Superwarfarin poisoning often underreported, HAI told

Superwarfarin poisoning is frequently underreported, the Haematology Association of Ireland Annual Meeting heard.

Dr Tanna Tan, University Hospital Galway, presented a clinical vignette titled ‘A vitamin K a day keeps the INR at bay: A case of superwarfarin poisoning’. She described the case of a previously healthy patient who was suspected of superwarfarin exposure.

The patient presented with bilateral flank pain, urinary frequency, and visible haematuria. There was no significant medical or surgical history and physical examination was unremarkable.

Initial investigations revealed a significantly prolonged prothrombin time (PT) and international normalised ratio (INR) and, to a lesser extent, activated partial thromboplastin time.

“Our working diagnosis was that the patient had been exposed to anticoagulant compounds, likely warfarin,” Dr Tan said. The patient was treated with IV vitamin K (10mg BD), resulting in improved PT and INR by day four of admission.

However, following a trial withdrawal of vitamin K, PT and INR levels again increased, raising suspicion of exposure to a long-acting anticoagulant, such as superwarfarin. Superwarfarin levels were subsequently sent to a laboratory on day five.

Vitamin K therapy was restarted, and the patient was discharged six days later on 20mg of oral vitamin K.

The patient re-presented on day 18 with headaches and evidence of rebound coagulopathy. Clinicians were concerned about the possible ongoing systemic circulation of superwarfarin compounds. Vitamin K was reintroduced, additional history was obtained, and further investigations were carried out.

Laboratory confirmation was later received showing very high concentrations of brodifacoum – a type of superwarfarin – in the patient’s serum, confirming the diagnosis.

The patient remained stable and was discharged home on oral vitamin K. Follow-up was conducted in the day ward on a fortnightly basis. The patient required varying doses of oral vitamin K, ranging from 10mg to 70mg, over seven months, remaining completely asymptomatic with no evidence of recurrent bleeding during this period.

The meeting heard that superwarfarins are second-generation, long-acting anticoagulant compounds used as rodenticides.

They were developed in the 1970s to address warfarin-resistant rat populations.

Brodifacoum inhibits VKORC1 in a similar manner to warfarin but has a tenfold higher binding affinity. It concentrates in the liver following ingestion and has a half-life of 16 to 36 days.

Dr Tan said that superwarfarin poisoning is often underreported, noting that between 2023 and 2024, 89 cases were reported to the UK National Poisons Information Service.

Routes of exposure include oral, inhalational, and transcutaneous. Superwarfarin poisoning typically presents 24 to 48 hours after ingestion and may involve mucocutaneous, central nervous system, gastrointestinal, or genitourinary bleeding.

Flank pain and haematuria are common, as are recurrent bleeding events.

The treatment of choice is vitamin K1 (phytomenadione), often requiring prolonged therapy. PCC, red blood cell transfusion and plasma have also been used in cases of severe, life-threatening bleeding. As part of the case learning, Dr Tan advised colleagues to consider superwarfarin exposure in patients with markedly deranged PT/INR who exhibit rebound coagulopathy following vitamin K withdrawal. She also recommended obtaining a detailed social, recreational, occupational, forensic, and psychiatric history to help identify potential sources of exposure. Extended high-dose vitamin K therapy may be required to prevent recurrent bleeding, Dr Tan added