C-peptide index (CPR index): how the C-peptide to glucose ratio works

The C-peptide index (CPR index) is serum C-peptide in ng/mL divided by plasma glucose in mg/dL, multiplied by 100. A C-peptide of 1.0 ng/mL with a glucose of 100 mg/dL gives 1.0, which is below the paper's cut-off of 2.02.

Not a medical device. Not a diagnosis or dosing recommendation.

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Saisho Y et al. · 2011
The CPR index formula
Worked examples
Frequently asked questions

What is the C-peptide index?

C-peptide is released with insulin, so it reflects how much insulin the pancreas is making. Dividing it by glucose puts that output in the context of the blood sugar at the same moment. Saisho and colleagues studied this ratio in people with type 2 diabetes and published it in Endocrine Journal in 2011.

Magent's calculator returns the index to 2 decimals and a below_best_cutoff flag that is true when the index is below 2.02.

This page explains the arithmetic and the flag. It doesn't say what a value means for a particular person.

The CPR index formula

CPR index = C-peptide (ng/mL) ÷ glucose (mg/dL) × 100
rounded to 2 decimals

below_best_cutoff = true when CPR index < 2.02

Exact expression the calculator uses:

cpri = c_peptide_ng_ml / glucose_mg_dl * 100

Inputs

Input	Meaning	Accepted range
`c_peptide_ng_ml`	Serum C-peptide, ng/mL	0 to 40
`glucose_mg_dl`	Plasma glucose, mg/dL	50 to 1000
`timing`	When the sample was taken	`fasting` or `postprandial`

How the calculation works

Divide C-peptide in ng/mL by glucose in mg/dL.
Multiply by 100 and round to 2 decimals.
Flag: below_best_cutoff is true when the result is below 2.02. A result of exactly 2.02 is not below the cut-off.

The timing label is required, but fasting and postprandial samples use the same arithmetic.

Units

C-peptide must be in ng/mL and glucose in mg/dL. The calculator doesn't convert glucose from mmol/L, so convert before entering. A missing value, one outside its range or a timing other than fasting or postprandial is rejected with an error.

Worked examples

These use Magent's own expected values for the calculator.

Example 1: exactly at the cut-off

Postprandial C-peptide 2.02 ng/mL, glucose 100 mg/dL.

2.02 ÷ 100 × 100 = 2.02.
2.02 is not below 2.02 → below_best_cutoff false.

Example 2: below the cut-off

Postprandial C-peptide 1.0 ng/mL, glucose 100 mg/dL.

1.0 ÷ 100 × 100 = 1.0.
1.0 is below 2.02 → below_best_cutoff true.

Example 3: a fasting sample

Fasting C-peptide 3.0 ng/mL, glucose 90 mg/dL.

3.0 ÷ 90 × 100 rounds to 3.33.
3.33 is not below 2.02 → below_best_cutoff false.

History of the C-peptide index

The calculator cites Saisho and colleagues, Endocrine Journal, 2011 (volume 58, issue 4, pages 315–322). The abstract describes a retrospective study of consecutive inpatients with type 2 diabetes. Fasting and postprandial serum and urinary C-peptide had been measured during the admission, and insulin use at the last clinic visit was taken from the medical records. In the paper, an index below 2.02 was associated with later insulin use, with 80.8% sensitivity and 63.3% specificity. That is a reported association, not a rule for starting insulin.

When it's used

The index is calculated for adults with type 2 diabetes from a C-peptide and a glucose taken at the same time, either fasting or after a meal.

Where it's used

The index needs a serum C-peptide result and a plasma glucose result from the same sample time. No other measurements go into it.

Who uses it

The person using the calculator enters C-peptide, glucose and the sample timing. The calculator works out the ratio and sets the flag.

Limits of the calculation

The calculator doesn't measure C-peptide or glucose.
It doesn't establish diabetes, isn't HOMA-IR and isn't C-peptide on its own.
The flag uses the 2.02 cut-off. The paper also notes a cut-off of 1.5 (about 80% specificity and 60% sensitivity); the calculator doesn't return a flag for 1.5.
The flag isn't an insulin order, and this page doesn't say what to do with a value.

Clinical decisions belong to a qualified clinician who knows the patient.

HOMA-IR combines fasting insulin and fasting glucose to estimate insulin resistance. The CPR index uses C-peptide, not insulin.
A1c to eAG converts HbA1c to an estimated average glucose. It doesn't use C-peptide.

Glossary

C-peptide: a fragment released in equal amounts with insulin, measured in ng/mL.
Postprandial: after a meal.
ROC cut-off: the value a study picks to separate two groups, with its sensitivity and specificity.

Common mistakes

Entering glucose in mmol/L. The calculator expects mg/dL and doesn't convert.
Expecting a different formula for fasting samples. The timing label doesn't change the arithmetic.
Treating 2.02 as below the cut-off. The flag needs a value strictly below 2.02.
Reading the flag as advice. It reports a published association, nothing more.

Source

Saisho Y, et al. Postprandial serum C-peptide to plasma glucose ratio as a predictor of subsequent insulin treatment in patients with type 2 diabetes. Endocr J. 2011;58(4):315-322. DOI: 10.1507/endocrj.K10E-399. PMID: 21415555

Frequently asked questions

How do you calculate the C-peptide index?

Divide C-peptide in ng/mL by glucose in mg/dL and multiply by 100.

What does the 2.02 cut-off mean?

It is the cut-off Saisho and colleagues reported for an association with later insulin use. The flag is true below 2.02.

Does fasting or postprandial timing change the formula?

No. Timing is a required label, but the arithmetic is the same.

Can I enter glucose in mmol/L?

No. Convert glucose to mg/dL first.

Is the CPR index the same as HOMA-IR?

No. HOMA-IR uses insulin and glucose; the CPR index uses C-peptide and glucose.