Proinsulin Unit Converter
Proinsulin Unit Converter
Convert proinsulin between pmol/L, pg/mL, ng/mL and nmol/L. Unlike insulin, proinsulin has no international unit, so the conversion is an exact molar one — and that is the only easy thing about this test.
Proinsulin converter
pmol/L ⇄ pg/mLProinsulin 22 pmol/L — the top of Mayo’s reference interval
The conversion, and where the molecular weight comes from
pmol/L = pg/mL × 0.106513
ng/mL = pmol/L ÷ 106.513
ng/mL = µg/L — the same quantity, no arithmetic
- 9,388.5
- proinsulin’s molar mass in daltons. No test catalogue states it, so it is computed from the sequence: UniProt P01308 residues 25–110, 86 residues, three disulfide bonds. The same arithmetic on the same sequence returns 5,807.6 for insulin against the canonical 5,808 and 3,020.3 for the 31-residue C peptide, so the method is checked against two known answers before being trusted on this one
- no international unit
- this is why proinsulin converts cleanly and insulin does not. Insulin is standardised by biological potency, so its µIU/mL to pmol/L factor is a convention that depends on which reference preparation is meant. Proinsulin is reported as an amount of substance, so one molar mass settles it
- intact or total
- the real source of disagreement between proinsulin results. Some assays measure intact proinsulin only; others measure total proinsulin immunoreactivity, which includes the des-31,32 and des-64,65 split products. The two are not interchangeable and the gap between them is far larger than any unit conversion
Worked example
Proinsulin 22 pmol/L — the top of Mayo's reference interval
22 ÷ 106.513 = 0.2065 ng/mL
= 206.5 pg/mL = 0.2065 µg/L — ng/mL and µg/L are numerically identical
At the top of Mayo's fasting interval of 3.6–22 pmol/L, and exactly its insulinoma cut-off during documented hypoglycaemia
Mayo's more sensitive 5 pmol/L cut-off is 46.9 pg/mL: above 95% sensitive for insulinoma, but only about 40% specific
Proinsulin in both scales, with the two insulinoma cut-offs
| Figure | pmol/L | pg/mL | ng/mL |
|---|---|---|---|
| Lower reference limit (Mayo) | 3.6 | 33.8 | 0.0338 |
| Sensitive insulinoma cut-off (Mayo) | 5.0 | 46.9 | 0.0469 |
| Upper reference limit, and the specific cut-off | 22 | 206.5 | 0.2065 |
| A markedly raised value | 100 | 938.9 | 0.9389 |
Why insulin, C peptide and proinsulin disagree about units
| Analyte | Reported as | Convertible by molar mass? |
|---|---|---|
| Insulin | µIU/mL or pmol/L | No — µIU is biological potency, so the factor is a standard-dependent convention |
| C peptide | ng/mL or nmol/L | Yes — a defined 31-residue peptide, 3,020.3 Da |
| Proinsulin | pmol/L, sometimes pg/mL | Yes — a defined 86-residue peptide, 9,388.5 Da |
What proinsulin adds to an insulin and a C peptide
Proinsulin is the single-chain precursor the beta cell makes before it cuts out the connecting peptide and releases insulin and C peptide in equal amounts. A small amount escapes processing and reaches the circulation intact, so a plasma proinsulin is a direct readout of how much unfinished hormone the beta cell is releasing. In health that is a few per cent of the circulating insulin; the proportion rises when the beta cell is under strain.
The first use is insulinoma. Endogenous hyperinsulinaemic hypoglycaemia is diagnosed on a set of results from one hypoglycaemic moment — glucose, insulin, C peptide, proinsulin, beta-hydroxybutyrate and a sulphonylurea screen — and proinsulin earns its place because it is the most specific of them. Mayo reports a proinsulin above 22 pmol/L with a glucose below 60 mg/dL as around 75% sensitive and close to 100% specific, and offers 5 pmol/L for sensitivity at the cost of specificity. The reason it is specific is mechanical rather than statistical: exogenous insulin contains no proinsulin at all, so surreptitious insulin administration suppresses it, which is exactly where insulin and C peptide can mislead.
The second use is metabolic rather than diagnostic. A disproportionately high proinsulin relative to insulin is a marker of beta-cell dysfunction and appears in type 2 diabetes, in obesity and in the years before diabetes is diagnosed. It also rises in chronic kidney disease, because the kidney clears it, so a raised value in renal failure usually means reduced clearance rather than excess secretion.
The trap is not the units. It is that assays differ in what they measure: some detect intact proinsulin only, others total proinsulin immunoreactivity including the des-31,32 and des-64,65 split products. That difference is far larger than the arithmetic on this page, it is not correctable, and it means proinsulin results from two laboratories should not be compared. Reference intervals are method- and laboratory-dependent, and the interval printed on your own report takes precedence over any figure here.
Frequently asked questions
How do I convert proinsulin from pmol/L to pg/mL?
Multiply the pmol/L value by 0.106513, or divide the pg/mL value by the same number to go the other way. So 22 pmol/L is 206.5 pg/mL. The factor comes from proinsulin’s molar mass of 9,388.5 Da; almost every clinical laboratory reports pmol/L, so the mass units are mostly seen on research immunoassay kits.
Why does insulin need a conversion convention when proinsulin does not?
Because insulin is standardised by biological potency. A µIU is a unit of activity defined against an international reference preparation, not an amount of substance, so its pmol/L factor depends on which preparation and which molar mass you mean — which is why 6.00, 6.945 and 7.175 are all in circulation. Proinsulin has no international unit and is reported as an amount of substance, so one molar mass settles it. See the insulin unit converter for that story.
What is a normal proinsulin level?
Mayo gives 3.6 to 22 pmol/L for fasting plasma proinsulin. Reference intervals are method- and laboratory-dependent, and the interval printed on your own report takes precedence over any figure here. It also matters whether your laboratory’s assay measures intact proinsulin or total proinsulin immunoreactivity, because the two give systematically different numbers from the same sample.
Why is proinsulin more specific than insulin for insulinoma?
Because manufactured insulin contains no proinsulin. In surreptitious or accidental insulin administration the insulin is high while C peptide and proinsulin are suppressed, so proinsulin separates an insulin-secreting tumour from injected insulin in a way the insulin level cannot. A sulphonylurea, by contrast, raises all three, which is why the screen is part of the same panel.
Can proinsulin be raised without an insulinoma?
Yes, and most raised values are not tumours. A high proinsulin relative to insulin is a marker of beta-cell stress in insulin resistance, obesity and type 2 diabetes. Chronic kidney disease raises it through reduced clearance. Rare proprotein convertase 1/3 deficiency raises it markedly, with insulin and C peptide low, because the precursor cannot be cut at all.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: PINS — Proinsulin, Plasma. Reference value 3.6–22 pmol/L. Rochester, MN. Accessed October 2026.
- UniProt Consortium. P01308 — INS_HUMAN, insulin. UniProtKB entry: signal peptide 1–24, B chain 25–54, C peptide 55–89, A chain 90–110. Accessed October 2026.
- National Institute for Biological Standards and Control. WHO International Standard Insulin, Human, NIBSC code 83/500, instructions for use: 0.03846 mg in one International Unit (26.0 IU/mg). Accessed October 2026.
- Commission on Isotopic Abundances and Atomic Weights (IUPAC). Standard Atomic Weights, 2021. Used to re-derive every molecular weight from its formula.
- Assay package inserts and the issuing laboratory’s own report — reference intervals are method-dependent and the local interval governs.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/