Vitamin K is essential for the formation of coagulation (clotting) factors, which help the blood clot to prevent excessive bleeding. Prothrombin is one of the coagulation factors which is vitamin K-dependent and aids in blood clot formation. Vitamin K test is helpful in detecting the deficiency as well as the toxicity of vitamin K. It is used to check the status of vitamin K in blood as vitamin K deficiency increases the risk of uncontrolled haemorrhage (bleeding).