Vitamin C (Ascorbic Acid) Unit Converter
Vitamin C (Ascorbic Acid) Unit Converter
Convert plasma ascorbate between mg/dL, mg/L and µmol/L — 1.0 mg/dL is 56.8 µmol/L — and read the result knowing that a mishandled, unstabilised sample is the commonest cause of an apparent vitamin C deficiency.
Vitamin C (Ascorbic Acid) converter
Mass ⇄ molarPlasma ascorbic acid 0.9 mg/dL
The conversion
mg/dL = µmol/L ÷ 56.7795
mg/L = mg/dL × 10
because 56.7795 = 1 mg/dL ÷ 176.12 g/mol, the molecular weight of L-ascorbic acid
- MW 176.12
- L-ascorbic acid, C₆H₈O₆. Only the L-enantiomer has vitamin activity
- mg/dL, mg/L, µg/mL
- mg/L and µg/mL are the same number as each other, and both are ten times the mg/dL figure. North American reports usually print mg/dL; µmol/L is standard in the United Kingdom and Europe
- the check worth remembering
- 1.0 mg/dL = 56.8 µmol/L. If a converted figure is out by a factor of ten, the mg/dL and mg/L columns have been confused
- what is measured
- reduced ascorbate. Dehydroascorbate, its oxidised form, is not counted by most methods — which is precisely why an oxidised, badly handled sample reads low
Worked example
Plasma ascorbic acid 0.9 mg/dL
0.9 mg/dL = 9.00 mg/L = 9.00 µg/mL
0.9 × 56.7795 = 51.1 µmol/L
51.1 µmol/L sits inside Mayo's 0.4–2.0 mg/dL interval, which is 22.7–113.6 µmol/L
The sanity check: 1.0 mg/dL would be 56.8 µmol/L, so 0.9 mg/dL being just over 51 is right
But before reading that as adequate vitamin C status, ask two questions — was the sample stabilised, chilled and kept dark on its way to the laboratory, and has the patient eaten fruit in the last few days? A yes to the second can mask depleted tissue stores
Sample handling, and the deficiency it invents
| Requirement | Why | What happens if it is missed |
|---|---|---|
| Stabilise the plasma — classically with metaphosphoric acid | Ascorbate is oxidised to dehydroascorbate in plasma within minutes to hours; acid precipitation of protein with a reducing environment preserves it | Ascorbate is lost and the result is falsely low. Most assays measure reduced ascorbate only |
| Chill immediately — wet ice from the moment of collection | Oxidation is temperature-dependent | A warm sample loses ascorbate on the bench and in transit |
| Protect from light | Ascorbate is photolabile | Further loss |
| Separate quickly and freeze — Mayo require processing within two hours and freezing, ideally at or below −60 °C | Once frozen and light-protected the analyte is stable for days to weeks | Delay at ambient temperature is the single commonest reason a well-nourished patient is reported as vitamin C deficient |
Plasma against leucocyte ascorbate
| Measurement | What it reflects | Behaviour |
|---|---|---|
| Plasma ascorbate | Intake over recent days | Responds within days to a change in intake; saturates at about 70–80 µmol/L because absorption and renal reabsorption are both saturable |
| Leucocyte ascorbate | Tissue stores | Falls late and recovers slowly — the better index of body stores, though technically demanding and not widely available |
| Clinical assessment | Scurvy itself | Perifollicular haemorrhage, corkscrew hairs, swollen bleeding gums, poor wound healing, bruising, fatigue, arthralgia and, in children, bone pain |
An unstable analyte, and the deficiency the transport system invents
Plasma vitamin C is reported as ascorbic acid in milligrams per decilitre, milligrams per litre (identical to micrograms per millilitre) or micromoles per litre. The molecular weight of L-ascorbic acid is 176.12, so one milligram per decilitre is 56.7795 micromoles per litre. The figure worth remembering is that 1.0 mg/dL is 56.8 µmol/L; if a conversion comes out ten times too large or too small, mg/dL and mg/L have been confused.
