Cystine Unit Converter
Cystine Unit Converter
Convert cystine between mg/dL and µmol/L, with fasting reference intervals by age — and note that two large laboratories publish adult ceilings five times apart, because how long the sample stood decides how much cysteine has become cystine.
Cystine converter
mg/dL ⇄ µmol/LCystine 2.40 mg/dL on a fasting plasma amino acid profile, read against the adult interval
Formula and conversion factor
mg/dL = µmol/L ÷ 41.6146
- 41.6146
- derived from the molecular weight of L-cystine, 240.30 Da (C6H12N2O4S2, PubChem CID 67678)
- why it is half
- cystine is two cysteines joined through their sulphur atoms: 2 × 121.16 less the 2.02 Da of hydrogen lost to the disulphide bond gives 240.30 exactly. So cysteine's factor, 82.5355, is almost exactly twice this one, and swapping them doubles or halves the answer
- half-cystine
- some analysers and most intracellular measurements are reported as half-cystine — cysteine residues rather than dimers — and one mole of cystine is two moles of half-cystine. Leucocyte cystine in cystinosis is quoted in nmol half-cystine/mg protein for exactly this reason
- the tube decides
- free cystine in plasma is the oxidised half of an equilibrium that keeps moving after the blood is drawn. Blood for amino acids goes on ice immediately, or the plasma is separated and frozen within 30 minutes
Worked example
Cystine 2.40 mg/dL on a fasting plasma amino acid profile, read against the adult interval
2.40 × 41.6146 = 99.88 µmol/L, reported as 100 µmol/L
Within the Mayo adult interval of 8–310 µmol/L used here — unremarkable
And well above Labcorp's adult interval of 13.5–60.2 µmol/L, where the same number is more than 60 per cent over the upper limit. One result, two published verdicts
Converted with cysteine's factor by mistake, 2.40 mg/dL would read 198 µmol/L — double, and above Labcorp's ceiling three times over. Check which analyte the report names before converting anything
Two published intervals, five times apart at the ceiling
| Source | Adult (µmol/L) | Children (µmol/L) | Younger bands (µmol/L) |
|---|---|---|---|
| Mayo Clinic Laboratories — LC-MS/MS, fasting | 8 – 310 (≥18 y) | 3 – 151 (2–17 y) | 6 – 131 (<24 months) |
| Labcorp — plasma amino acid intervals | 13.5 – 60.2 (>15 y) | 8.8 – 36.3 (2–15 y) | 5.4 – 29.4 (31 d–23 m); 6.1 – 38.7 (0–30 d) |
Three disorders, three different samples, and only one of them plasma
| Disorder | What goes wrong | The sample that carries the diagnosis |
|---|---|---|
| Cystinuria — SLC3A1 (50–70%), SLC7A9 (25–45%) | Renal and intestinal transport of cystine and the dibasic amino acids fails, so cystine — which is poorly soluble — is lost in the urine and forms stones. Urinary lysine, arginine and ornithine are raised alongside it | Urine. Cystine stones and a raised 24-hour urinary cystine excretion establish the clinical diagnosis, with stone composition by infrared spectroscopy or x-ray diffraction rather than a chemical assay. Treatment is high fluid intake, urinary alkalinisation with potassium citrate or bicarbonate, a low-sodium low-protein diet, and tiopronin or D-penicillamine |
| Cystinosis — CTNS | Cystine cannot leave the lysosome, so it accumulates inside cells: renal Fanconi syndrome, corneal crystals typically present after 16 months of age, and later extrarenal disease | White cells. Diagnosis needs an elevated cystine concentration in polymorphonuclear leucocytes — 3.0–23.0 nmol half-cystine/mg protein in nephropathic cystinosis against a normal of 0.2 or less. Cysteamine is titrated against that same leucocyte measurement, taken 5–6 hours after a dose, to below 1.0 |
| Homocystinuria — CBS deficiency | Cystathionine beta-synthase fails, so the transsulphuration pathway that makes cysteine from homocysteine is blocked. Homocysteine and methionine accumulate behind the block and cystine is low in front of it | Plasma. A markedly raised plasma total homocysteine, often over 100 µmol/L against a normal under 15, with a high or borderline high methionine. A low cystine supports the picture; it does not make the diagnosis, and it is not what is monitored |
Two cysteines, one bond, and a number that depends on the tube
Cystine is the oxidised dimer of cysteine: two cysteine molecules joined through their sulphur atoms by a disulphide bond. Its molecular weight is 240.30 Da — twice cysteine’s 121.16 less the 2.02 Da of hydrogen lost to the bond — so 1 mg/dL is 41.61 µmol/L, almost exactly half the cysteine factor. That near-doubling is the arithmetic trap on this pair of pages, and it runs both ways: a cystine converted as cysteine reads twice too high, and a cysteine converted as cystine reads half of what it is.
