Warfarin deliberate self poisoning

The three most important things to know are:

• Is there evidence of active bleeding?
• What is the magnitude of the elevation in INR?
• Does the patient have a therapeutic indication for anticoagulation, and what is it?

Without knowing the answers to these 3 questions, therapeutic over-anticoagulation or poisoning with warfarin cannot be treated appropriately.

Two other important points to emphasize are:

• Don’t give the patient vitamin K before checking the INR!
• Children who have ingested warfarin are managed differently to adults – we don’t usually have to check an INR (see Toxicology Conundrum 015).

Using the “Resus-RSI-DEAD” approach:

• Resuscitation
  • Nil. Clinically significant anticoagulation will be delayed >6 hours. There should be no active bleeding.
• Risk assessment
  • Potential for clinically significant delayed anticoagulation.
• Supportive care and monitoring
  • Routine measures.
• Investigations
  • Screening tests — ECG, glucose, paracetamol level.
  • Check INR on arrival (should be normal if performed within 6 hours of ingestion).
• Decontamination
  • Although activated charcoal may reduce the absorption of the ingested warfarin if administered within 1 hour, I would not attempt decontamination. The potential for adverse effects (e.g. vomiting) and the availability of a safe, effective antidote outweigh the small benefit from decontamination.
• Enhanced elimination
  • Nil.
• Antidotes
  • Administer vitamin K 20 mg po (or 10 mg IV if nausea or vomiting).
• Disposition
  • If the patient is otherwise well they may be medically discharged following the administration of vitamin K.
  • Psychiatry review is essential prior to discharge.
  • A follow up INR is required 48 hours post-ingestion. This can be performed as an outpatient.

Again, using the “Resus-RSI-DEAD” approach:

• Resuscitation
  • Nil. Clinically significant anticoagulation will be delayed >6 hours. There should be no active bleeding.
• Risk assessment
  • Potential for clinically significant delayed anticoagulation.
• Supportive care and monitoring
  • Routine measures.
• Investigations
  • Screening tests – ECG, glucose, paracetamol level.
  • Check INR on arrival (should be normal if performed within 6 hours of ingestion) and periodically thereafter according to INR and bleeding risk.
• Decontamination
  • In contrast to the scenario in Q2, I would administer 50g activated charcoal orally within 1 hour of ingestion.
  • Decontamination is indicated because further management of a patient with a therapeutic requirement for warfarin will be easier if the extent of over-anticoagulation is limited.
• Enhanced elimination
  • Nil.
• Antidotes
  • Vitamin K should only be given if the INR is sufficiently elevated and the bleeding risk is sufficiently high (see Q5):
  • If INR 5-9 and bleeding risk is:
    • low – monitor INR, no vitamin K required.
    • high – monitor INR, administer 0.5-1mg vitamin K IV.
  • If INR >9 and bleeding risk is:
    • low – monitor INR, administer 1mg vitamin K IV.
    • high – monitor INR, administer 1mg vitamin K IV, consider the administration of FFP/ prothrombinex.
• Disposition
  • Admit the patient to a ward environment to monitor INR until it has stabilised. The patient will probably require admission for about 48 hours post-ingestion.
  • Psychiatry review is essential prior to discharge.

A normal INR performed 48 hours after ingestion excludes significant warfarin overdose.

In this case, it would suggest that the history is spurious – either a different medication was taken or she did not actually swallow as many tablets as reported.

In patients with warfarin poisoning or therapeutic over-anticoagulation the management is not solely dependent on the INR. If the patient is not actively bleeding but has an INR >5 we need to determine if the patient has low or high bleeding risk to decide on appropriate therapy.

Risk of bleeding is considered high in the presence of any of these factors:

• major surgery within the previous 2 weeks.
• thrombocytopenia
• antiplatelet drugs (e.g. aspirin, clopidogrel)
• active gastrointestinal disease (e.g. inflammatory bowel disease, peptic ulcer disease)

Warfarin results in anticoagulation by inhibiting vitamin K metabolism, which depletes the reduced form of vitamin K needed for the synthesis of clotting factors (factors II, VII, IX, X, as well as proteins C and S).

The delay in the anticoagulative effect of warfarin reflects the degradation half-lives of the preformed clotting factors. Warfarin prevents their resynthesis but does not alter the rate of degradation. The respective half-lives are:

• factor II: 60 h
• factor VII: 6h
• factor IX: 24h
• factor X: 40h

The net result of the varying half lives is a lag in INR elevation following warfarin administration (typically 8-12 hours, always >6h) with a peak effect at about 72 hours.

Interestingly, the half-lives of protein C and S, which are vitamin-K dependent inhibitors of coagulation, are intermediate in this range. This explains the importance of using alternative anticoagulation (e.g. heparin, enoxaparin) when starting treatment with warfarin. There may be a paradoxical coagulative effect after warfarin administration following depletion of the coagulation inhibitors (protein C and S) before complete degradation of the vitamin K-dependent clotting factors (factors II, VII, IX, X).

• Baker RI, Coughlin PB, Gallus AS et al. Warfarin reversal: consensus guidelines, on behalf of the Australasian Society of Thrombosis and Haemostasis. Medical Journal of Australia 2004; 181(9):492-487. [fulltext]
• Isbister GK, Hackett LP, Whyte IM. Intentional warfarin overdose. Therapeutic Drug Monitoring 2003; 25(6):715-722. PMID: 14639058