Ornithine Unit Converter
Ornithine Unit Converter
Convert ornithine between mg/dL and µmol/L, with fasting reference intervals by age — a urea cycle intermediate rather than a protein amino acid, where a raised result is read with the ammonia beside it.
Ornithine converter
mg/dL ⇄ µmol/LOrnithine 1.00 mg/dL on a fasting plasma amino acid profile, read against the adult interval
Formula and conversion factor
mg/dL = µmol/L ÷ 75.6659
- 75.6659
- derived from the molecular weight of L-ornithine, 132.16 Da (C5H12N2O2, PubChem CID 6262)
- not a protein amino acid
- ornithine is a urea cycle intermediate, not one of the twenty amino acids found in proteins. It accepts a carbamoyl group to become citrulline and is regenerated when arginine is cleaved to urea, so it is read beside the ammonia, the citrulline and the arginine
- read with lysine and arginine
- the three dibasic amino acids share the y+L transporter, so a transport defect moves all three — down in plasma and up in urine in lysinuric protein intolerance, and up in urine in cystinuria alongside the cystine
- asparagine, coincidentally
- 132.12 Da, a factor of 75.6888, which matches ornithine's to four significant figures. The two analytes are unrelated; the masses simply happen to coincide, and a shared factor is not a relationship
Worked example
Ornithine 1.00 mg/dL on a fasting plasma amino acid profile, read against the adult interval
1.00 × 75.6659 = 75.67 µmol/L, reported as 76 µmol/L
Within the Mayo adult interval of 39–154 µmol/L used here, and within Labcorp's 30.5–131.4 µmol/L
Far below either hyperornithinaemia: HHH syndrome has been reported at 200 to 1,915 µmol/L at diagnosis, and a UK gyrate atrophy cohort had a median of 800 µmol/L with a range of 458 to 1,244
On its own, though, this number answers little. Ornithine is read beside the ammonia, the citrulline, the arginine and the lysine from the same fasting, promptly separated sample — and the ammonia is what would separate the two raised patterns
Two published intervals, and the ranges the literature uses
| Source | Adult (µmol/L) | Children (µmol/L) | Younger bands (µmol/L) |
|---|---|---|---|
| Mayo Clinic Laboratories — LC-MS/MS, fasting | 39 – 154 (≥18 y) | 32 – 148 (2–17 y) | 32 – 171 (<24 months) |
| Labcorp — plasma amino acid intervals | 30.5 – 131.4 (>15 y) | 27.3 – 135.6 (2–15 y) | 25.7 – 131.3 (31 d–23 m); 30.5 – 205.7 (0–30 d) |
A raised ornithine: two disorders, separated by the ammonia
| Pattern | Disorder | What confirms it, and what is done |
|---|---|---|
| Raised ornithine with hyperammonaemia | HHH syndrome — hyperornithinaemia-hyperammonaemia-homocitrullinuria, from biallelic SLC25A15 variants affecting the mitochondrial ornithine transporter | Homocitrulline in the urine is the key additional feature, with episodic or postprandial mild to moderate hyperammonaemia — published median ammonia around 300 µmol/L in neonatal presentations and 120 µmol/L in childhood ones. Plasma ornithine at diagnosis has ranged from 200 to 1,915 µmol/L against a stated normal of 30–110. Management is an age-appropriate protein-restricted diet with citrulline supplementation, preferred to arginine, under a metabolic service |
| Very raised ornithine with a characteristically normal ammonia | Gyrate atrophy of the choroid and retina — ornithine aminotransferase (OAT) deficiency | Progressive ocular disease carries the diagnosis: myopia and night blindness early, then chorioretinal atrophy, with findings present in every participant over six years of age in a UK cohort of 18, and pre-senile cataracts in two thirds. That cohort’s plasma ornithine was a median of 800 µmol/L, range 458–1,244, against a reference range of 40–150. Treatment is an arginine-restricted low protein diet with amino acid supplementation, and a pyridoxine trial of at least 200–400 mg daily for two weeks in newly diagnosed patients. Transient neonatal hyperammonaemia is described, so a normal ammonia is the usual picture rather than an invariable one |
| Modestly raised ornithine, nothing else | Ornithine supplements, a non-fasting sample, delayed separation | Repeating properly — fasting, on ice, plasma separated promptly — before anything else |
A urea cycle intermediate, read with the ammonia beside it
Ornithine is not one of the twenty amino acids found in proteins. It is an intermediate of the urea cycle: ornithine transcarbamylase adds a carbamoyl group to make citrulline, and ornithine is regenerated at the end of the cycle when arginase cleaves arginine into urea. Its molecular weight is 132.16 Da, so 1 mg/dL is 75.67 µmol/L. That is within a hair of asparagine’s factor, 75.6888, purely because the two molecules happen to weigh almost the same — a coincidence of mass and not a relationship of any kind.
