Vitamin B6 (PLP) Unit Converter
Vitamin B6 (PLP) Unit Converter
Convert plasma pyridoxal 5'-phosphate between µg/L, ng/mL and nmol/L — the active B6 vitamer, lowered by inflammation independently of B6 status, and raised into neurotoxicity by pyridoxine supplements.
Vitamin B6 (PLP) converter
Mass ⇄ molarPlasma pyridoxal 5'-phosphate 20 µg/L
The conversion, and which vitamer it is
µg/L = nmol/L ÷ 4.04629
because 4.04629 = 1 µg/L ÷ 247.14 g/mol, the molecular weight of pyridoxal 5′-phosphate
- MW 247.14 — pyridoxal 5'-phosphate
- PLP, the active coenzyme form of vitamin B6 and the cofactor for well over a hundred enzymes, transaminases and decarboxylases among them. It is the vitamer plasma assays measure and the right one to measure
- the other vitamers
- pyridoxine (169.18), pyridoxal (167.16), pyridoxamine (168.19) and their phosphates all count as vitamin B6. Pyridoxine is what supplements contain; PLP is what the body uses, after hepatic conversion. A factor derived from pyridoxine does not belong on a PLP result
- µg/L = ng/mL
- identical concentrations. Watch for nmol/mL on some reports, which is 1,000 times a nmol/L
- the sample
- fasting plasma, protected from light. PLP is photolabile, and the reference interval assumes no vitamin supplement in the preceding 24 hours
Worked example
Plasma pyridoxal 5'-phosphate 20 µg/L
20 µg/L = 20.00 ng/mL — the same number
20 × 4.04629 = 80.9 nmol/L
80.9 nmol/L sits inside Mayo's 5–50 µg/L interval, which is 20.2–202.3 nmol/L
Going back: 80.9 ÷ 4.04629 = 20.0 µg/L
Now the interpretation the arithmetic cannot give you. If this patient has a CRP of 80, the PLP is being pulled down by the inflammation and may be understating true B6 status; if the patient takes 100 mg of pyridoxine daily, the question is not deficiency but the neuropathy that dose can cause
Why a low PLP is not always a low B6
| Cause of a low plasma PLP | Mechanism | What to do |
|---|---|---|
| Inflammation | PLP falls during an acute-phase response independently of vitamin B6 intake or tissue stores — mobilisation to sites of inflammation and altered distribution rather than depletion | Measure CRP alongside, and interpret a low PLP in an inflamed patient with caution. Repeat when the inflammation has settled |
| True dietary deficiency | Low intake, rare in isolation in high-income settings | Usually part of broader undernutrition |
| Alcohol use | Acetaldehyde displaces PLP from its binding protein and accelerates its degradation | Treat the alcohol use; replace B6 |
| Isoniazid, hydralazine, penicillamine, cycloserine | React with PLP to form inactive hydrazones | Prophylactic pyridoxine with isoniazid is routine |
| Chronic kidney disease and dialysis | Increased losses and altered metabolism | Supplementation is usual |
| Coeliac disease and other malabsorption | Reduced absorption | Investigate the malabsorption |
And why a high PLP matters too
| Point | Detail |
|---|---|
| Excess pyridoxine causes a sensory peripheral neuropathy | A dose-dependent sensory ganglionopathy: paraesthesiae, numbness, a glove-and-stocking sensory loss and sensory ataxia, with motor function typically spared |
| It was first described at very high doses | Schaumburg’s 1983 series reported severe sensory neuropathy in people taking 2–6 g of pyridoxine daily, some of it reversible on stopping |
| It is not confined to gram doses | Cases are described at more modest long-term supplemental intakes. The tolerable upper intake level for adults was set at 100 mg per day |
| The treatment is to stop the pyridoxine | Recovery is usual but can be slow and incomplete. The important step is identifying the source — often an over-the-counter B-complex, an energy drink or a high-dose B6 preparation the patient does not think of as a drug |
The active vitamer, and two ways to misread it
Vitamin B6 is not one molecule. Pyridoxine, pyridoxal and pyridoxamine and their 5′-phosphates all count, and which one is measured decides the conversion factor. Plasma assays measure pyridoxal 5′-phosphate — PLP — because that is the active coenzyme, the form that serves as cofactor for well over a hundred enzymes including the transaminases and the decarboxylases of neurotransmitter synthesis. Its molecular weight is 247.14, so one microgram per litre is 4.04629 nanomoles per litre and a PLP of 20 µg/L is 80.9 nmol/L. Micrograms per litre and nanograms per millilitre are the same number; watch out for nmol/mL on some reports, which is a thousand times a nmol/L.
