Arginine Unit Converter

Convert arginine between mg/dL and µmol/L — and read the direction, because arginine is low in most urea cycle disorders and high in arginase deficiency, and it falls faster than any other amino acid in a sample left to stand.

Last updated 12 Sep 2026

Arginine converter

mg/dL ⇄ µmol/L

Arginine

Multiply mg/dL by 57.41 to get µmol/L. Arginine is the amino acid most damaged by delayed separation — it falls about 25% in two hours at room temperature — so a low result on a sample that travelled is checked before it is believed.

Reference groupPublished intervals assume a fasting sample separated promptly — Mayo states its values are for fasting patients, and amino acids shift with feeding and again in the tube if the blood is left to stand. In an infant, the sample is taken immediately before a feed or at the interval the metabolic service specifies. Arginine especially: blood for arginine goes on ice or is separated within 30 minutes. The groups below are the Mayo fasting intervals with the Labcorp neonatal band; the two laboratories disagree by 13 µmol/L at the adult lower limit.

86µmol/LExample

Arginine 1.5 mg/dL on a fasting, promptly separated sample, read against the adult interval

Formula and conversion factor

µmol/L = mg/dL × 57.4053
mg/dL = µmol/L ÷ 57.4053

57.4053: derived from the molecular weight of L-arginine, 174.20 Da — the heaviest of the six amino acids on these pages, so the smallest factor
conditionally essential: arginine is made by the urea cycle, so when the cycle is blocked it can no longer be made in sufficient quantity and has to be supplied. That is why it is given therapeutically in most urea cycle disorders
the one exception: arginase 1 deficiency, where arginine is what accumulates. Arginine supplementation is contraindicated there, so the direction of the result is not a detail
43% in 24 hours: the fall in arginine in whole blood at room temperature before separation — 6% at 30 minutes, 25% at 2 hours, 43% at 24 hours, and under 10% over 24 hours if the sample is kept on ice

Worked example

Arginine 1.5 mg/dL on a fasting, promptly separated sample, read against the adult interval
1.5 × 57.4053 = 86.11 µmol/L, reported as 86 µmol/L
Within the Mayo adult interval of 45–144 µmol/L used here, and within Labcorp's 32–150 µmol/L
Now the pre-analytical arithmetic, which matters more here than for any other amino acid: the same blood left two hours at room temperature before separation would have given about 65 µmol/L, and after 24 hours about 49 µmol/L
The 24-hour figure is still inside the adult interval, but it is 43% lower than the truth and would read as a low arginine in a child, where the lower limit is 28 µmol/L
On ice, the change over 24 hours is under 10%. That is the whole argument for putting the tube on ice or separating it within 30 minutes

Low or high — the direction says which urea cycle disorder

Arginine becomes essential in every urea cycle disorder except arginase 1 deficiency, because it is made by the cycle itself — which is why it, or citrulline, is supplemented in the others and contraindicated in that one. Arginase 1 deficiency is also the disorder least likely to present with hyperammonaemia, so a raised arginine may be the finding that leads to it. None of this is decided on one result: a metabolic service confirms it with enzyme and molecular testing.
Disorder	Plasma arginine	Ammonia	Arginine supplementation
CPS1, OTC and NAGS deficiency	Low	Can be very high in crisis	Given — as arginine or as its precursor citrulline
Argininosuccinate synthetase deficiency (citrullinaemia type 1)	Low, with a high citrulline	Can be very high in crisis	Given
Argininosuccinate lyase deficiency	Low, with argininosuccinic acid present	Can be very high in crisis	Given
Arginase 1 deficiency	High — three- to fourfold the upper limit of normal is highly suggestive	Acute hyperammonaemia above 150 µmol/L is uncommon	Contraindicated
Unaffected infant	Up to twofold the upper limit has been reported	Normal	Not indicated

Arginine in blood that was not separated promptly

Measured by Davis and colleagues in stored whole blood. Arginine falls faster than the other amino acids and in the direction that manufactures the abnormality being looked for — a spurious low arginine in a child being investigated for a urea cycle disorder. The fix is simple and belongs on the request: on ice, or separated within 30 minutes. A low arginine from a sample that travelled at room temperature is repeated before it is interpreted.
Time before separation	Room temperature	On ice	What an 86 µmol/L sample would read
30 minutes	about 6% lower	essentially unchanged	about 81 µmol/L
2 hours	about 25% lower	essentially unchanged	about 65 µmol/L
24 hours	about 43% lower	under 10% change	about 49 µmol/L

Made by the cycle it reports on

Arginine is the last amino acid of the urea cycle: ornithine, citrulline, argininosuccinate, arginine, and then arginase splits arginine into urea and ornithine to begin again. It has the heaviest molecular weight of the amino acids on these pages, 174.20 Da, and so the smallest conversion factor — 1 mg/dL is 57.41 µmol/L. Because arginine is made by the cycle rather than only eaten, it is described as conditionally essential: normally the body makes enough, but when the cycle is blocked it cannot, and arginine has to be supplied.

