Vitamin K1 (phylloquinone) is a fat-soluble vitamin essential for blood clotting and bone health. This test measures vitamin K1 levels in the blood to evaluate nutritional status and diagnose deficiency. Vitamin K1 serves as a cofactor for the gamma-carboxylation of glutamate residues in proteins, creating Gla-proteins that are essential for blood coagulation factors (II, VII, IX, X) and bone proteins like osteocalcin and matrix Gla-protein. Deficiency can cause excessive bleeding, impaired blood clotting, and increased risk of osteoporosis and arterial calcification. Vitamin K1 deficiency is most common in patients with malabsorption syndromes, liver disease, those on long-term antibiotics, and newborns who are at risk for hemorrhagic disease. This test is particularly valuable for evaluating unexplained bleeding disorders, monitoring patients on anticoagulant therapy, and assessing nutritional status in malabsorption conditions.