Vitamin K
Vitamin K, a fat soluble vitamin, is a generic name for a group of quinine compounds with a methylated naphthoquinone structure. These include the naturally occurring form phylloquinone (vitamin K1), menaquinones, and a synthetic form, menadione. Dietary phylloquinone (2-methyl-3 phytyl-1,4-naphthoquinone) is the main source of vitamin K.
Phylloquinone plays an important role in coagulation, bone metabolism, and vascular health. Phylloquinone acts as a cofactor for a specific microsomal enzyme, carboxylase, which catalyzes the post-translational carboxylation of glutamic acid (Glu) to ɣ-carboxy glutamic acid (Gla) in vitamin K-dependent proteins. This step confers calcium & phospholipid binding properties to the vitamin K dependent proteins, and so is essential for their functional activity.
The main form of vitamin K1 (phylloquinone) is transported primarily by chylomicrons. Consequently plasma vitamin K concentrations are influenced by plasma triglycerides concentration and reflect recent dietary intake. It is also associated with serum lipoproteins mainly transported in the triglyceride-rich VLDL.
Deficiency and Toxicity
Patients who are at risk of developing vitamin K deficiency are those on long-term broad-spectrum antibiotics or long-term hyperalimentation, and patients with chronic liver disease, biliary obstruction, or fat malabsorption. Poor vitamin K status is associated with impaired blood clotting (causing excessive bleeding) and impaired bone metabolism in the young and the elderly resulting in defective bone formation and bone disease. Oral anticoagulants such as warfarin act by antagonizing the metabolism of vitamin K.
Haemorrhagic disease of the newborn can develop readily as a result of poor placental transfer of vitamin K, liver immaturity and poor vitamin K content of breast milk. This condition is routinely prevented by prophylactic administration of vitamin K1.
No toxic manifestations have been noted on ingestion of large amounts of vitamin K over extended period of time. However administration of Menadione (not phylloquinone) may cause haemolytic anaemia, hyperbilirubinaemia and kernicterus in newborn.
Assessment of status
Assessment of phylloquinone status is useful in patients with obstructive liver disease, malabsorption due to coeliac disease, or pancreatitis and in patients on long-term parenteral nutrition. Phylloquinone status can be assessed by direct measurement of its circulating concentration or by functional assays such as prothrombin time. The direct quantification of phylloquinone concentration in plasma is probably the best indicator of vitamin K status. However, plasma vitamin K concentrations are influenced by plasma triglycerides concentration as most of the circulating vitamin K1 is associated with VLDL.85 Phylloquinone population reference intervals should therefore be expressed as a ratio of the triglyceride concentration.85
In addition, work done by STEMDRL has shown that phylloquinone concentrations in plasma fall transiently by almost 50% after an acute inflammatory insult.85 When results were adjusted for the decrease in triglyceride concentration over the same study period, however, the decrease in phylloquinone concentration was no longer significant over the period of injury (Table 17). These results suggest that the changes in circulating phylloquinone concentration during acute injury may simply reflect the passive process of inflammation-driven redistribution of the plasma lipid fraction.85 Therefore, in the presence of systemic inflammatory response, plasma phylloquinone concentrations are unlikely to be a reliable measure of status, and during such a response the plasma phylloquinone:triglyceride ratio provides a more reliable measurement of phylloquinone status.85
We routinely only report vitamin K1:triglyceride ratio for assessment of vitamin K status. Results are expressed as nmoles of vitamin K1 per mmol triglyceride.
Recommended Daily Allowance
Adults: 120 ug/day.
Effect of Systemic Inflammatory Response on Plasma Vitamin K Concentrations
Table: Baseline, peak/ trough and day 7 concentrations of CRP, phylloquinone, triglyceride and phylloquinone:triglyceride ratio after elective surgery for knee arthroplasty (n = 10).85 Median (range).
|
Baseline values (0 h) |
Peak/trough (48 h) |
Final values (168 h) |
Friedman P (0–168 h) |
|
| CRP (mg/L) | <6 (<6–<6) | 155 (83–242) | 36 (<6–88) | <0.001 |
| Plasma phylloquinone (nmol/L) |
0.68 (0.44-1.9) |
0.28 (0.15-0.55) |
0.54 (0.29-1.74) |
<0.001 |
|
Plasma triglyceride (mmol/L) |
2.6 (0.70-8.80) | 1.20 (0.60-4.60) | 1.75 (0.90-3.90) | <0.001 |
|
Phylloquinone:triglyceride (nmol/mmol) |
0.33 (0.22-0.69) |
0.26 (0.13-0.57) |
0.30 (0.15-0.51) |
0.24 |
Sample Requirements and Reference Ranges for Vitamin K
| Sample Type | Plasma/serum or whole blood. (Fasting sample preferred*). |
|---|---|
| Container | Lithium heparin (non-gel) or EDTA. |
| Precautions | Light-sensitive; wrap in tin foil. Send by first class post within 72 hours of collection. If delivery to Glasgow is outwith 72 hours, separate sample and store plasma frozen until sending and then send by first class post. (ice or dry ice not required). |
| Minimum volume** | 500 µL plasma/serum or 1 mL whole blood (this is sufficient for vitamins A and E to be analysed on the same sample). |
| Reference range |
0.2 to 2.2 nmol/mmol triglyceride. (STEMDRL derived)85 <0.2 nmol/mmol triglyceride (At risk of deficiency) |
| Mean turnaround time | 5.9 days (see this page for STEMDRL TAT targets) |
| Method | Liquid chromatography - tandem mass spectrometry |
| Traceability | EQA. Please contact laboratory for current EQA performance. |
| Intermediate Precision (CV) | See this page for latest data |
| Measurement Uncertainty, U | See this page for latest data |
| Analytical Goals (CV) |
See this page for latest data Plasma vitamin K goal is based on biological variation71 |
| EQA Scheme | UK NEQAS, Birmingham (four times per year). |
| Included in UKAS scope? | Yes |
* Ideally a fasting sample should be collected, especially if the patient is receiving oral or parenteral vitamin K supplementation. If this is not possible, sample should be taken at least 8 hours post treatment for patients receiving oral supplementation or TPN.
** Absolute minimum volume; this volume is insufficient to carry out repeat analysis if analysis fails.
Please note that this assay is not currently UKAS accredited.