That is why the direction of an abnormal arginine is more informative than its size. In carbamoyl phosphate synthetase 1, ornithine transcarbamylase, N-acetylglutamate synthase, argininosuccinate synthetase and argininosuccinate lyase deficiency, arginine is low, and the guidelines have it supplemented — as arginine itself, or as its precursor citrulline. In arginase 1 deficiency, the last step of the cycle, arginine is what accumulates: a plasma arginine three- to fourfold the upper limit of normal is described as highly suggestive, while up to twofold has been reported in unaffected infants. Arginine supplementation is contraindicated there, so the direction is not a detail.

Arginase 1 deficiency is also the urea cycle disorder that least often announces itself with ammonia. Acute hyperammonaemia above 150 µmol/L is uncommon in it, and it presents instead with progressive spastic diplegia, developmental regression and seizures over years rather than with a neonatal crisis. A raised arginine may therefore be the finding that leads to the diagnosis rather than a confirmation of one already suspected, and it is a secondary rather than a core condition on some newborn screening panels.

One pre-analytical fact deserves more weight for arginine than for any other amino acid on these pages. In whole blood left at room temperature before the plasma is separated, arginine falls by about 6 per cent at 30 minutes, 25 per cent at two hours and 43 per cent at 24 hours, while on ice the change over 24 hours stays under 10 per cent. The error runs downwards — towards exactly the low arginine that a urea cycle work-up is looking for. Plasma amino acids are taken fasting, in an infant immediately before a feed or at the interval the metabolic service specifies, and the tube goes on ice or is separated within half an hour. A low arginine from a sample that travelled warm is repeated, not interpreted.

Frequently asked questions

How do I convert arginine from mg/dL to µmol/L?

Multiply by 57.4053, derived from the molecular weight of arginine, 174.20 Da. An arginine of 1.5 mg/dL is 86 µmol/L. To go the other way, divide the µmol/L figure by 57.4053.

What is a normal arginine level?

Mayo publishes 45–144 µmol/L for fasting adults and Labcorp 32–150 µmol/L for anyone over 15, with wider bands in infancy reaching about 164–171 µmol/L. The interval printed on your own report, for that age and on a properly handled sample, is the one to use.

Why is arginine low in urea cycle disorders?

Because arginine is made by the urea cycle itself, so a block anywhere upstream means it can no longer be synthesised in sufficient quantity. That makes it conditionally essential in those disorders, and it is given therapeutically — as arginine or as its precursor citrulline — in all of them except arginase 1 deficiency.

What does a high arginine mean?

It points to arginase 1 deficiency, the urea cycle disorder in which arginine cannot be broken down. A plasma arginine three- to fourfold the upper limit of normal is described as highly suggestive, while up to twofold occurs in unaffected infants. Arginine supplementation is contraindicated in that disorder, which is why the direction matters so much.

Why does the sample have to go on ice?

Because arginine falls faster in stored blood than the other amino acids: about 6% at 30 minutes, 25% at two hours and 43% at 24 hours at room temperature, against under 10% over 24 hours on ice. The error runs downwards, manufacturing exactly the low arginine a urea cycle work-up is looking for, so blood is put on ice or separated within 30 minutes.

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References

Häberle J, Burlina A, Chakrapani A, et al. Suggested guidelines for the diagnosis and management of urea cycle disorders: first revision. J Inherit Metab Dis. 2019;42(6):1192–1230.
Sun A, Crombez EA, Wong D. Arginase 1 Deficiency. In: GeneReviews. Seattle: University of Washington; 2004, updated 2020.
Davis JS, Darcy CJ, Piera K, et al. Ex-vivo changes in amino acid concentrations from blood stored at room temperature or on ice: implications for arginine and taurine measurements. BMC Clin Pathol. 2009;9:10.

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.