A vitamin K blood test measures the level of vitamin K in your bloodstream, a fat-soluble vitamin essential for blood clotting and bone health. Unlike routine blood tests, vitamin K testing is typically requested only when there is clinical suspicion of deficiency or when monitoring specific medical conditions. In UK clinical practice, healthcare professionals often assess vitamin K status indirectly through clotting tests such as prothrombin time (PT) or international normalised ratio (INR), as these functional tests reflect how well vitamin K-dependent clotting factors are working. Direct measurement requires specialised laboratory techniques and is not routinely available in all NHS laboratories.

What Is a Vitamin K Blood Test?

A vitamin K blood test is a laboratory investigation that measures the level of vitamin K in your bloodstream. Vitamin K is a fat-soluble vitamin essential for blood clotting and bone health. Unlike many other vitamins, vitamin K is not routinely measured in standard blood panels, and specific testing is typically requested only when there is clinical suspicion of deficiency or when monitoring certain medical conditions.

The test usually measures vitamin K1 (phylloquinone), which comes from dietary sources such as green leafy vegetables. While vitamin K2 (menaquinone) is produced by bacteria in the gut, direct K2 testing is rarely available in UK clinical settings. In practice, healthcare professionals more commonly assess vitamin K status indirectly through clotting tests such as the prothrombin time (PT) or international normalised ratio (INR), as these functional tests reflect how well vitamin K-dependent clotting factors are working.

Direct measurement of vitamin K levels requires specialised laboratory techniques, typically high-performance liquid chromatography (HPLC), which is not available in all NHS laboratories. This means the test may need to be sent to a reference laboratory, potentially resulting in longer turnaround times for results. The test requires a blood sample, usually taken from a vein in your arm. Important pre-analytical requirements often include fasting for 8-12 hours before the test, and the sample must be protected from light immediately after collection as vitamin K is photolabile (degrades when exposed to light). Always follow your healthcare provider's specific instructions for test preparation.

Why You Might Need a Vitamin K Blood Test

Your GP or specialist may request a vitamin K blood test if you have symptoms or conditions that suggest vitamin K deficiency. Common clinical scenarios include:

• Unexplained bleeding or bruising – Easy bruising, nosebleeds, bleeding gums, or prolonged bleeding from minor cuts may indicate impaired clotting function related to vitamin K deficiency.

• Malabsorption disorders – Conditions such as coeliac disease, Crohn's disease, ulcerative colitis, or chronic pancreatitis can impair fat absorption, including fat-soluble vitamins like vitamin K.

• Liver disease – The liver produces clotting factors that depend on vitamin K, so chronic liver conditions may affect vitamin K metabolism and utilisation.

• Long-term antibiotic use – Prolonged courses of broad-spectrum antibiotics can disrupt gut bacteria that produce vitamin K2, particularly in the context of poor dietary intake or malabsorption.

• Cholestatic conditions – Biliary obstruction, cystic fibrosis, post-bariatric surgery states, and prolonged parenteral nutrition can all impair vitamin K absorption.

Additionally, patients taking anticoagulant medications such as warfarin, which works by interfering with vitamin K metabolism, may require monitoring of their clotting status through INR testing rather than direct vitamin K measurement. If you are taking warfarin, you should not start or change vitamin K supplements or significantly alter your diet without consulting your anticoagulation service or GP.

It's important to note that all newborns in the UK are offered vitamin K prophylaxis at birth to prevent vitamin K deficiency bleeding (VKDB), rather than undergoing screening tests for vitamin K levels.

How the Vitamin K Blood Test Works

The vitamin K blood test is performed using a standard venous blood sample. A healthcare professional, typically a phlebotomist or practice nurse, will collect blood from a vein in your arm using a sterile needle and collection tube. The procedure takes only a few minutes and is similar to any routine blood test you may have had previously.

The testing process involves:

• Sample collection – A tourniquet is applied to your upper arm to make the veins more visible, the skin is cleaned with an antiseptic wipe, and blood is drawn into a specialised tube. The sample must be protected from light immediately after collection, as vitamin K can degrade when exposed to ultraviolet light.

• Laboratory analysis – The blood sample is sent to a laboratory where vitamin K levels are measured using advanced techniques such as HPLC or liquid chromatography-mass spectrometry (LC-MS). These methods can distinguish between different forms of vitamin K and provide precise measurements.

