From the Guidelines
Urine C-peptide is a valuable diagnostic test for distinguishing between type 1 and type 2 diabetes, assessing residual beta cell function, and investigating suspected cases of insulinoma or factitious hypoglycemia, as recommended by the most recent guidelines 1. The test measures the amount of C-peptide excreted in urine, providing insight into the body's insulin production. C-peptide is produced in equal amounts to insulin when proinsulin is cleaved in the pancreas, making it an excellent marker for endogenous insulin secretion.
Key Points to Consider
- The test is particularly useful in ambiguous cases, such as individuals who have a type 2 phenotype but present in ketoacidosis 1.
- For collection, a 24-hour urine sample is typically required, though some protocols may use shorter collection periods or spot samples with creatinine correction.
- Normal values generally range from 20-120 μg/day, though reference ranges vary by laboratory 1.
- Lower levels suggest decreased insulin production as seen in type 1 diabetes, while normal or elevated levels in a diabetic patient typically indicate type 2 diabetes with insulin resistance.
- The test offers advantages over serum C-peptide measurement, including being less invasive, reflecting insulin production over time rather than at a single moment, and being unaffected by exogenous insulin administration.
- However, results can be affected by kidney function since C-peptide is primarily excreted renally.
Clinical Applications
- Distinguishing between type 1 and type 2 diabetes: C-peptide measurements may help distinguish type 1 from type 2 diabetes in ambiguous cases 1.
- Assessing residual beta cell function: The test can help assess residual beta cell function in patients with diabetes.
- Investigating suspected cases of insulinoma or factitious hypoglycemia: C-peptide measurements can aid in the diagnosis of insulinoma or factitious hypoglycemia 1.
From the Research
Urine C-peptide Measurement
- Urine C-peptide measurement is a non-invasive alternative to blood measures of beta-cell function 2, 3
- The urine C-peptide creatinine ratio is a stable and reproducible measure of endogenous C-peptide 4, 5
- This measurement can be used to assess insulin secretion and classify diabetes subtype 2, 6
Clinical Utility
- Urine C-peptide measurement can assist in the clinical management of patients with diabetes, particularly in distinguishing between Type 1 and Type 2 diabetes 2, 6
- The measurement can also be used to assess the potential for insulin deficiency and guide treatment choices 4, 6
- A stimulated urine C-peptide creatinine ratio cut-off of 0.3 nmol/mmol can identify subjects without clinically significant endogenous insulin secretion 4
Comparison to Serum C-peptide
- Urine C-peptide creatinine ratio is well correlated with serum C-peptide 4, 5
- The correlation between urine and serum C-peptide is similar in patients with and without moderate renal impairment 5
- Urine C-peptide creatinine ratio may be a more practical alternative to serum C-peptide measurement due to its stability and ease of collection 3, 4