Tests for vitamin K levels are not widely available and are rarely ordered. They are not typically used to screen for or help diagnose vitamin K deficiencies because a lack of vitamin K is usually discovered when unexpected or excessive bleeding or easy bruising occurs. The primary test used to investigate the bleeding is a prothrombin time (PT). If the result of the PT is prolonged and it is suspected to be due to low levels of the vitamin, then oral supplements or injections of vitamin K are administered. If the bleeding is resolved and the PT/INR results return to normal, then a vitamin K deficiency is assumed to be the cause.
For more on this, see the article on Vitamin K Deficiency.