Warfarin 🩸

• 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).

• 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.

• 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).

• ↑ 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.

• 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.

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