# Situation: A 63-year-old man with a deep vein thrombosis of the left leg is being discharged on warfarin because a direct oral anticoagulant is not affordable for him. He lives alone, prepares his own meals, and will have his international normalized ratio (INR) checked weekly at the outpatient clinic. His INR was within his target range in weeks 1 and 2. At week 3, the clinic reports that his INR has risen well above the target range; he has no bleeding. Which of his statements BEST explains the rise in his INR?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=629852  
> language: ko  
> subject: Nursing Practice III — Care of Clients with Problems in Surgery, Oxygenation, Fluid and Electrolytes, Infectious, Inflammatory and Immunologic Response, Cellular Aberrations

## 문제

Situation: A 63-year-old man with a deep vein thrombosis of the left leg is being discharged on warfarin because a direct oral anticoagulant is not affordable for him. He lives alone, prepares his own meals, and will have his international normalized ratio (INR) checked weekly at the outpatient clinic.

His INR was within his target range in weeks 1 and 2. At week 3, the clinic reports that his INR has risen well above the target range; he has no bleeding. Which of his statements BEST explains the rise in his INR?

## 보기

1. "Since last week I have eaten a big bowl of green leafy vegetables every day."
2. "Twice this week I forgot a tablet, so I took two the next day." **✔ 정답**
3. "I moved my tablet from 8 in the morning to 5 in the afternoon."
4. "I took one paracetamol tablet for a headache on Sunday."

**정답: 2**

## 해설

Taking a double dose after a missed dose, twice in one week, adds extra warfarin and explains a rise in the INR. Clients are taught not to skip or double doses. A sudden increase in vitamin K–rich vegetables works in the opposite direction and lowers the INR, changing the dosing time is acceptable if then kept consistent, and a single paracetamol dose does not raise the INR the way regular high doses can.

## 심화 해설

Clinical context Warfarin has a narrow therapeutic index, so small changes in dose, diet, or interacting drugs can move the INR out of range quickly. When the INR rises above target without bleeding, the first step in nursing reasoning is to identify what increased the anticoagulant effect or decreased warfarin clearance.

Why option 2 is the best explanation The client missed a dose twice in one week and then took two tablets the next day on each occasion. This creates two days with a 200% dose, which is not the same as maintaining a steady daily dose. Warfarin inhibits vitamin K epoxide reductase, reducing the regeneration of active vitamin K and slowing production of factors II, VII, IX, and X. Because factor VII has a short half-life of about 6 hours, a higher-than-prescribed dose lowers factor VII activity quickly, and the INR, which is most sensitive to factor VII, rises within days. Doubling a missed warfarin dose is a high-risk behavior because it produces a supratherapeutic peak rather than simply replacing the omitted dose. The standard teaching is to take the missed dose as soon as remembered only if it is the same day, and never to take two doses at once.

Why the other statements do not explain the rise

| Statement | Expected effect on INR | Reasoning |
| --- | --- | --- |
| 1. Large daily intake of green leafy vegetables since last week | Lowers INR | Green leafy vegetables are rich in vitamin K, which provides more substrate for clotting factor synthesis and antagonizes warfarin. A sudden increase in vitamin K intake would be expected to reduce the INR, not raise it. |
| 3. Moving the dose from 8 AM to 5 PM | No meaningful rise | Warfarin has a long half-life of approximately 40 hours, so a one-time shift of a few hours does not cause a supratherapeutic INR. The key is to then keep the new time consistent. |
| 4. One paracetamol tablet for headache | No meaningful rise | A single dose of paracetamol does not significantly potentiate warfarin. Clinically relevant interaction is associated with regular, high-dose use, typically more than 2 g/day over several days, which can raise the INR through inhibition of warfarin metabolism or altered vitamin K handling. |

Nursing process application Key point! The assessment priority is to determine the exact warfarin-taking pattern over the past week, because unintentional double dosing is the most direct explanation for a rapid INR rise. The nursing diagnosis of risk for bleeding is activated whenever the INR exceeds the target range, even if the client currently has no bleeding. Priority interventions include holding the next dose as ordered, repeating the INR, assessing for occult bleeding such as dark stools, hematuria, gum bleeding, or easy bruising, and reinforcing safe administration teaching. The client should be instructed to use a pill organizer or calendar and to contact the clinic before making any dose change.

Safety teaching for warfarin Clients living alone and managing their own medication need clear, written instructions. The core messages are to take the dose at the same time each day, never double a dose after forgetting one, keep vitamin K intake consistent rather than avoiding it entirely, and report any sign of bleeding or an INR outside the target range. Watch out! A missed dose is safer than a doubled dose, because the risk of a temporary subtherapeutic INR is generally lower than the risk of hemorrhage from a supratherapeutic INR. The safest response to a missed warfarin dose is to take it only if remembered within the same day; if it is already the next day, the missed dose is skipped and the regular schedule resumes.

## 임상 시나리오

Warfarin Missed Dose SafetyNever double up after a missed dose
Teach patients to take a missed warfarin dose only if remembered the same day, and never to take two doses at once. Doubling creates a supratherapeutic peak that raises INR within days because factor VII has a half-life of about 6 hours.

When INR rises above target without bleeding, first assess for dosing errors, increased anticoagulant effect, or decreased warfarin clearance. A sudden increase in vitamin K-rich vegetables lowers INR, not raises it.

CautionA single paracetamol dose does not significantly raise INR, but regular high doses can. Reinforce that changing the administration time is acceptable only if kept consistent.

## 핵심 개념

- **Warfarin** — Vitamin K antagonist that inhibits clotting factor synthesis; narrow therapeutic index
- **INR** — International normalized ratio; monitors warfarin effect, sensitive to factor VII
- **Vitamin K epoxide reductase** — Enzyme inhibited by warfarin, reducing regeneration of active vitamin K
- **Factor VII** — Clotting factor with short half-life (~6 hours); INR most sensitive to it
- **Supratherapeutic INR** — INR above target range, increasing bleeding risk without acute hemorrhage

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