Vitamin K is High Yield for the USMLE Step 1

• Vit. K is needed for the γ-carboxylation of clotting factors 2, 7, 9 and 10
• This gives them a Ca binding site
• This explains why the vit. K dependent factors are the same factors that are dependent on calcium
• Deficiency:

o Primary problem is an inability to γ-carboxylate factors 2, 7, 9 and 10

• therefore, factors ARE made but can NOT be activated by Ca
• Involves several factors, including factor 7 which:
• has the shortest t½ of all the clotting factors
• therefore, factor 7 is the first factor unable to be activated
• Since factor 7 is specific to the extrinsic pathway the extrinsic pathway is affected first
• Since the prothrombin time (PT) is a measure of the extrinsic pathway, it is the value expected to be elevated first
• Leads to ↑↑PT (AND eventually ↑PTT) – therefore, PT is most sensitive since its ↑’d 1^st
• Bleeding time, a measure of platelet function, is normal since platelets are not affected
• Deficiency can be seen in the following patients:
• Newborns born at home
• Breast milk has low levels of vit K
• All babies in the US are given vit. K shots @ birth in the hospital to prevent hemorrhagic disease of the newborn
• Pts on broad spectrum antibiotics
• Bacteria in colon synthesize much of our vit. K
• Pts w/ steatorrhea
• Loss of fat in the stools also leads to a loss of the fat soluble vitamins, including vit. K
• Vitamin E toxicity
• Inhibits vitamin K dependent carboxylation of clotting factors
• Treatment
• Vitamin K Injection
• If bleeding is serious then give fresh frozen plasma