The vitamin K1 test measures phylloquinone, the main dietary form of vitamin K found in leafy greens, vegetable oils, and some fruits. It is most often used when a clinician suspects low vitamin K intake, poor fat absorption, bile flow problems, long antibiotic exposure, or unexplained bleeding with abnormal clotting tests. Vitamin K1 helps the liver activate clotting factors, so severe deficiency can raise bleeding risk. Mildly low blood levels, however, do not always mean a person is actively bleeding or has a dangerous deficiency.
This test needs careful interpretation because vitamin K1 in blood changes after meals, supplements, and changes in blood lipids. A single result usually works best when combined with diet history, medication review, liver and bile duct markers, prothrombin time, INR, and sometimes PIVKA-II, a functional marker of vitamin K use.
- Vitamin K1 is usually measured in serum or plasma as phylloquinone, often by LC-MS/MS.
- A common adult reference range is about 0.10–2.20 ng/mL, but ranges vary by laboratory.
- Low vitamin K1 usually suggests low recent intake, poor absorption, or impaired bile flow, especially when symptoms or clotting tests also point to deficiency.
- A high vitamin K1 result most often comes from recent supplements, a nonfasting sample, a vitamin K-rich meal, or high lipoprotein levels.
- A 12-hour fast is commonly required, and the blood sample often needs protection from light.
- Urgent care is needed for serious bleeding, black stools, vomiting blood, sudden severe headache, or neurologic symptoms.
Table of Contents
- What the Vitamin K1 Test Measures
- Vitamin K1 Normal Range and Result Meaning
- Low Vitamin K1: Causes and Deficiency Signs
- High Vitamin K1 Results
- How to Prepare for the Test
- Follow-Up Tests and Related Markers
- Improving Low Vitamin K1
- When to Seek Medical Care
What the Vitamin K1 Test Measures
The vitamin K1 test measures the amount of phylloquinone circulating in the blood. Phylloquinone is vitamin K1, the plant-based form of vitamin K. It comes mainly from green vegetables such as spinach, kale, collards, broccoli, and Brussels sprouts, with smaller amounts from vegetable oils and certain fruits.
Vitamin K1 is fat-soluble. This means it depends on normal fat digestion, bile flow, pancreatic function, and intestinal absorption. After absorption, it travels in lipoproteins, the same particle family that carries triglycerides and cholesterol. Because of this, vitamin K1 blood levels can rise after a meal and may be harder to interpret when a specimen is lipemic, meaning visibly fatty.
The test does not directly measure how well blood clots. It measures the circulating supply of vitamin K1. The body uses vitamin K to activate several proteins, including clotting factors II, VII, IX, and X and the natural anticoagulant proteins C and S. A broader vitamin K blood test discussion may include vitamin K1, vitamin K2 forms, and functional markers, but many clinical labs focus on vitamin K1 because it is the most measurable dietary form.
Vitamin K1 testing is most useful when the result answers a specific clinical question, such as:
- Is low vitamin K intake contributing to abnormal clotting tests?
- Is fat malabsorption causing low levels of fat-soluble vitamins?
- Is bile duct obstruction or cholestasis limiting vitamin K absorption?
- Did a supplement, meal replacement, or tube-feeding formula provide too much or too little vitamin K?
- Is a person on long-term antibiotics, parenteral nutrition, or a highly restricted diet at risk?
The test is less useful as a general wellness screen. A normal result does not rule out every problem with vitamin K-dependent proteins, and a low result should not be interpreted without symptoms, medications, diet, liver function, and clotting results.
Vitamin K1 Normal Range and Result Meaning
A commonly used adult serum vitamin K1 reference range is 0.10–2.20 ng/mL. The same numeric value may also be written as 0.10–2.20 mcg/L, because 1 ng/mL equals 1 mcg/L. Pediatric ranges may not be established by some laboratories, so children’s results need age-aware interpretation by the ordering clinician.
