The sample that would not clot

Vitamin K is a vital component within both the intrinsic and extrinsic coagulation cascade as certain factors (II, VII, IX, X and protein C and S) utilize vitamin K as a cofactor during post translational modification. Deficiency of vitamin K can result in the inability to properly form blood clots, both in vivo and in vitro, due to reduced vitamin K dependent factor levels and function. Vitamin K deficiency can result from congenital causes, such as VKOR or CYP2C9 mutations, or acquired causes, such as nutritional deficiencies, antibiotic therapy, or supra-therapeutic warfarin dosing. In this case we present a patient with multifactorial vitamin K deficiency (due to nutritional defects and multiple genetic mutations in VKOR and CYP2C9) that was exacerbated by antibiotic and warfarin therapy during her hospital admission. This case displays the importance of genetic testing prior to warfarin dosing and the role antibiotics play in the coagulation cascade. •Vitamin K deficiency can severely impact the ability to form a blood clot (both in vivo and in vitro).•Certain antibiotics can contribute to vitamin K deficiency (and thus prolong the INR).•Genetic testing is essential to understand a patient's warfarin drug metabolism.