Vitamin K catalyzes the post-translational carboxylation of coagulation proteins C, S, and factors II, VII, XI, and X. Detection of the noncarboxylated forms allows an indirect and specific measure of the vitamin K deficiency found in early, classic, and late hemorrhagic disease of the newborn (HDN), malabsorption syndromes, and drug related (warfarin, anticonvulsants, and antibiotics) states. Idiopathic late HDN (CNS bleeding) occurs in exclusively breast-fed infants and is prevented by appropriate parenteral and oral vitamin K prophylaxis given at birth. All newborn infants and older infants with malabsorption syndromes should receive prophylactic vitamin K.