Vitamin K: what it is, what it tells you and when it’s tested

Your liver needs vitamin K to make the proteins that let your blood clot. It also plays a part in your bones. It comes in two forms: K1, in leafy greens, and K2, in fermented foods and some cheeses.

When it’s tested

It is rarely measured. Doctors order it when they suspect a shortage: with bleeding or bruising for no reason, with conditions that absorb fat poorly, such as coeliac disease, cystic fibrosis or bile duct disease, or in people who eat very little and take antibiotics for a long time.

Usually the doctor looks first at clotting tests such as the INR. These are read in context, often in hospital. If you have bleeding that will not stop, call 112 or go to the emergency department.

It is not a test for people who feel well: in healthy adults a shortage is rare, and the value depends mostly on what you ate in the days before. That is why Rosa does not test it, and it does not run clotting tests either. It does measure platelets, haemoglobin and your liver, which makes the clotting proteins.

What it tells you, and what it does not

It shows how much vitamin K1 is in your blood right now. It mostly reflects your recent meals, and it does not measure K2.

To see whether your liver has enough to do its work, some labs measure a protein called PIVKA-II, which rises when vitamin K is short. The INR, by contrast, only rises with a marked shortage.

That is why it is read together with other tests:

Normal vitamin K levels

There is no single range. Few labs run this test, and different methods give different numbers. The range printed next to your result is what counts.

As a guide, experts consider a vitamin K1 value below 0.15 µg/L a shortage.

Fasting matters: a meal with leafy greens can raise the value for a few hours.

If your vitamin K is low or high

Who is at risk of a vitamin K shortage

In healthy adults it is rare. It is more common in these cases:

Symptoms of a shortage

A shortage shows mainly through bleeding:

If you have blood in your urine or black stools, vomit blood or have bleeding that will not stop, call 112 or go to the emergency department.

Vitamin K and blood thinners

Warfarin and acenocoumarol work by blocking vitamin K. That is why how much vitamin K you eat can change their effect and your INR.

You do not need to cut out green vegetables. What matters is eating similar amounts week after week, without big sudden changes. If you change your diet a lot, tell your doctor.

Before starting or stopping a supplement that contains vitamin K, K2 included, talk to your doctor. Never change the dose of your blood thinner on your own: your doctor sets the INR you should reach. If you take another kind of blood thinner, ask your doctor whether diet matters for you too.

Where it is found: what to eat

Vitamin K is found mostly in vegetables. For example:

A diet with vegetables every day is usually enough. Ask your doctor before taking supplements, especially if you take blood thinners.

What can skew the result

A few things change the value without changing your stores.

When to worry

Talk to your doctor if:

Frequently asked questions

What does vitamin K do? Your liver needs it to make the proteins that let your blood clot. It also plays a part in your bones.

Which foods contain vitamin K? Mostly leafy greens and cabbages: spinach, chard, lettuce, rocket, broccoli, cavolo nero, turnip tops. K2 is in aged cheese, egg yolk and fermented foods.

Can I eat green vegetables if I take warfarin? Yes. You do not need to cut them out: what matters is eating similar amounts each week. If you change your diet a lot, tell your doctor, who can check your INR. Never change the dose on your own.

Is vitamin K2 good for bones and arteries? Its role is being studied, but the evidence is not enough to recommend it to healthy people. If you take blood thinners, do not start it without talking to your doctor.

Why are newborns given vitamin K? Newborns are born with small stores, and breast milk contains little. A dose at birth prevents rare but serious bleeding.

Does Rosa test vitamin K? No, and it does not run INR or aPTT either. Rosa measures platelets, haemoglobin and ALT and AST. If your doctor wants vitamin K or clotting tests, they order them separately.

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