Vitamin K test: what it shows and how much adults need

Vitamin K is a fat-soluble vitamin whose main role is to take part in the production of blood clotting factors (prothrombin among them) in the liver. Without enough vitamin K, the body cannot properly stop bleeding. There are several forms of vitamin K: K1 (phylloquinone) comes from plant foods, especially green leafy vegetables, while the K2 form is made by gut bacteria and is found in fermented products. A small share of vitamin K is stored in the liver and used when needed. The amount of vitamin K in blood, or an indirect sign of a shortage of it, is most often assessed through blood clotting tests (for example prothrombin time), because measuring vitamin K in blood directly is done less often. Newborns are born with very low vitamin K stores, so it is usual practice to give them a preventive dose of vitamin K. For people taking medicines that affect blood clotting, the amount of vitamin K in the diet and in the body carries particular significance, because it can affect how those medicines work.

• When increased bleeding of unclear origin, or easy bruising, appears
• Before planned surgery, to assess blood clotting status
• In newborns at risk of bleeding due to a lack of vitamin K
• When there are digestive tract conditions that interfere with the absorption of fats (and with them the fat-soluble vitamins)
• When assessing liver function and the production of blood clotting factors
• When monitoring patients who take medicines that affect blood clotting

An excess of vitamin K from food or from usual supplement doses is rare. Per the NHS, taking up to 1 mg of vitamin K supplements a day is considered unlikely to cause harm, while there is not enough evidence about the effect of higher doses. Clinically more important is the interaction with medicines that affect blood clotting (for example warfarin) — a raised amount of vitamin K can weaken how those medicines work.

Causes listed for a raised level:

• Large doses of vitamin K supplements
• Interaction with medicines that affect blood clotting, when the amount of vitamin K in the diet or in supplements changes suddenly

Symptoms:

• Usually no clear symptoms — the main risk is a changed effect of medicines that affect blood clotting

What to discuss with a doctor:

• Discussing with your doctor the supplements and the blood-clotting medicines you take
• Blood clotting tests (for example prothrombin time / INR), if you take such medicines

A lowered vitamin K level (deficiency)

A lowered vitamin K level, or a shortage of it in the body, shows up as an increased tendency to bleed and as easy bruising, because there is not enough raw material to make blood clotting factors. Newborns are especially vulnerable: they are born with very low stores, so they are usually given a preventive dose. The risk group also includes people with fat absorption disorders (MedlinePlus).

Causes listed for a lowered level:

• Not enough vitamin K from food, or poor absorption of it
• Fat absorption disorders in the digestive tract that interfere with taking up fat-soluble vitamins
• Very low stores in the newborn period
• Medicines that affect blood clotting and suppress the action of vitamin K in the body

Symptoms:

• An increased tendency to bleed
• Bruises that appear easily
• Bleeding from cuts or wounds that is hard to stop quickly

What to discuss with a doctor:

• Discussing with your doctor all the medicines you take that could affect blood clotting
• Blood clotting tests (for example prothrombin time) for a more precise assessment
• An assessment of diet and of fat absorption
• For newborns — preventive measures as recommended by the doctor

Per the NHS, vitamin K is found mainly in green leafy vegetables such as broccoli and spinach, and also in vegetable oils and cereal grains. Vitamin K that is not used is stored in the liver for later use.

Related pages

Prothrombin time test · vitamin D test · how to read blood test results.