Warfarin 🩸
Concise reference for warfarin: indications, INR targets, initiation/maintenance dosing, interactions, and safety. Use the embedded tools for dosing and elevated INR/bleeding management.
About Warfarin (Quick Overview)
- Vitamin K antagonist (VKA). DOACs are preferred for many VTE and non-valvular AF indications; warfarin remains first-line when DOACs are unsuitable (e.g. mechanical heart valves, some hypercoagulable states, or patient preference).
- Narrow therapeutic index — individualise dosing; use a nomogram plus clinical judgement. Always follow local protocols and anticoagulation clinic guidance.
- Embedded tools: initiation/maintenance dosing (UNMH nomogram) and elevated INR / bleeding–reversal (Irish PCC practice — follow local haematology protocol).
INR Targets (common ranges)
- Most AF / VTE / NVAF indications: target INR 2.5 (range 2.0–3.0).
- Mechanical aortic bileaflet (NSR, normal LA size) or Medtronic Hall: usually 2.5 (2.0–3.0) ± low-dose aspirin; some centres use 2.5–3.5 for the first 3 months.
- Mechanical mitral (bileaflet/tilting disk), caged-ball/disk, or aortic + mitral mechanical: typically 3.0 (2.5–3.5) ± aspirin.
- On-X aortic valve: 2.5 (2.0–3.0) for first 3 months, then selected patients without other thromboembolic risks may use 1.5–2.0 (+ aspirin) per labelling — confirm with cardiology.
- Bioprosthetic aortic/mitral: often 2.5 (2.0–3.0) for 3–6 months if anticoagulated, then aspirin alone unless other indications.
- Triple therapy (VKA + dual antiplatelet): consider targeting INR 2.0–2.5 while on triple therapy and cover with a PPI.
Initiation & Monitoring
- Obtain baseline INR (and albumin for new starts). Day 1: 5–7.5 mg standard, or 2.5 mg if warfarin-sensitive (age >75, decompensated CHF, poor nutrition, thyrotoxicosis, liver disease, interacting drugs, malignancy, high bleed risk).
- Days 2–6: adjust from the initiation nomogram (see dosing tool). Rules of thumb: if INR rises >0.5 consider reducing the dose; if INR rises ≥1 consider holding one dose.
- Maintenance (≥1 week): adjust from the maintenance nomogram; check adherence, illness, drug interactions, and diet before changing dose. Prefer the patient’s home tablet strength at discharge.
- Until stable: check INR frequently (daily–every few days). When stable: often every 4–8 weeks (sooner after illness, drug/diet change, or reversal).
Key Interactions & Factors
- ↑ INR / less warfarin needed: amiodarone, azoles, co-trimoxazole (Bactrim), metronidazole, macrolides, quinolones, many antibiotics, SSRIs, PPIs, fibrates, steroids, thyroid replacement, acute illness, diarrhoea, poor intake, low albumin, broad-spectrum antibiotics.
- ↓ INR / more warfarin needed: rifamycins, carbamazepine, barbiturates, azathioprine, nafcillin; increased vitamin K intake; starting tube feeds (binding / Vit K content).
- ↑ Bleeding risk (even without large INR change): aspirin, P2Y12 inhibitors, NSAIDs, UFH/LMWH, fondaparinux.
- Diet: keep vitamin K intake consistent. Illness (fever, CHF, malignancy, infection) and GI changes commonly shift INR — recheck sooner.
- Run a drug-interaction check for every warfarin patient when starting or stopping interacting medicines.
Patient Advice & Safety
- Missed dose: take when remembered on the same day; if next day, skip — do not double. Record and inform clinic.
- Warning signs: unusual bruising/bleeding, black stools, haematuria, severe headache/neuro signs — seek urgent review.
- Pregnancy: generally contraindicated — urgent specialist advice if pregnancy planned or suspected.
- Carry an anticoagulant alert card/bracelet and keep an up-to-date medication list.
Warfarin Bleeding & Reversal Tool
For trained clinicians. Always follow local/hospital protocols.
Bleeding Risk Factors
Higher risk when INR >5:
- Age >70
- Previous bleeding complications
- GI ulcers/haemorrhage
- Prior CVA
- Recent surgery
- Uncontrolled blood pressure
- Recent initiation of anticoagulants
Reversal Plan
Additional Information
No-bleed INR adjustments follow the UNMH maintenance nomogram (target 2–3). Major bleed / ICH / urgent procedure reversal follows Irish PCC practice (e.g. Octaplex®) — always use your hospital haematology protocol.
Most over‑anticoagulated patients return to therapeutic range within ~3 days of stopping warfarin.
Monitor: repeat INR per above; more frequent if PCC given (30–60 min, ~6 h, then as guided).
Discuss prosthetic valve cases with cardiology/haematology before vitamin K or full reversal.
Warfarin Dosing Tool
Initiation & maintenance (UNMH nomogram; target INR 2–3 / 2.5–3.5)
Dosing Plan
Based on the UNMH Warfarin Dosing Guideline (initiation Days 1–6 and maintenance ≥1 week). Guidance only — individualise to the clinical scenario and follow local anticoagulation clinic policy.
Prefer home tablet strength at discharge; if changed, counsel and update pharmacy records. Before adjusting maintenance dose, check adherence, illness, drug interactions, and diet.
Educational reference only. Medical disclaimer