Reference intervals are age-dependent and laboratory-specific, and ornithine is one of the analytes where the published sets sit close together: Mayo gives 39–154 µmol/L for fasting adults, Labcorp 30.5–131.4 µmol/L for anyone over 15, and the clinical literature on the hyperornithinaemias quotes normal ranges of 30–110 and 40–150 µmol/L. Both laboratory sets assume a fasting sample separated promptly, and the interval that applies is the one printed on your own report for that age.
A raised ornithine is one of the more informative findings on a plasma amino acid profile, because it points at two specific disorders — and the ammonia on the same request tells them apart. Raised ornithine with episodic or postprandial hyperammonaemia and homocitrulline in the urine is the pattern of HHH syndrome, where the mitochondrial ornithine transporter encoded by SLC25A15 fails and ornithine cannot reach the enzyme that uses it; plasma ornithine at diagnosis has been reported from 200 to 1,915 µmol/L. Very high ornithine with a characteristically normal ammonia, alongside progressive myopia, night blindness and chorioretinal atrophy, is the pattern of gyrate atrophy of the choroid and retina, from ornithine aminotransferase deficiency: a UK cohort found a median of 800 µmol/L with a range of 458 to 1,244. Transient neonatal hyperammonaemia has been described there too, so the normal ammonia is the usual picture rather than an absolute rule.
The low direction matters as well, and never in isolation. Ornithine shares the y+L transporter with lysine and arginine, so all three fall together in lysinuric protein intolerance, where they leak into the urine and the presentation is hyperammonaemia after a protein-rich meal with a normal fasting ammonia — and there the urine amino acids, not the plasma, are what show it. The same three appear in the urine of cystinuria, alongside the cystine that forms the stones. A converted ornithine is therefore the beginning of a question: it is read beside the ammonia, the citrulline, the arginine and the lysine from the same fasting sample, and interpreted by a metabolic service.
Frequently asked questions
How do I convert ornithine from mg/dL to µmol/L?
Multiply by 75.6659, derived from the molecular weight of ornithine, 132.16 Da. An ornithine of 1.00 mg/dL is 76 µmol/L. To go the other way, divide the µmol/L figure by 75.6659.
What is a normal ornithine level?
Mayo publishes 39–154 µmol/L for fasting adults and Labcorp 30.5–131.4 µmol/L for anyone over 15, with different bands for children and infants; published clinical series use normal ranges of 30–110 and 40–150 µmol/L. Use the interval your own laboratory prints for that age, on a fasting sample separated promptly.
What does a high ornithine mean?
Two disorders raise it, and the ammonia separates them. With hyperammonaemia — typically episodic or after protein — and homocitrulline in the urine, the pattern is HHH syndrome, reported at 200 to 1,915 µmol/L at diagnosis. With a characteristically normal ammonia and progressive visual loss from chorioretinal atrophy, the pattern is gyrate atrophy from ornithine aminotransferase deficiency, with a reported median of 800 µmol/L. A modest rise is more often a supplement or a non-fasting sample.
Is ornithine a protein amino acid?
No. It is a urea cycle intermediate, made and consumed within that cycle rather than incorporated into proteins, which is why it is read beside the ammonia, the citrulline and the arginine rather than as one of a protein profile.
What does a low ornithine mean?
Read with a low lysine and a low arginine, it raises lysinuric protein intolerance, where the three dibasic amino acids leak into the urine; there the diagnosis rests on heavy urinary excretion of the cationic amino acids and hyperammonaemia after protein, with a fasting ammonia that is usually normal. In isolation it is most often low protein intake, prolonged fasting or a dilute profile.
Why is a pyridoxine trial given in gyrate atrophy?
Because a minority of ornithine aminotransferase variants respond to high-dose vitamin B6. In the UK cohort every newly diagnosed patient had a trial of at least 200–400 mg of pyridoxine daily for a minimum of two weeks, alongside the arginine-restricted low protein diet that is the mainstay of lowering plasma ornithine. Dosing and monitoring are set by the metabolic service.
Related calculators
References
- Camacho J, Rioseco-Camacho N. Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome. In: GeneReviews. Seattle: University of Washington; 2012, updated 2025.
- Palmer E, Stepien KM, Campbell C, et al. Clinical, biochemical and molecular analysis in a cohort of individuals with gyrate atrophy. Orphanet J Rare Dis. 2023;18:265.
- Mayo Clinic Laboratories. Test ID: AAQP — Amino Acids, Quantitative, Plasma. Liquid chromatography tandem mass spectrometry. Reference values are for fasting patients, in three age bands (under 24 months, 2–17 years, 18 years and over). Accessed 2026.
- Labcorp. Plasma Amino Acid Reference Intervals — four age bands from 0–30 days to over 15 years, µmol/L. Accessed 2026.
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