The first way to misread the result is to treat a low PLP as proof of vitamin B6 deficiency. Plasma PLP falls during an acute-phase response independently of intake or tissue stores — an effect that has been described repeatedly and is mechanistically explained by redistribution of the vitamer to sites of inflammation rather than by depletion. The practical consequence is that a PLP measured during an acute illness, in active rheumatoid arthritis, in inflammatory bowel disease or after surgery can read low in a patient whose vitamin B6 status is entirely adequate. A CRP measured at the same time is what lets you tell the difference, and where the CRP is raised the sensible course is to repeat the PLP once the inflammation has settled rather than to commit to replacement.
The second way to misread it is to assume a high result is harmless. It is not. Chronic excess pyridoxine causes a sensory peripheral neuropathy — a dose-dependent sensory ganglionopathy with paraesthesiae, numbness, glove-and-stocking sensory loss and sensory ataxia, with motor function typically spared. Schaumburg’s original series described severe cases in people taking two to six grams a day, but neuropathy is also reported at more modest long-term supplemental intakes, and the tolerable upper intake level for adults was set at 100 mg daily. The treatment is to find and stop the source, which is frequently an over-the-counter B-complex or an energy drink that the patient does not think of as medication. Recovery is usual but may be slow and incomplete.
Sample handling is straightforward but not optional: fasting plasma, collected into an amber tube or otherwise protected from light, separated promptly. PLP is photolabile and a light-exposed sample is reported falsely low. Laboratories generally ask that vitamin supplements are avoided for 24 hours before collection, because a dose taken the previous evening can move the result well above the interval.
Frequently asked questions
How do you convert vitamin B6 (PLP) from µg/L to nmol/L?
Multiply by 4.04629, which is one microgram per litre divided by the molecular weight of pyridoxal 5′-phosphate, 247.14 g/mol. A PLP of 20 µg/L is 80.9 nmol/L. Divide by the same factor to go back. µg/L and ng/mL are numerically identical, but nmol/mL is a thousand times a nmol/L.
Why is PLP the right form of vitamin B6 to measure?
Because it is the active coenzyme. Pyridoxine, pyridoxal and pyridoxamine and their phosphates all count as vitamin B6, but pyridoxal 5′-phosphate is the form that acts as cofactor for more than a hundred enzymes, and it is what plasma assays are calibrated to report. A conversion factor derived from pyridoxine, molecular weight 169.18, does not belong on a PLP result.
Can inflammation cause a low PLP?
Yes, and this is the commonest reason a low PLP is over-interpreted. Plasma PLP falls during an acute-phase response independently of vitamin B6 intake or tissue stores, through redistribution rather than depletion. Measure a CRP alongside: where it is raised, a low PLP may simply reflect the inflammation, and repeating the measurement once the inflammation has settled is more informative than starting replacement.
Is a high vitamin B6 level harmful?
It can be. Chronic excess pyridoxine causes a sensory peripheral neuropathy — numbness, paraesthesiae, glove-and-stocking sensory loss and sensory ataxia, with motor function usually spared. It was first described at gram doses but is reported at more modest long-term supplemental intakes, and the tolerable upper intake level for adults is 100 mg per day. The management is to identify and stop the supplement, which is often an over-the-counter B-complex or an energy drink.
Does the sample need any special handling?
Yes. Fasting plasma, protected from light, separated promptly. PLP is photolabile, so a light-exposed sample is reported falsely low, and laboratories generally ask that vitamin supplements are avoided for 24 hours before collection because a recent dose can push the result above the interval.
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References
- Paul L, Ueland PM, Selhub J. Mechanistic perspective on the relationship between pyridoxal 5′-phosphate and inflammation. Nutr Rev. 2013;71(4):239–244. doi:10.1111/nure.12014
- Schaumburg H, Kaplan J, Windebank A, et al. Sensory neuropathy from pyridoxine abuse: a new megavitamin syndrome. N Engl J Med. 1983;309(8):445–448.
- Mayo Clinic Laboratories. Test ID: PLP — Pyridoxal 5-Phosphate, Plasma. Reference values 5–50 mcg/L; 12-hour fast required; amber vial, light protected; no vitamin supplements for 24 hours.
- Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington DC: National Academies Press; 1998. Vitamin B6 chapter — tolerable upper intake level 100 mg/day for adults.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.