Vitamin K deficiency is often discovered after unexpected or excessive bleeding occurs. A prothrombin time (PT/INR) is the main laboratory test used to investigate the bleeding. Low levels of vitamin K will have a prolonged result. Vitamin K will then be given by injection, and if the bleeding stops and the PT returns to normal, a vitamin K deficiency will be assumed as the cause.

This test as well as other coagulation tests may be performed to evaluate the symptoms of excessive bleeding and bruising. These tests are partial thromboplastin time (PTT), thrombin time, platelet count, platelet function tests, coagulation factor tests, fibrinogen, von Willebrand factor, and d-dimer.

Because measurements of the vitamin K level in the blood are rarely used to diagnose if a deficiency exist, this is not a routine test and will usually be performed in a reference laboratory. Result make take several days.