Reference ranges vary because labs use different methods, specimen types, fasting rules, and calibration standards. Always compare the number with the range printed on the report.
| Result pattern | Usual meaning | Important context |
|---|---|---|
| Below the lab range | Low circulating vitamin K1; possible insufficiency or deficiency | More concerning with bleeding, malabsorption, bile duct disease, poor intake, or abnormal PT/INR |
| Within the lab range | Circulating vitamin K1 is in the expected fasting range | Does not fully measure vitamin K use in tissues or long-term status |
| Above the lab range | Often recent vitamin K intake, supplements, nonfasting sample, or high lipoproteins | Usually not “toxicity,” but it can interfere with warfarin anticoagulation |
A low vitamin K1 result means the blood sample contained less phylloquinone than expected. It may reflect poor recent intake, poor absorption, or a temporary low level after dietary changes. It becomes more clinically meaningful when other findings point in the same direction.
A normal result means the blood level was within the lab’s expected fasting range at the time of collection. Because vitamin K1 reflects recent intake and circulates with lipids, it is not a perfect measure of total body vitamin K sufficiency. People with liver disease, anticoagulant use, or serious illness may still need other tests.
A high result usually means the sample was collected after vitamin K intake or supplementation. Vitamin K1 does not have a standard “toxic” blood range in the way some drugs do. The main clinical issue is interaction with vitamin K antagonist medicines, especially warfarin.
The prothrombin time and INR show whether the blood is taking longer than expected to clot through vitamin K-dependent pathways. These tests usually change only when vitamin K deficiency is more significant, when liver synthesis is impaired, or when medication affects clotting.
Low Vitamin K1: Causes and Deficiency Signs
Low vitamin K1 means the measured phylloquinone level is below the laboratory’s reference range. It does not automatically mean severe deficiency, but it deserves attention when the person has bleeding symptoms, abnormal clotting tests, or risk factors for poor absorption.
Vitamin K deficiency can develop faster than many people expect because the body stores relatively small amounts compared with some other fat-soluble vitamins. A person with poor intake plus poor absorption can become deficient within days to weeks, especially during illness, antibiotic treatment, or inadequate nutrition.
Common causes of low vitamin K1 include:
- Low dietary intake: very limited vegetable intake, prolonged poor appetite, restrictive diets, eating disorders, or inadequate tube-feeding formulas.
- Fat malabsorption: celiac disease, inflammatory bowel disease, short bowel syndrome, cystic fibrosis, chronic pancreatitis, pancreatic insufficiency, and some bariatric surgeries.
- Reduced bile flow: cholestasis, bile duct obstruction, obstructive jaundice, advanced liver or biliary disease.
- Medication effects: long antibiotic courses, bile acid sequestrants such as cholestyramine, orlistat, mineral oil, and some anticonvulsants.
- Critical illness or hospitalization: poor intake, inflammation, antibiotics, altered gut flora, and increased physiologic demand.
- Newborn risk: low placental transfer, low vitamin K in breast milk, immature gut flora, and lack of vitamin K prophylaxis at birth.
Severe vitamin K deficiency mainly affects bleeding risk. Symptoms may include easy bruising, nosebleeds, bleeding gums, heavy menstrual bleeding, blood in urine, black or bloody stools, prolonged bleeding from cuts, or unusual bleeding after surgery, dental work, or injections.
A low vitamin K1 result with a prolonged PT result is more concerning than a low vitamin K1 result alone. PT often rises before aPTT because factor VII has a short half-life. In advanced deficiency, multiple vitamin K-dependent clotting factors can fall.
Vitamin K deficiency is not the only cause of bruising or bleeding. Low platelets, platelet function problems, liver disease, inherited clotting factor disorders, anticoagulant medicines, kidney failure, and blood vessel fragility can cause similar symptoms. That is why a low vitamin K1 result is usually interpreted as part of a broader clotting and nutrition evaluation.
High Vitamin K1 Results
High vitamin K1 usually reflects recent intake rather than harm from vitamin K itself. A person who takes a vitamin K supplement, eats a large serving of leafy greens, drinks a meal replacement shake, or has blood drawn without fasting may have a higher result.
Vitamin K1 also travels with lipoproteins. A lipemic sample can make interpretation difficult because vitamin K1 concentration often rises with triglyceride-rich particles. Some laboratories reject lipemic specimens for this reason.
