Warfarin in Primary Care: Practical Basics, Dose Adjustments, and When to Choose a DOAC

Many of us inherit patients who are already on warfarin (Coumadin). Some are managed by a coumadin clinic, others rely on their PCP. While DOACs (e.g., apixaban, rivaroxaban) are often easier to manage, warfarin isn’t going away—especially for patients with mechanical valves, antiphospholipid syndrome, severe kidney/liver disease, or cost barriers. Here’s a quick, clinic-friendly guide you can use today.

DOACs vs. Warfarin (at a glance)

Common Indications for Warfarin

• Atrial fibrillation

• Venous thromboembolism (DVT/PE) treatment and secondary prevention

• Mechanical (prosthetic) heart valves

• Antiphospholipid syndrome

• Selected cases: heart failure, post-ACS, stroke (per specialist guidance)

For most nonvalvular AF/VTE patients, a DOAC is preferred when affordable and clinically appropriate.

Targets, Monitoring, and Workflow

INR goal:

• 2.0–3.0 for most indications

• 2.5–3.5 for some mechanical valves or per cardiology/hematology

Standing orders help. I place a standing INR for 12 months so patients can draw as needed.

Frequency:

• Stable patients: every 4 weeks

• After any dose change / out of range: weekly until therapeutic

• Remember: it takes ~48–72 hours for an INR to reflect a dose change.

Patient education:

• Keep vitamin K intake consistent (don’t ban greens—keep them steady).

• Review new meds (antibiotics, antifungals, amiodarone, SSRIs, NSAIDs, herbals).

• Discuss bleeding signs and when to call.

“Percent Change” Approach to Warfarin Adjustments

There is no single universal protocol. A simple method is to adjust the total weekly dose by a percentage based on the INR, then redistribute across the week.

Typical adjustment ranges (guide, not gospel):

• INR ≤1.5: ↑ total weekly dose ~15%

• INR 1.51–1.99: ↑ ~10%

• INR 2.0–3.0: No change

• INR 3.01–4.0: ↓ ~10%

• INR 4.01–4.99: Hold 1 dose, then restart ↓ ~10%

• INR 5–8.99: Hold until INR 2–3, then restart ↓ ~15%
  (Always use clinical judgment and consider bleeding risk; urgent management if major bleeding or INR ≥9.)

Example 

• Current total: 30 mg/week with INR 1.8

• Plan: Increase by 10% → 30 × 0.10 = +3 mg

• New weekly total: 33 mg/week

• How to schedule: 5 mg on 6 days and 3 mg on 1 day (or 5 mg on 5 days + 4 mg on 2 days).

• Recheck INR: in 1 week.

Tip: Document the weekly total in your note and message the patient with a day-by-day plan to reduce errors.

Starting Warfarin (when you must)

I rarely initiate warfarin in primary care, but when needed:

• Consider a validated tool like WarfarinDosing.org for starting doses.

• Avoid loading doses in most outpatients.

• Overlap with parenteral anticoagulation for acute VTE until INR is therapeutic per guideline.

When I Try to Switch to a DOAC

• Nonvalvular AF or VTE where renal/hepatic function is adequate, no strong drug interactions, and cost/coverage is reasonable.

• Shared decision-making: fewer interactions, no INR checks, often lower intracranial bleeding risk.

Safety Reminders

• Pregnancy: Warfarin is teratogenic (avoid; use specialist guidance).

• Procedures: Have a peri-procedural plan (hold times/bridging per risk and specialty input).

• Alcohol: Heavy use raises bleeding risk and destabilizes INR.

Follow-Up Script You Can Paste in Your EMR

“Your goal INR is 2.0–3.0. Keep your vitamin K intake consistent (don’t make big changes in greens). Report any new meds, antibiotics, or supplements. Watch for bleeding (black stools, nosebleeds, easy bruising). We adjusted your weekly dose to __ mg/week:
Sun __ / Mon __ / Tue __ / Wed __ / Thu __ / Fri __ / Sat __.
Recheck INR in 1 week.”

References & Resources

• CHEST/ACCP guidance on antithrombotic therapy (warfarin management, peri-procedural care).

• AHA/ACC/HRS guidance for atrial fibrillation anticoagulation.

• ISTH guidance on DOAC use.

• UpToDate: Practical warfarin management; DOAC selection and dosing.

• WarfarinDosing.org – evidence-based initiation and adjustment support.

(Cite your preferred local/proprietary resources as applicable.)

Final Word

There are no universally accepted outpatient dosing protocols—use a consistent method, communicate clearly, and recheck INRs promptly after adjustments. When appropriate, discuss switching to a DOAC.

Come back weekly for practical primary-care pearls. I’d love to connect—[email protected]  or DM me on social.

Curious about 1:1 mentorship for case discussions, lab reviews, or “phone-a-friend” questions? I offer a structured 3-month program built for new primary-care NPs. Email me to book a free 15-minute call.