Vitamin K
Fat-soluble vitamin the liver needs to make blood clotting factors
Also known as
Definition
Vitamin K is a fat-soluble vitamin stored in the liver and other tissues such as the brain, heart, pancreas and bone. The liver needs it to make clotting factors II, VII, IX and X; without vitamin K, blood would not clot. There are several forms, including phylloquinone (K1) and menaquinone (K2). Vitamin K is found mainly in green leafy vegetables and cabbage-family vegetables, and bacteria in the lower intestine also make it.
| Parameter | Value |
|---|---|
| Unit | varies by lab and method |
| Reference Range | No uniform reference range in this glossary: labs measure vitamin K with different methods and set their own ranges. The reference range on your own report is what counts; discuss it with your doctor. |
Agree on your personal target range with your doctor.
↓ What a low value means
Vitamin K deficiency is very rare. It can occur when the intestine does not absorb the vitamin properly, or after long-term antibiotic treatment. People with a deficiency bruise and bleed more easily. In the PT/INR clotting test, a deficiency shows up as a longer clotting time, that is, a higher INR. The same change can, however, also be linked to liver disease or bleeding disorders; discuss any deviation with your doctor.
↑ What a high value means
A high vitamin K intake from food or supplements can lower the INR, meaning blood clots faster. This matters above all with blood-thinning medicines such as warfarin: vitamin K affects how they work, and sudden changes in intake can shift their effect. If you take such medicines, ask your doctor what to watch for with vitamin K-containing foods, and always mention supplements.
When to test
Vitamin K deficiency can be checked indirectly with the PT/INR clotting test. It is ordered, for example, for unexplained bleeding or bruising, before surgery, when liver problems are suspected and to monitor treatment with warfarin. Alcohol, high-fat diets, foods high in vitamin K and soybean and canola oils can affect the result; ask your practice what to watch for before the test.
Frequently asked questions
How are vitamin K and the PT/INR value related? +
Vitamin K is needed for several clotting factors. Without enough of it, clotting takes longer and the INR rises. A diet very high in vitamin K or supplements can, by contrast, lower the INR. An abnormal INR can have many causes, though, such as liver disease, bleeding or clotting disorders or medicines.
What does vitamin K mean with blood thinners such as warfarin? +
Vitamin K and vitamin K-containing foods can affect how warfarin works. According to MedlinePlus, a consistent intake from day to day may then be needed, because sudden changes can shift the effect. Discuss what this means for your diet with your doctor, and always mention vitamin K supplements.
Which forms of vitamin K are there and where are they found? +
Vitamin K comes in several forms, including phylloquinone (K1) and menaquinone (K2). Green leafy vegetables such as kale and spinach and cabbage-family vegetables such as broccoli and Brussels sprouts contain a lot; fish, liver, meat, eggs and cereals contain smaller amounts. Bacteria in the lower intestine also make vitamin K.
How common is vitamin K deficiency? +
Very rare. It occurs mainly when the intestine cannot absorb the vitamin properly, or after long-term antibiotic treatment. Signs can include a greater tendency to bruise and bleed.
Vitamin K
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.