What this test measures
Vitamin K is the cofactor needed to produce active forms of clotting factors II (prothrombin), VII, IX and X, as well as proteins C and S and bone proteins (osteocalcin, matrix Gla protein). Vitamin K1 (phylloquinone) comes from green leafy vegetables; vitamin K2 (menaquinone) is made by gut bacteria.
Direct measurement of serum vitamin K1 is uncommon in routine practice — vitamin K status is usually inferred from prothrombin time (PT) / INR, which is sensitive to vitamin K-dependent factor activity. The vitamin K assay is reserved for specific cases — refractory coagulopathy of unclear cause, neonatal haemorrhagic disease workup, and research.
Why it matters
In India, clinically relevant vitamin K deficiency is most often seen in: newborns (Vitamin K Deficiency Bleeding — VKDB, prevented by routine IM vitamin K at birth as per Indian Academy of Paediatrics), chronic cholestatic liver disease (fat malabsorption, including biliary obstruction), prolonged antibiotic use (suppresses gut menaquinone production), severe malabsorption (cystic fibrosis, short bowel, IBD), and warfarin overdose (warfarin is a vitamin K antagonist).
For most clinical questions, PT/INR is the right test — vitamin K assay adds nothing. Direct vitamin K is occasionally ordered in unexplained INR derangement or in research settings.
How to prepare
Fasting morning sample preferred — vitamin K is lipid-bound and recent fatty meals can shift values. Avoid vitamin K supplements for at least 24 hours. Avoid eating large amounts of green leafy vegetables in the 24 hours before. Mention all medications, especially warfarin, anti-epileptics, broad-spectrum antibiotics.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Vitamin K1 (Phylloquinone) (ng/mL)[1][2] | 0.13 – 1.19 (method-dependent — confirm with lab) | Deficiency — newborns (limited placental transfer, low breast milk content, sterile gut), chronic cholestasis (impaired fat absorption), prolonged broad-spectrum antibiotic use (suppresses gut menaquinone), chronic fat malabsorption (cystic fibrosis, short bowel), warfarin therapy (antagonist effect — not a true substrate deficiency). Clinically presents as easy bruising, mucosal bleeding, prolonged PT / INR, and (in newborns) catastrophic intracranial bleed. | High vitamin K is usually from recent supplementation or a vitamin K-rich meal. No known toxicity from natural vitamin K — the body excretes excess readily. Synthetic menadione (K3) is not used in humans because of toxicity. |
How to assess vitamin K status — PT/INR or direct assay
| Test | Best use | Notes |
|---|---|---|
| PT / INR | First-line for any suspected vitamin K deficiency or warfarin monitoring | Cheap, fast, widely available; corrects with vitamin K replacement |
| PIVKA-II (DCP / des-γ-carboxy prothrombin) | Sensitive for early vitamin K deficiency before PT changes; also a hepatocellular carcinoma marker | Niche use |
| Direct Vitamin K1 | Refractory coagulopathy of unclear cause; research | Expensive, slow; PT / INR usually enough |
| Vitamin K Challenge | IV / IM vitamin K → repeat PT; corrects vitamin K deficiency but not liver disease | Useful in distinguishing causes of prolonged PT |
Frequently asked questions
Why isn't vitamin K routinely measured?
Prothrombin time (PT / INR) is a cheaper, faster, more clinically meaningful test — it reflects the functional consequence of vitamin K deficiency. Direct vitamin K assay is reserved for refractory or unexplained cases.
Do I need to fast?
A fasting morning sample is preferred. Avoid vitamin K supplements and large amounts of green leafy vegetables for 24 hours.
Why is vitamin K given to all newborns?
Newborns have limited placental transfer of vitamin K, low breast milk content, and a sterile gut that does not produce menaquinone — so they are at risk of Vitamin K Deficiency Bleeding (VKDB), including catastrophic intracranial haemorrhage. A single IM dose at birth (1 mg) prevents this and is standard of care in Indian and international guidelines.
How does warfarin work?
Warfarin blocks the enzyme that recycles vitamin K (vitamin K epoxide reductase), reducing the activity of factors II, VII, IX, X and proteins C, S. Effect is monitored by PT / INR. Reversal uses vitamin K (slow) or fresh frozen plasma / prothrombin complex concentrate (fast).
What foods are high in vitamin K?
Green leafy vegetables — spinach, kale, methi, coriander, broccoli — are the richest sources. Patients on warfarin should aim for a consistent (not zero) intake, because abrupt changes upset INR.
Can long-term antibiotics cause vitamin K deficiency?
Yes — broad-spectrum antibiotics for several weeks can suppress gut bacteria that produce menaquinone (K2), occasionally causing clinically meaningful deficiency, especially when intake is poor or fat absorption is impaired.
Is vitamin K supplementation needed in adults?
Not for healthy adults with a normal diet. Indicated in chronic cholestasis, fat malabsorption, prolonged antibiotic use with poor intake, and as warfarin reversal.
Related Vitamins / Nutrition tests
Tests commonly ordered alongside VITAMIN K, or that help interpret an unexpected result.
Sources & references
- NIH Office of Dietary Supplements — Vitamin K · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Vitamin K Deficiency · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Laboratories — Vitamin K1, Serum · accessed 2026-05-30T00:00:00.000Z
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