W
Warfarin
DEFINITION
Commonly used rodenticide; testing is typically for retrospective confirmation (e.g., identification and prevention of future reintoxication or for legal purposes). Note: animals suspected of active hemorrhage due to ingestion of warfarin or other anticoagulant toxicosis should be evaluated with prothrombin time (PT) or assessment of proteins inhibited by vitamin K absence/antagonism (PIVKA), because turnaround time to warfarin assay results is typically long, and it is less widely available.
TYPICAL NORMAL RANGE
Consult laboratory for reference range and for inclusion of both first- and second-generation compounds in test panel.
PHYSIOLOGY
Well- but slowly-absorbed following oral administration. Peak plasma levels in 6-12 hours. Most is plasma protein bound, but high concentrations in liver, spleen, kidney. Metabolism occurs in the liver. Elimination rate depends on compound, amount ingested; may accumulate if small amounts ingested over several days. Warfarin half-life in dog plasma is 14.5 hours; the half-life of diphacinone is suspected to be days. Brodifacoum is assumed to be similar or longer than that of diphacinone. The differences in residual half-lives have important therapeutic implications. Compounds interfere with coagulation by decreasing coagulation factors II, VII, IX, and X (competitive inhibition of vitamin K epoxide-reductase).
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