INR 6.2 Without Bleeding: Warfarin, TMP-SMX, and a Borrowed NSAID

Ranked contributors

1. TMP-SMX - the pharmacokinetic engine. It inhibits CYP2C9, the principal metabolic pathway for S-warfarin, displaces warfarin from albumin, and suppresses gut flora that synthesize vitamin K. The three-day interval fits: warfarin's half-life is roughly 40 hours, so an enzyme inhibitor typically surfaces on days 3–5. This alone explains most of the jump from 2.4 to 6.2.
2. Naproxen - the bleeding amplifier. Its effect on the INR number is modest, but it irreversibly inhibits platelet COX-1 and injures gastric mucosa. Combined with warfarin it multiplies gastrointestinal bleeding risk at any INR, including a therapeutic one.
3. Reduced vitamin K intake from feeling unwell and eating less - a real but minor contributor here.

Why not liver disease or a new illness: ALT is 24 U/L, he is afebrile, and his INR was 2.4 twelve days ago on an unchanged dose. A stable baseline followed by an abrupt rise is nearly always exogenous - a drug, an illness, or a diet change - so I would not start a malignancy workup today.

The most dangerous factor in the next 48 hours is naproxen, not the INR. The INR quantifies the anticoagulant effect; it does not quantify bleeding risk. Once naproxen is stopped, platelet function does not fully recover for about 5–7 days (aspirin-like irreversible effect on circulating platelets, replaced gradually). A falling INR will therefore not make him safe. My teaching and follow-up plan must target the NSAID, not just the number.

Do now

• Hold tonight's warfarin dose and notify the physician with the full medication timeline, not just the INR.
• Focused bleeding survey: neurological status and headache, skin and mucosal surfaces, gums, stool color and hemoccult if indicated, urine, and any injection or venipuncture sites. Document a negative exam explicitly - "no evidence of bleeding" is a clinical finding that supports withholding reversal.
• Avoid all intramuscular injections and any elective invasive procedure until the INR is documented below 4.0.
• Fall and injury precautions, soft toothbrush, electric razor.
• Recheck INR in 24 hours (some protocols 48), and again after the antibiotic course ends, because the interaction persists as long as TMP-SMX is on board.

Deliberately avoided

• IV vitamin K. For an INR of 6.2 with no bleeding, guidelines support withholding warfarin rather than routine vitamin K. IV phytomenadione carries anaphylactoid risk and, more importantly, produces prolonged warfarin resistance - you then need days of higher dosing and bridging in a patient whose stroke risk from atrial fibrillation is the reason he is anticoagulated.
• IM vitamin K. Same overcorrection problem plus a hematoma at the injection site.
• Oral vitamin K 1–2.5 mg is not automatic either. It becomes reasonable if he has additional risk factors - a recent bleed, an impending procedure, or thrombocytopenia - and that decision belongs to the prescriber.

Framing for the physician: "INR 6.2, no bleeding, TMP-SMX started three days ago and naproxen started yesterday. I have held the warfarin dose, completed a bleeding assessment that is negative, and I am requesting an INR in 24 hours. Do you want oral vitamin K given the NSAID exposure?" That gives a decision-ready recommendation rather than a raw number.

The core message must be that feeling well is not evidence of safety.

Content

• Stop the naproxen today and discard it. Acetaminophen up to 2 g/day is his analgesic, and he needs to know the ceiling because "extra strength" labels vary. If knee pain becomes intolerable, he should call rather than borrow or restart an NSAID; ibuprofen and aspirin are the same trap.
• Explain the antibiotic interaction in plain terms: "This antibiotic slows the liver's ability to clear warfarin, so the blood thinner piled up. That is why your INR is 6.2." Patients who understand the mechanism are far less likely to repeat it with the next antibiotic.
• Any new prescription - including over-the-counter - gets checked against warfarin. Ask the pharmacist, every time.
• Keep vitamin K foods consistent (leafy greens, broccoli); consistency, not avoidance.
• Red-flag list with an action attached: black or tarry stool, blood in urine, vomiting blood or coffee-ground material, unusual bruising, severe headache, sudden weakness or vision change, confusion, a fall. For any of these, call emergency services rather than drive.
• Written plan: warfarin held until the physician says otherwise, INR recheck date and time, who to call after hours, and a wallet card stating he takes warfarin.

Confirming comprehension - teach-back, not "do you understand?" Ask him to state, in his own words: why the antibiotic caused this, which pain medicine he will use, what he will do if his knee is still sore, and the three symptoms that send him to the emergency department. Then have him physically show me the naproxen bottle and put it aside. If he cannot name an alternative analgesic unprompted, the teaching has not landed and I redo it before he leaves.

Documentation: the bleeding assessment, the medications reviewed, the teach-back result, the withheld dose, and the follow-up interval.