Warfarin Sensitivity

Diagnosis

Indications for Testing

  • Not enough evidence to recommend widespread adoption of genotyping
  • May be indicated in
    • Warfarin-naïve individuals
    • Individuals with personal or family history of difficulty with anticoagulation
    • Individuals who are therapy adherent but difficult to treat (eg, those requiring <21 mg per week or >49 mg per week to maintain therapeutic INR)
    • Individuals who might be placed on warfarin prophylactically after surgery

Laboratory Testing

  • CYP2C9 and VKORC1 genotyping – for clinical dosing
    • Many algorithms and models for dosing are available
      • Algorithms predict maintenance dose but do not necessarily consider pharmacokinetic differences that may influence the time required to achieve a steady state
      • Algorithms typically offer no guidance regarding dosing intervals or indicate when interpretation of an international normalized ratio (INR) result is appropriate
      • Dose revision algorithms after INR response is observed are also available (Lenzini, 2010)
    • See warfarin dosing calculator for therapeutic dose estimates – other models for dosing are also available
    • Dosing equation example from Sconce, 2005
      • Dose (mg/d) = [0.628 – 0.0135 (age in years) – 0.240 (a) – 0.370 (b) – 0.241 (c) + 0.0162 (height in cm)]2
      • a = CYP2C9*2, input 0, 1, 2 based on number of alleles
      • b = CYP2C9*3, input 0, 1, 2 based on number of alleles
      • c = VKORC1, input 1 for GG, 2 for GA, 3 for AA
    • 2010 revision of the Coumadin label includes expected maintenance dosing based on genotype
    • Warfarin Dosing (md/d) Based on Genotype
      Warfarin Dosing (md/d) Based on Genotype

      CYP2C9

      mutations

      VKORC1 GG calculated initial dose

      % reduced from 5.6

      VKORC1 AG calculated initial dose

      % reduced from 5.6

      VKORC1 AA calculated initial dose

      % reduced from 5.6

      None

      5.6

       

      4.5

      20%

      3.5

      38%

      CYP2C9*2

      4.5

      20%

      3.5

      38%

      2.7

      52%

      CYP2C9*3

      4.0

      29%

      3.1

      45%

      2.3

      59%

      CYP2C9*2/*2

      3.5

      38%

      2.7

      52%

      2.0

      64%

      CYP2C9*2/*3

      3.1

      45%

      2.3

      59%

      1.6

      71%

      CYP2C9*3/*3

      2.6

      54%

      1.9

      66%

      1.3

      77%

    • When dosing, also consider warfarin drug interactions
    • Pharmacogenetic testing does not replace the need to monitor warfarin therapy through INR testing

Clinical Background

Warfarin (Coumadin) is one of the most widely used anticoagulants worldwide, with potentially severe hemorrhagic or thrombotic consequences if dosed incorrectly. The labeling for a popular formulation of warfarin was revised in August 2007 and February 2010 to include information about how the three most widely studied pharmacogenetic mutations (CYP2C9*2, CYP2C9*3, and VKORC1 -1639G>A) affect dose requirements (NDA 9-218/S-105).  These mutations account for as much as 45% of variation in warfarin response in Caucasians and 30% in African Americans.

Epidemiology

  • Incidence
    • CYP2C9*2 (c.430C>T)

      • Caucasian – 0.08-0.13
      • Asian – 0.02-0.06
      • African American – <0.01
    • CYP2C9*3 (c.1075A>C)
      • Caucasian – 0.06-0.10
      • Asian – <0.01
      • African American – 0.01-0.04
    • VKORC1 (c.-1639G>A)
      • Caucasian – 0.42
      • Asian – 0.89
      • African American – 0.08
      • The VKORC1 (c.-1639G>A) mutation is in very strong linkage disequilibrium with the VKORC1 variant c.173+1000C>T

Risk Factors

  • CYP2C9 mutations reduce warfarin clearance
    • *2 allele (430C>T)
      • Reduces metabolism of S-warfarin by ~30%
      • Extends half-life – requires a longer time to achieve steady state
      • Average daily warfarin requirement reduced (see table in Diagnosis tab)
    • *3 allele (1075A>C)
      • Reduces metabolism of S-warfarin by ~80%
      • Extends half-life – requires a longer time to achieve steady state
      • Average daily warfarin requirement reduced more than with the CYP2C9*2 allele (see table in Diagnosis tab)
  • VKORC1 mutations affect response to warfarin
    • Many mutations are known and exist in linkage disequilibrium
    • Genotyping to detect a common promoter mutation (-1639G>A) predicts warfarin sensitivity haplotypes
    • Average daily warfarin requirement reduced (see table in Diagnosis tab)
  • Combinations of mutations from one or more genes further reduce the average daily warfarin requirement

Pathophysiology

  • Warfarin prescribed for the following
    • Atrial fibrillation
      • Patients in therapeutic range ~55% of the time
    • Venous thromboembolism
    • Mechanical heart valves
    • Postoperative prophylaxis
  • Warfarin inhibits production of vitamin K-dependent coagulation factors through inhibition of vitamin K epoxide reductase (VKOR)
    • Response to warfarin therapy routinely assessed through prothrombin time/international normalized ratio (INR) testing
  • Warfarin is a racemic mixture of R and S enantiomers
    • S is 3-5 times more potent than R
    • S-warfarin is metabolized primarily by CYP2C9
    • Warfarin has a narrow therapeutic index that is influenced by a variety of drugs and illnesses
  • Pharmacogenetics affect dosing
    • CYP2C9 genotype accounts for as much as 18% of the variability in warfarin dosing
    • VKORC1 genotype accounts for as much as 29% of the variability in warfarin dosing
    • Combining genotypes with clinical factors may account for 50-70% of variability in warfarin dosing

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Warfarin Sensitivity (CYP2C9 and VKORC1 ) 3 Mutations 0051370
Method: Polymerase Chain Reaction/DNA Hybridization/Electrochemical Detection

Consider ordering in warfarin-naive patients or patients with a history of difficulty with anticoagulation in order to confirm genotype affecting warfarin metabolism

Clinical sensitivity – 90% in Caucasians; less well characterized in other populations

Rare mutations within primer or probe regions may interfere with the assay  
Cytochrome P450 2C9 (CYP2C9) 2 Variants 0051103
Method: Polymerase Chain Reaction/DNA Hybridization/Electrochemical Detection

Consider ordering in warfarin-naive patients or patients with a history of difficulty with anti-coagulation in order to confirm genotype affecting warfarin metabolism

Clinical sensitivity – 90% in Caucasians; not well characterized in other ethnicities

Only the two common CYP2C9 mutations are detected

Additional mutations in this or other genes are not detected

Mutation detection is not a substitute for therapeutic drug or other clinical monitoring

Nongenetic factors may also affect drug metabolism

 
Prothrombin Time/International Normalized Ratio 0030224
Method: Electromagnetic Mechanical Clot Detection

Monitor warfarin therapy

   
Warfarin Genotyping Plus 2004358
Method: Polymerase Chain Reaction/DNA Hybridization/Electrochemical Detection

Preferred test if individualized warfarin dosage planning is required

Individualized dosing recommendations are provided when specific individual parameters are submitted

   
Additional Tests Available
 
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Warfarin, Urine 0091230
Method: High Performance Liquid Chromatography
Warfarin, Serum or Plasma 0090805
Method: High Performance Liquid Chromatography