tinzaparin were added ex-vivo in concentrations of 0.0, 0.5, 1.0 and 1.5 anti-FXa international units (IU)/mL, to blood from 10 volunteers. aPTT was measured using two whole blood methods (Free oscillation rheometry (FOR) and Hemochron Jr (HCJ)) and an optical plasma method using two different reagents (ActinFSL and PTT-Automat). Anti-FXa activity was quantified using a chromogenic assay. Thrombin generation (Endogenous Thrombin Potential, ETP) was measured on a Ceveron Alpha instrument using the TGA RB and more tissue-factor rich TGA RC reagents.
  
  
  RESULTS
  Methods' mean aPTT at 1.0 IU/mL LMWH varied between 54s (SD 11) and 69s (SD 14) for enoxaparin and between 101s (SD 21) and 140s (SD 28) for tinzaparin. ActinFSL gave significantly shorter aPTT results. aPTT and anti-FXa generally correlated well. ETP as measured with the TGA RC reagent but not the TGA RB reagent showed an inverse exponential relationship to the concentration of LMWH. The HCJ-aPTT results had the weakest correlation to anti-FXa and thrombin generation (Rs0.62-0.87), whereas the other aPTT methods had similar correlation coefficients (Rs0.80-0.92).
  
  
  CONCLUSIONS
  aPTT displays a linear dose-response to LMWH. There is variation between aPTT assays. Tinzaparin increases aPTT and decreases thrombin generation more than enoxaparin at any given level of anti-FXa activity, casting doubt on anti-FXa's present gold standard status. Thrombin generation with tissue factor-rich activator is a promising method for monitoring LMWH's.
authors.affiliation: Medical Faculty, University of Lund, Lund, Sweden; Department of Paediatric Anaesthesia and Intensive Care, SUS Lund University Hospital, Lund, Sweden.
authors.forename: Owain
authors.initials: O
authors.lastname: Thomas
authors.affiliation: Medical Faculty, University of Lund, Lund, Sweden.
authors.forename: Emanuel
authors.initials: E
authors.lastname: Lybeck
authors.affiliation: Medical Faculty, University of Lund, Lund, Sweden; Coagulation Laboratory, Dept. of Clinical Chemistry, Division of Laboratory Medicine, Skåne University Hospital, Malmö, Sweden.
authors.forename: Karin
authors.initials: K
authors.lastname: Strandberg
authors.affiliation: Department of Clinical Immunology and Transfusion Medicine, Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden; Department of Clinical Chemistry, Department of Clinical Experimental Medicine, Linköping University, Linköping, Sweden.
authors.forename: Nahreen
authors.initials: N
authors.lastname: Tynngård
authors.affiliation: Medical Faculty, University of Lund, Lund, Sweden; Department of Anaesthesia and Intensive Care, SUS Lund University Hospital, Lund, Sweden.
authors.forename: Ulf
authors.initials: U
authors.lastname: Schött
mesh_terms: D000328:Adult; D004305:Dose-Response Relationship, Drug; D017984:Enoxaparin / Q000494:pharmacology*; D065427:Factor Xa Inhibitors / Q000494:pharmacology*; D005343:Fibrinolytic Agents / Q000494:pharmacology*; D006495:Heparin, Low-Molecular-Weight / Q000494:pharmacology; D006801:Humans; D008297:Male; D008875:Middle Aged; D010314:Partial Thromboplastin Time; D059748:Proteolysis; D013917:Thrombin / Q000378:metabolism; D000078222:Tinzaparin; D055815:Young Adult
chemical_list: D017984:Enoxaparin; D065427:Factor Xa Inhibitors; D005343:Fibrinolytic Agents; D006495:Heparin, Low-Molecular-Weight; D000078222:Tinzaparin; D013917:Thrombin