It is also why a fasting plasma amino acid profile reports cystine rather than free cysteine. The thiol oxidises on standing, so the split between the two analytes keeps moving after the blood is drawn, and cystine is the more stable thing to measure. The cost shows up in the reference intervals. Mayo publishes 8–310 µmol/L for fasting adults; Labcorp publishes 13.5–60.2 µmol/L for anyone over 15. Those ceilings are five times apart, which is the largest gap between these two laboratories anywhere in the profile, and the reason is not a mistake at either — it is that each interval has its own collection and handling practice built into it. Blood for amino acids belongs on ice immediately, or the plasma separated and frozen within thirty minutes. Total cysteine, measured after the sample is chemically reduced, is a third quantity again at about 220–320 µmol/L in adults.
Clinically, cystine appears in the name of two disorders and the pathway of a third, and none of the three is a plasma cystine diagnosis. Cystinuria is a defect of renal and intestinal transport, from variants in SLC3A1 or SLC7A9, that spills poorly soluble cystine into the urine and forms stones; the urinary lysine, arginine and ornithine rise with it, and the diagnosis is made on urinary cystine excretion and stone composition. Cystinosis is a lysosomal disorder in which cystine cannot leave the lysosome; it is diagnosed on the cystine content of polymorphonuclear leucocytes, and cysteamine is titrated against that same measurement.
The one place a plasma cystine earns its place is beside the methionine and the total homocysteine. In homocystinuria from cystathionine beta-synthase deficiency the transsulphuration pathway is blocked, so homocysteine and methionine accumulate behind the block and cystine is low in front of it. Even there the cystine supports the picture rather than establishing it: what establishes it is a total homocysteine often over 100 µmol/L against a normal under 15, with a high or borderline high methionine, interpreted by a metabolic service alongside the rest of the profile.
Frequently asked questions
How do I convert cystine from mg/dL to µmol/L?
Multiply by 41.6146, derived from the molecular weight of cystine, 240.30 Da. A cystine of 2.40 mg/dL is 100 µmol/L. To go the other way, divide the µmol/L figure by 41.6146.
Why is the cystine factor about half the cysteine factor?
Because cystine is two cysteines joined by a disulphide bond. Twice 121.16 Da less the two hydrogens lost to the bond is 240.30 Da, so a mole of cystine weighs about twice a mole of cysteine and the mass-to-molar factor is about half. Using the wrong one doubles or halves the answer.
Why do two laboratories publish such different cystine intervals?
Because free cystine in plasma is the oxidised half of an equilibrium that keeps shifting after the blood is drawn, so how quickly a sample is chilled and separated changes the result — and each laboratory’s interval carries its own practice inside it. Mayo publishes 8–310 µmol/L for fasting adults and Labcorp 13.5–60.2 µmol/L for over-15s. Read your result against your own laboratory’s interval only.
Does a high plasma cystine mean cystinuria?
No. Cystinuria is a renal transport defect and a urine diagnosis: cystine stones and a raised 24-hour urinary cystine excretion, with stone composition confirmed by infrared spectroscopy or x-ray diffraction. Plasma cystine is not how it is found.
Is cystinosis diagnosed on a plasma cystine?
No. Cystinosis needs an elevated cystine concentration in polymorphonuclear leucocytes — 3.0–23.0 nmol half-cystine per mg protein against a normal of 0.2 or less — and cysteamine treatment is monitored on the same measurement, taken 5–6 hours after a dose.
What does a low cystine mean?
Read beside a raised methionine and a markedly raised total homocysteine, a low cystine fits homocystinuria from cystathionine beta-synthase deficiency, where the pathway that makes cysteine is blocked. On its own it is far more often dietary, a dilute profile or a handling effect, and it is not what the diagnosis rests on.
Related calculators
References
- Singer HS, Mink JW, Gilbert DL, Jankovic J. Disorders of amino acid metabolism. In: Movement Disorders in Childhood. 3rd ed. Elsevier; 2022.
- Spasiano A, Halbritter J, Ferraro PM. Cystinuria. In: GeneReviews. Seattle: University of Washington; 2025.
- Nesterova G, Gahl WA. Cystinosis. In: GeneReviews. Seattle: University of Washington; 2001, updated 2025.
- 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.
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.