Does an International Normalized Ratio (INR) of 1.4 after taking 2mg of warfarin (coumarin) rule out a vitamin K deficiency?

No, an INR of 1.4 measured 18 hours after a single 2mg warfarin dose does NOT rule out vitamin K deficiency—in fact, it may suggest it.

Understanding the Pharmacokinetics

The timing and response pattern in this scenario is actually concerning for vitamin K deficiency rather than reassuring against it. Here's why:

• Warfarin taken at 6pm with INR measured at 12:15pm the next day represents only an 18-hour interval 1
• When warfarin is stopped in patients with normal vitamin K stores and a baseline therapeutic INR of 2.0-3.0, the INR falls to normal range over 4-5 days 1
• A single 2mg dose of warfarin in a vitamin K-replete individual should produce minimal INR elevation within 18 hours, as warfarin requires 48-72 hours to deplete existing vitamin K-dependent clotting factors 1

Why This Pattern Suggests Possible Vitamin K Deficiency

Patients with vitamin K deficiency demonstrate hypersensitivity to even small doses of warfarin:

• In vitamin K-deficient patients, even low-dose vitamin K supplements (25 mcg daily) can cause significant INR changes, demonstrating their baseline depleted state 2
• Approximately 12% of ambulatory anticoagulated patients have very low plasma vitamin K levels (<0.1 ng/mL), making them oversensitive to warfarin 2
• The key issue: if this person had pre-existing vitamin K deficiency, their baseline INR before the warfarin dose would already be elevated 2, 3

Critical Missing Information

To properly interpret this scenario, you must know:

• What was the baseline INR before the 2mg warfarin dose? If the baseline INR was already 1.4 or higher before warfarin administration, this would strongly suggest vitamin K deficiency 2
• If the baseline INR was normal (0.9-1.1) and rose to 1.4 within 18 hours after a single 2mg dose, this would represent an unusually rapid response potentially indicating vitamin K deficiency 2
• Dietary history: vitamin K intake varies dramatically (breast milk contains 0.3 μg/100 kcal vs commercial formula 8-16 μg/100 kcal; dark green vegetables contain 100-570 μg/serving) 1

Clinical Approach to Assess Vitamin K Status

If vitamin K deficiency is suspected, consider:

• Measure plasma vitamin K levels directly (levels <0.1 ng/mL indicate deficiency) 2
• Assess dietary intake: very low vitamin K in breast milk, minimal intake of dark green vegetables, use of certain cooking oils 1, 4
• Review medications and malabsorption conditions that impair vitamin K absorption 1
• A therapeutic trial: administer 25-100 mcg oral vitamin K and recheck INR in 24 hours—if INR normalizes, this confirms deficiency 2, 3

Common Pitfall

The major error is assuming that any "normal-range" INR rules out vitamin K deficiency. The INR of 1.4 is at the upper limit of normal and could represent either:

1. Baseline vitamin K deficiency (INR was 1.4 before warfarin)
2. Hypersensitivity to warfarin due to depleted vitamin K stores
3. Normal variation

Without knowing the pre-warfarin baseline INR, this measurement is uninterpretable for ruling out vitamin K deficiency 2, 3.