Warfarin Flashcards
Warfarin MOA
Inhibits vitamin K oxidase which prevent prothrombin precursor getting converted to prothrombin
- Does not affect existing clotting factors
- Affects factor II, IV, IX, X, protein C and S
Half-life of Warfarin
36-42 hours while prothrombin half-life is 60-72 hours
Warfarin PK
Highly plasma protein bound
Metabolized by the liver
Substrate of CYP450
Warfarin AE
Bleeding
CI in pregnancy
Induced skin necrosis in those with a protein C or S deficiency
Purple toe syndrome dur to microembolization
VTE (DVT/PE) INR Range
2.0-3.0
A Fib INR Ranges
2.0-3.0
AVR INR Ranges
2.0-3.0
AVR + A Fib/Low EF/hypercoagulable INR Range
2.5-3.5
MVR- mitral valve replacement INR Range
2.5-3.5
Warfarin Initial Dose and Bridge Therapy
5 mg once daily
Avoid large loading doses >10 mg
When initiating warfarin for treatment of acute thrombotic event (eg DVT), overlap IV heparin or LMWH until INR in range for 2 consecutive days (allows for adequate decrease in Factors II and X)
Warfarin Drug Interactions
Bactrim (50%) Metronidazole (50%) Amiodarone (15-60%) Rifampin (50%) Herbals/OTC
Warfarin Increased Sensitivity
• Hyperthyroidism • Fever • HF exacerbations • ESRD Diarrhea • Hepatic dysfunction
Warfarin Decreased Sensitivity
Hypothyroidism
Warfarin Food Interactions
Vitamin K
Grapefruit
Alcohol
Illness (N/V/D)
Define Anticoagulation Management Services (AMS)
Each step is done by one person or small group of people to prevent mix ups
AMS Benefits
Improved time in therapeutic range
Reduced frequency of bleeding complication and thromboembolic events
Reduced costs
INR Monitoring - When first initiating therapy
- Daily after 2 days on therapy until goal range obtained
- Then 2-3 times over the next week or two to assess stability
- In outpatient setting: Every 2-3 days until stable
INR Monitoring - After dosage adjustments
• 5-7 days after change
INR Monitoring- Stable patient with last INR in range
- First follow-up 2-4 weeks
* Then, every 4-6 weeks if consistently stable
Things to assess with Warfarin Management
Verify indication and target INR range to use
Duration of therapy (e.g. post DVT)
Assess compliance
Check for any drug-drug interactions (including OTCs and herbals)
Changes in lifestyle (significant diet changes, alcohol use, cigarette smoking)
Any bleeding complications (mild, moderate, severe) – watch for bruises
Any signs/symptoms of thromboembolic event
Any potential causes?
- Alcohol
- Drug or food interaction
- Compliance (taking too much or too little)
- Illness (acute illness often raises INRs)
***If INR has been very stable and INR is only 0.5 over/under goal range
o No change in dose, recheck INR 1-2 week
o May consider booster dose or 1x dosage decrease
***If INR is 0.2-0.5 over or under goal range and patient hasn’t been very stable in recent past or have 2 INRs in a row out of range
o Increase / decrease weekly dose by 5-10%
o Hold 1 dose if supratherapeutic
***If INR is 0.6-1.0 over or under goal range
o Increase / decrease weekly dose by 10-15%
o Hold 1-2 doses if supratherapeutic
o Consider extra dose(s) if INR under goal