The dominant practical issue with this test is not the arithmetic but the stability of the analyte. Ascorbate is a reducing agent, which is the whole basis of what it does in the body, and in a tube of plasma it oxidises to dehydroascorbate within minutes to hours — faster when warm, faster in light. Most assays measure reduced ascorbate only, so whatever has oxidised has simply disappeared from the result. Laboratories therefore ask for the plasma to be stabilised, classically by precipitating protein with metaphosphoric acid, placed on wet ice at collection, protected from light and separated and frozen promptly — Mayo specify processing within two hours and freezing ideally at or below minus 60 degrees. A sample that has crossed a hospital in a clear tube at room temperature, with a couple of hours on a bench at each end, can lose a large part of its ascorbate before analysis. That, rather than the patient’s diet, is the commonest reason for an unexpectedly low vitamin C, and it is the first thing to check when a low result does not fit the patient.
The second thing to understand is what a plasma level represents. Ascorbate absorption is carrier-mediated and saturable, and renal reabsorption is too, so plasma concentrations plateau around 70 to 80 micromoles per litre no matter how much is taken by mouth — the pharmacokinetic work behind the recommended intakes rests on exactly that observation. The consequence is that plasma ascorbate tracks intake over recent days rather than the state of the tissues. Leucocyte ascorbate falls later and recovers more slowly and is the better index of body stores, though it is technically demanding and not widely offered.
Scurvy remains a clinical diagnosis that the level supports. Perifollicular haemorrhage and corkscrew hairs, swollen bleeding gums in a patient with teeth, easy bruising, poor wound healing, fatigue, arthralgia and, in children, bone pain and pseudoparalysis make the diagnosis; a low plasma ascorbate corroborates it, and the response to replacement — often striking within days — completes the picture. A patient who has eaten fruit in the week before the blood test may have a plasma ascorbate in the reference interval and scurvy nonetheless.
Frequently asked questions
How do you convert vitamin C from mg/dL to µmol/L?
Multiply by 56.7795, which is one milligram per decilitre divided by ascorbic acid’s molecular weight of 176.12 g/mol. So 1.0 mg/dL is 56.8 µmol/L, and a plasma ascorbate of 0.9 mg/dL is 51.1 µmol/L. Milligrams per litre and micrograms per millilitre are the same number as each other and ten times the mg/dL figure.
Why do vitamin C samples need special handling?
Because ascorbate is a reducing agent and oxidises to dehydroascorbate in plasma within minutes to hours, faster when warm or exposed to light, and most assays measure only the reduced form. Laboratories ask for stabilisation — classically with metaphosphoric acid — immediate chilling on wet ice, protection from light, and prompt separation and freezing. A sample that misses those steps loses ascorbate and is reported falsely low.
What is the commonest cause of a low vitamin C result?
A mishandled sample. An unstabilised, warm, light-exposed or delayed specimen can lose a large fraction of its ascorbate before analysis, and that is a far commoner explanation for an unexpected “deficiency” than the patient’s diet. When a low result does not fit the clinical picture, check the collection and transport before acting on it.
Does a normal plasma vitamin C exclude scurvy?
No. Plasma ascorbate reflects intake over recent days and saturates around 70–80 µmol/L, so a patient who has eaten fruit in the days before the test can have a level inside the reference interval while tissue stores remain depleted. Leucocyte ascorbate reflects stores better. Scurvy is a clinical diagnosis — perifollicular haemorrhage, corkscrew hairs, bleeding gums, poor wound healing — supported by the level and by the response to treatment.
Can a vitamin C level be too high?
Supplementation raises it, but absorption and renal reabsorption are both saturable, so oral dosing cannot push plasma much past the mid-70s µmol/L; a markedly high value suggests a recent intravenous dose. The relevant concern at high intakes is oxalate: ascorbate is metabolised to oxalate, so high doses matter in calcium oxalate stone formers and in renal impairment. Very high ascorbate concentrations also interfere with some glucose meters and other assays.
Related calculators
References
- Levine M, Conry-Cantilena C, Wang Y, et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad Sci USA. 1996;93(8):3704–3709.
- Mayo Clinic Laboratories. Test ID: VITC — Ascorbic Acid (Vitamin C), Plasma. Reference values 0.4–2.0 mg/dL; below 0.2 mg/dL significant deficiency, 0.2–0.4 mg/dL moderate risk. Wet ice, process within 2 hours, freeze protected from light.
- Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington DC: National Academies Press; 2000.
- Maxfield L, Daley SF, Crane JS. Vitamin C deficiency. In: StatPearls. Treasure Island FL: StatPearls Publishing; 2024.
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.