Common reasons for a high vitamin K1 result include:
- Vitamin K1 supplements or multivitamins
- Combination supplements containing vitamin D, calcium, and vitamin K
- Recent intake of leafy greens, green powders, meal replacements, or nutrition shakes
- Nonfasting blood draw
- High triglycerides or visibly lipemic sample
- Parenteral nutrition or enteral formulas containing vitamin K
- Recent medical vitamin K treatment
A high vitamin K1 result is usually not managed as vitamin K toxicity. Natural vitamin K1 from food has a low toxicity profile, and no standard tolerable upper intake level has been set for vitamin K from foods or ordinary supplements. The more important issue is medication interaction.
People taking warfarin should not suddenly increase or sharply decrease vitamin K intake unless their clinician gives that instruction. Warfarin works by blocking vitamin K recycling. A large increase in vitamin K can lower INR and reduce anticoagulant effect, while a sudden drop in vitamin K intake can raise INR and increase bleeding risk. Consistency matters more than avoidance.
High vitamin K1 does not mean the blood is “too thick.” Clotting risk depends on the whole clinical picture, including medications, medical conditions, inherited risks, inflammation, cancer, immobility, and clotting history. A high vitamin K1 result by itself is rarely enough to explain a clot.
How to Prepare for the Test
Vitamin K1 testing is sensitive to timing, food, supplements, and sample handling. The most common preparation is a 12-hour fast, though the ordering lab’s instructions should be followed exactly. Infants may need blood collected before the next feeding.
Because vitamin K1 is affected by recent intake, preparation should focus on avoiding unusual intake rather than trying to “improve” the number before testing. Do not suddenly stop a prescribed medication or anticoagulant. Do not stop medically directed vitamin K unless the ordering clinician tells you to.
A practical preparation checklist:
- Fast for the required time, often 12 hours.
- Drink water unless told otherwise.
- Ask whether to hold nonessential supplements before the draw.
- Tell the clinician about multivitamins, vitamin K drops, green powders, nutrition shakes, and meal replacements.
- Report warfarin, antibiotics, bile acid sequestrants, orlistat, anticonvulsants, and recent vitamin K treatment.
- Avoid changing your usual diet dramatically in the days before the test unless instructed.
- Make sure the lab knows the sample requires light protection if the test is being sent out.
The collection team may place the serum in an amber tube or wrap it to protect it from light. The sample may need timely centrifugation and separation. These handling details matter because vitamin K1 is measured at low concentrations, and poor sample handling can reduce reliability.
If the result does not match the clinical picture, repeating the test under strict fasting conditions may help. For example, a high result after a nonfasting draw may normalize when repeated correctly. A low result in someone who recently changed diet may need confirmation with diet review and functional clotting markers.
Follow-Up Tests and Related Markers
Follow-up depends on why the vitamin K1 test was ordered. A low level in a healthy person with no symptoms is handled differently from a low level in someone with jaundice, chronic diarrhea, black stools, or an elevated INR.
The most useful follow-up tests often include clotting tests, liver and bile duct tests, a blood count, and sometimes nutritional markers.
| Test | Why it helps |
|---|---|
| PT and INR | Show whether vitamin K-dependent clotting pathways are prolonged |
| PIVKA-II | Shows undercarboxylated prothrombin, a functional sign of poor vitamin K use |
| CBC and platelet count | Look for anemia from bleeding or low platelets as another bleeding cause |
| Liver enzymes, bilirubin, ALP, and GGT | Assess liver injury, cholestasis, and bile flow problems |
| Fat-soluble vitamins A, D, and E | Help identify broader fat malabsorption |
| Stool fat or pancreatic tests | May be used when chronic diarrhea, weight loss, or pancreatic insufficiency is suspected |
The PIVKA-II test can be especially helpful because it reflects whether prothrombin was made without enough active vitamin K. A person may have low-normal vitamin K1 but abnormal vitamin K use, or a low vitamin K1 level without a major functional clotting problem.
Liver disease can complicate interpretation. The liver makes clotting factors, so PT and INR can rise from poor liver synthesis even when vitamin K intake is adequate. Bile flow problems can also reduce vitamin K absorption. In that situation, a liver function test panel helps separate poor absorption from impaired clotting factor production.
For suspected malabsorption, clinicians often look beyond vitamin K alone. Low vitamin A, low vitamin D, low vitamin E, low albumin, weight loss, chronic greasy stools, or persistent diarrhea may point toward a digestive or bile flow problem. A broader vitamin and mineral blood test panel may be useful when the history suggests more than one nutrient deficiency.