• Turnaround time – Because direct vitamin K testing requires specialised equipment not available in all laboratories, your sample may be sent to a reference centre. Turnaround times vary significantly depending on your location and the laboratory used; check with your healthcare provider for a more specific timeframe.

Many laboratories recommend fasting for 8-12 hours before the test due to post-prandial variation in vitamin K levels. It is important to inform your healthcare provider about all medications and supplements you are taking, especially vitamin K supplements, warfarin, or lipid-lowering therapies, as these may affect vitamin K levels or interact with the test interpretation. Always follow the specific instructions provided by your healthcare team or local laboratory.

Understanding Your Vitamin K Test Results

Interpreting vitamin K blood test results requires clinical context, as reference ranges vary between laboratories and testing methods. Normal vitamin K1 (phylloquinone) levels will be defined by your local laboratory's reference interval, which should appear alongside your results. Different laboratories may report results in different units such as nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL).

Your doctor will consider several factors when reviewing your results:

• Clinical symptoms – The presence of bleeding, bruising, or other signs of clotting dysfunction is more significant than the numerical value alone.

• Functional clotting tests – Your vitamin K result will be interpreted alongside PT/INR and activated partial thromboplastin time (APTT) results, which assess how well your blood actually clots. In vitamin K deficiency, PT typically becomes prolonged before APTT due to the shorter half-life of factor VII.

• Dietary intake – Your typical diet and any recent changes in eating patterns will be considered, as vitamin K is primarily obtained from food sources.

• Underlying conditions – Malabsorption disorders, liver disease, or medication use will influence how your results are interpreted.

It is important to understand that mildly reduced phylloquinone without coagulopathy may simply reflect recent dietary intake; the clinical significance is uncertain and should be interpreted alongside PT/INR and clinical findings. In some specialist centres, PIVKA-II (des-γ-carboxy prothrombin) may be measured as a sensitive marker of functional vitamin K deficiency. Your healthcare provider will explain what your specific results mean for your health and whether any intervention is needed. Never attempt to self-diagnose or self-treat based on test results alone—always discuss findings with your GP or specialist.

What Happens If Your Vitamin K Levels Are Abnormal

If your vitamin K blood test reveals abnormal levels, your doctor will develop a management plan based on the severity of the deficiency and your individual circumstances. Treatment approaches typically include:

• Dietary modification – Increasing consumption of vitamin K-rich foods such as kale, spinach, broccoli, Brussels sprouts, and other green leafy vegetables. The NHS recommends adults consume approximately 1 microgram of vitamin K per kilogram of body weight daily.

• Oral supplementation – Vitamin K supplements (usually vitamin K1 or phytomenadione) may be prescribed if dietary changes alone are insufficient. Dosing will be determined by your clinician according to your specific needs and in line with UK prescribing guidelines.

• Injectable vitamin K – In cases of severe deficiency with active bleeding or significantly prolonged clotting times, intravenous vitamin K may be administered in hospital settings for rapid correction. Intravenous administration carries a rare risk of hypersensitivity reactions and should be given slowly. Intramuscular injection may be avoided in patients with coagulopathy due to the risk of haematoma formation.

• Treating underlying causes – Addressing malabsorption disorders, optimising liver function, or reviewing medications that may interfere with vitamin K metabolism.

Patients with unexplained bleeding disorders should be referred appropriately for specialist haematology assessment according to local pathways. You should seek urgent medical attention if you experience:

• Severe or uncontrolled bleeding – call 999 or go to A&E

• Blood in urine or stools

• Persistent severe headaches (which could indicate intracranial bleeding)

• Sudden vision changes or neurological symptoms

Follow-up testing may be arranged to monitor your response to treatment, typically including repeat clotting studies rather than direct vitamin K measurement. Most patients with vitamin K deficiency respond well to treatment, with clotting function normalising within days to weeks. If you are taking warfarin, it is essential not to take vitamin K supplements or make significant changes to your vitamin K intake without consulting your anticoagulation service or GP, as this could interfere with your anticoagulation control.

If you experience any suspected adverse reactions to vitamin K treatment, report them to the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

Frequently Asked Questions