Calculate the insulin to C-peptide ratio from lab values in conventional or molar units, with results shown as conventional or molar ratio.
Disclaimer: For educational purposes only; not medical advice and not for diagnosis or medication changes. Unit conversions used here are approximate and can vary by laboratory/assay—use your lab’s reported units and reference ranges, and consult a qualified clinician for interpretation (especially with symptoms of hypoglycemia or concerns about diabetes/endocrine disorders).
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Insulin C Peptide Ratio Formula
The insulin C-peptide ratio compares the insulin concentration in a blood sample with the C-peptide concentration from the same sample. The calculator has two modes: conventional ratio and molar ratio.
Conventional ratio
R = I / C
- R = insulin to C-peptide ratio in µIU/mL per ng/mL
- I = insulin converted to µIU/mL
- C = C-peptide converted to ng/mL
Molar ratio
R_m = I_pmol / C_pmol
- Rm = molar insulin to C-peptide ratio in pmol/pmol
- Ipmol = insulin converted to pmol/L
- Cpmol = C-peptide converted to pmol/L
Unit conversions used
1 µIU/mL insulin = 1 mU/L insulin = 6.00 pmol/L insulin
1 ng/mL C-peptide = 331.1 pmol/L = 0.3311 nmol/L
In conventional mode, insulin is converted to µIU/mL and C-peptide is converted to ng/mL before division. In molar mode, both values are converted to pmol/L before division, which compares molecule-to-molecule concentration more directly.
Common Result Ranges and Unit Conversions
| Mode | Approximate result | General interpretation |
|---|---|---|
| Conventional ratio | Less than 1 | Often seen when endogenous insulin production and hepatic insulin clearance are not unusually high. |
| Conventional ratio | About 1 to 2 | Commonly cited range for fasting samples, depending on lab method and clinical context. |
| Conventional ratio | Greater than 2 | May be elevated. Possible reasons include exogenous insulin exposure or reduced insulin clearance. |
| Molar ratio | Around 0.1 fasting | Often consistent with normal hepatic insulin extraction in fasting conditions. |
| Molar ratio | 0.2 to 0.5 | Can be mildly elevated, especially after meals or with insulin resistance. |
| Molar ratio | Near 1 or higher | Can suggest exogenous insulin or reduced insulin clearance. Clinical review is needed. |
| Measurement | Conversion |
|---|---|
| Insulin | 1 µIU/mL = 1 mU/L |
| Insulin | 1 µIU/mL ≈ 6.00 pmol/L |
| C-peptide | 1 ng/mL ≈ 331.1 pmol/L |
| C-peptide | 1 nmol/L = 1000 pmol/L |
| C-peptide | 1 nmol/L ≈ 3.0203 ng/mL |
Example Calculations
Example 1: Conventional ratio
You have an insulin result of 10 µIU/mL and a C-peptide result of 2.0 ng/mL.
R = 10 / 2.0 = 5.0
The conventional insulin to C-peptide ratio is 5.0 µIU/mL per ng/mL.
Example 2: Molar ratio
You have an insulin result of 60 pmol/L and a C-peptide result of 600 pmol/L.
R_m = 60 / 600 = 0.100
The molar insulin to C-peptide ratio is 0.100 pmol/pmol.
FAQ
What is the insulin C-peptide ratio used for?
The ratio helps compare circulating insulin with C-peptide. Since the pancreas releases insulin and C-peptide together, an unusual mismatch can give clues about insulin source, insulin clearance, or timing of the blood draw. It is not a diagnosis by itself.
Why are there conventional and molar ratio options?
Conventional units use common lab reporting units, such as µIU/mL for insulin and ng/mL for C-peptide. The molar ratio converts both values to pmol/L before dividing, which compares the number of insulin molecules with the number of C-peptide molecules more directly.
Should fasting and non-fasting results be interpreted the same way?
No. Food intake can raise insulin and C-peptide and can change the ratio. Fasting samples are usually easier to compare with reference discussions. If your sample was taken after a meal, during illness, or during insulin treatment, interpretation should account for that context.