Improving Low Vitamin K1
Low vitamin K1 is corrected by treating the cause and restoring an appropriate vitamin K supply. The right approach depends on whether the problem is low intake, poor absorption, medication-related deficiency, bile flow disease, or active bleeding.
For adults, the adequate intake for total vitamin K is 120 mcg per day for men and 90 mcg per day for women. Pregnancy and lactation are also commonly listed at 90 mcg per day for adults. These values are intake targets, not blood test targets.
Food can provide enough vitamin K1 for many people. Examples include:
- ½ cup cooked collards: about 530 mcg
- 1 cup raw spinach: about 145 mcg
- 1 cup raw kale: about 113 mcg
- ½ cup cooked broccoli: about 110 mcg
- 1 tablespoon soybean oil: about 25 mcg
- 1 tablespoon canola oil: about 10 mcg
Eating vitamin K-rich vegetables with some dietary fat can improve absorption. For example, a salad with olive oil, avocado, eggs, nuts, or fish may provide better absorption than plain raw greens with no fat. This does not mean meals need to be high-fat; a modest amount is usually enough.
When malabsorption is present, food changes may not fully correct the problem. People with cholestasis, pancreatic insufficiency, short bowel syndrome, cystic fibrosis, or certain bariatric surgery histories may need medical treatment, targeted supplementation, or monitoring of several fat-soluble vitamins.
Supplement decisions should be individualized. Vitamin K1 is available as phylloquinone or phytonadione. Vitamin K2 supplements, such as MK-4 or MK-7, are also available, but a vitamin K1 blood test primarily reflects circulating phylloquinone. Taking vitamin K2 may not raise vitamin K1 in a predictable way.
People on warfarin need special guidance. They should usually aim for consistent vitamin K intake rather than avoiding greens. Sudden dietary changes can destabilize INR. Any supplement containing vitamin K should be reviewed with the anticoagulation clinic or prescribing clinician before use.
Do not self-treat possible severe deficiency when there is active bleeding, a high INR, liver disease, or suspected bile duct obstruction. Those situations require medical evaluation because the route, dose, urgency, and monitoring plan matter.
When to Seek Medical Care
A low vitamin K1 result should be discussed with the ordering clinician, especially when it appears with bleeding symptoms, abnormal PT/INR, jaundice, chronic diarrhea, unintentional weight loss, recent antibiotics, or known liver, pancreatic, or intestinal disease.
Seek urgent medical care for:
- Vomiting blood or material that looks like coffee grounds
- Black, tarry, or bloody stools
- Blood in urine with clots or heavy bleeding
- Severe headache, confusion, weakness, vision changes, or trouble speaking
- Bleeding that will not stop after firm pressure
- Large unexplained bruises or swelling after minor injury
- Heavy bleeding in a person taking warfarin or another anticoagulant
- Any bleeding concern in a newborn or young infant
Newborn vitamin K deficiency bleeding is a medical emergency because bleeding can occur internally, including in the brain. Vitamin K prophylaxis at birth prevents most cases. A vitamin K1 blood test should not replace urgent evaluation when an infant has bleeding, unusual sleepiness, seizures, poor feeding, or signs of illness.
For non-urgent low results, the next step is usually a structured review: diet, supplements, medications, gastrointestinal symptoms, liver and bile duct markers, and clotting tests. The result becomes more useful when the clinician can connect it to a likely cause and then confirm that treatment improves both the lab pattern and the symptoms.
References
- VITK1 – Overview: Vitamin K1, Serum 2026 (Lab Reference)
- Vitamin K – Health Professional Fact Sheet 2021 (Official)
- Vitamin K Deficiency: Diagnosis and Management 2025 (Review)
- Vitamin K – sources, physiological role, kinetics, deficiency, detection, therapeutic use, and toxicity 2022 (Review)
- Laboratory assessment of vitamin K status 2020 (Review)
- Vitamin K: a potential missing link in critical illness-a scoping review 2024 (Scoping Review)
Disclaimer
Vitamin K1 results should be interpreted by a qualified healthcare professional who can review symptoms, medications, diet, liver function, and clotting tests. Do not start or stop vitamin K supplements, warfarin, antibiotics, or other prescribed medicines based only on a blood test result. Seek urgent care for serious bleeding symptoms or any bleeding concern in a newborn or infant.
