Cysteine Unit Converter
Cysteine Unit Converter
Convert cysteine between mg/dL and µmol/L — and check first which cysteine you have, because a plasma amino acid profile reports cystine, the oxidised dimer, and total cysteine after reduction is a third quantity again.
Cysteine converter
mg/dL ⇄ µmol/LTotal cysteine 3.30 mg/dL on a reduced plasma sample, read against the adult total cysteine interval
Formula and conversion factor
mg/dL = µmol/L ÷ 82.5355
- 82.5355
- derived from the molecular weight of L-cysteine, 121.16 Da (C3H7NO2S, PubChem CID 5862)
- cystine is not cysteine
- the oxidised dimer, C6H12N2O4S2, 240.30 Da — two cysteines less the two hydrogens lost to the disulphide bond. Its factor is 41.6146, almost exactly half. Convert a cystine with cysteine's factor and the answer is about twice what it should be
- total cysteine
- everything cysteine-derived in the sample — free thiol, cystine, mixed disulphides and the large protein-bound fraction — released by reducing the plasma first. About 220–320 µmol/L in healthy adults, an order of magnitude more than the free thiol
- the tube decides
- cysteine oxidises to cystine on standing, so which analyte a delayed or unreduced sample is really reporting shifts with time. Blood for amino acids goes on ice immediately, or the plasma is separated and frozen within 30 minutes
Worked example
Total cysteine 3.30 mg/dL on a reduced plasma sample, read against the adult total cysteine interval
3.30 × 82.5355 = 272.37 µmol/L, reported as 272 µmol/L
Within the adult total cysteine range of 220–320 µmol/L used here
The same 3.30 mg/dL, if it were in fact a cystine, would be 137 µmol/L — the factor is 41.6146, not 82.5355, and using the wrong one doubles the answer
And a fasting plasma amino acid profile would not report either number as cysteine: it reports cystine, for which Mayo publishes 8–310 µmol/L in adults and Labcorp 13.5–60.2 µmol/L
Three quantities, one element of confusion
| Quantity | What it is | Published adult figures | Where it comes from |
|---|---|---|---|
| Free cysteine | The reduced thiol circulating unbound — a small fraction of the total, and the analyte this converter’s factor belongs to | Not published by either laboratory | Mayo’s AAQP analyte list and Labcorp’s plasma amino acid intervals both list cystine and no cysteine, so a routine profile does not report it |
| Cystine | The oxidised dimer of two cysteines, 240.30 Da. This is what a plasma amino acid profile reports | Mayo 8–310 µmol/L (≥18 y); Labcorp 13.5–60.2 µmol/L (>15 y) | Mayo Clinic Laboratories AAQP and Labcorp, both fasting plasma. The ceilings differ more than fivefold, which is itself a handling story |
| Total cysteine | Free thiol plus cystine plus mixed disulphides plus the protein-bound fraction, released by reducing the sample before analysis | About 220–320 µmol/L in healthy adults | Stipanuk, quoted by Schwinger et al. (2020), whose own cohort of children aged 6–30 months averaged 179 µmol/L with a standard deviation of 25 |
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 |
Which cysteine is on the report?
Cysteine is the sulphur-containing amino acid at the end of the transsulphuration pathway, made from methionine by way of homocysteine, and it is conditionally essential: the body makes it, unless that pathway is blocked. Its molecular weight is 121.16 Da, so 1 mg/dL is 82.54 µmol/L. That factor is the easy part of this page. The hard part is that three different quantities all get called cysteine on a report, and they differ by more than an order of magnitude.
Cysteine carries a free thiol group, and thiols oxidise. Two cysteines join through their sulphur atoms to form cystine, the disulphide dimer — 240.30 Da, which is twice 121.16 less the two hydrogens lost to the bond. That reaction happens in the tube as well as in the body, so the balance between the two analytes shifts with how long the blood stood before the plasma came off. This is why a fasting plasma amino acid profile reports cystine and not free cysteine: Mayo’s AAQP list and Labcorp’s plasma amino acid intervals each publish a cystine interval and neither publishes a cysteine one. Total cysteine — around 220–320 µmol/L in healthy adults — is a third quantity again, measured only after the sample has been chemically reduced to release everything cysteine-derived, including the large fraction bound to albumin.
So the first question a cysteine number raises is which of the three it is, and the second is how the sample was handled. Blood for plasma amino acids goes on ice immediately, or the plasma is separated and frozen within thirty minutes; intervals assume a fasting sample treated that way and differ by age, so the one that applies is printed on your own report for that age.
Clinically, the sulphur amino acids matter in three conditions easy to run together and with almost nothing in common. Cystinuria is a renal transport defect that spills poorly soluble cystine into the urine and forms stones; it is a urine diagnosis. Cystinosis is a lysosomal disorder in which cystine accumulates inside cells, diagnosed and monitored on the cystine content of white cells, in nmol of half-cystine per mg of protein. Homocystinuria is the one where a plasma profile is central — but there the findings that matter are a markedly raised total homocysteine and a high methionine, with a low cystine as supporting detail rather than the diagnosis. None of the three is settled by a converted number, and all three are managed by a metabolic service.
Frequently asked questions
How do I convert cysteine from mg/dL to µmol/L?
Multiply by 82.5355, derived from the molecular weight of cysteine, 121.16 Da. A cysteine of 3.30 mg/dL is 272 µmol/L. To go the other way, divide the µmol/L figure by 82.5355.
Is cysteine the same as cystine?
No. Cystine is two cysteine molecules joined by a disulphide bond, so it has roughly twice the molecular weight — 240.30 Da against 121.16 — and its own conversion factor of 41.6146. They are different analytes with different reference intervals, and converting one with the other’s factor doubles or halves the answer.
Why does my report show cystine and not cysteine?
Because cysteine oxidises to cystine on standing, so the free thiol is not a stable thing to report from a routine sample. Mayo’s plasma amino acid profile and Labcorp’s plasma amino acid intervals both publish cystine and neither publishes cysteine. If you need the cysteine-derived total, the laboratory has to reduce the sample first, and that result is reported as total cysteine.
What is a normal cysteine level?
It depends entirely on which quantity is measured. Total cysteine in healthy adults is about 220–320 µmol/L; free cysteine is a small fraction of that and is not routinely reported; cystine on a fasting plasma profile is published as 8–310 µmol/L by Mayo and 13.5–60.2 µmol/L by Labcorp for adults. Use the interval your own laboratory prints, for that quantity and that age.
Does a cysteine or cystine level diagnose cystinuria or cystinosis?
No. Cystinuria is diagnosed on urine — cystine stones and raised 24-hour urinary cystine excretion. Cystinosis is diagnosed on the cystine content of white cells, in nmol half-cystine per mg protein, and cysteamine treatment is monitored the same way. Neither is a plasma cysteine test.
Related calculators
References
- Nesterova G, Gahl WA. Cystinosis. In: GeneReviews. Seattle: University of Washington; 2001, updated 2025.
- Spasiano A, Halbritter J, Ferraro PM. Cystinuria. In: GeneReviews. Seattle: University of Washington; 2025.
- Schwinger C, Chowdhury R, Sharma S, et al. Association of plasma total cysteine and anthropometric status in 6–30 months old Indian children. Nutrients. 2020;12(10):3146.
- 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.