What laboratory tests should be obtained to monitor C‑peptide levels and assess beta‑cell function in a diabetic patient?

Laboratory Monitoring for C‑Peptide and β‑Cell Function

Core Testing Strategy

Order a random (non‑fasting) C‑peptide drawn within 5 hours of eating to assess β‑cell function in most clinical scenarios—this approach replaces formal stimulation tests and provides robust classification of diabetes type. 1

When to Measure C‑Peptide

• Measure C‑peptide in insulin‑treated adults when diabetes type is unclear, particularly if they present with ketoacidosis but exhibit a type 2 phenotype or are antibody‑negative, because the result directly influences therapeutic decisions. 1
• Obtain fasting C‑peptide when the concurrent fasting plasma glucose is ≤220 mg/dL (≤12.5 mmol/L) to satisfy insurance documentation requirements for insulin‑pump coverage. 1
• Use C‑peptide measurement when evaluating unexplained (nondiabetic) hypoglycemia to exclude surreptitious insulin administration. 1
• Do not test C‑peptide within 2 weeks of a hyperglycemic emergency, as results will be unreliable. 1

Optimal Sampling Method

• A random C‑peptide sample collected within 5 hours of a meal is acceptable for diabetes classification and eliminates the need for fasting or formal stimulation testing in routine practice. 1, 2
• When a formal stimulation test is required (e.g., for challenging diagnostic cases), an intravenous glucagon‑stimulated C‑peptide assay offers the best balance of sensitivity and practicality. 1, 3
• Random non‑fasting C‑peptide correlates strongly with mixed‑meal stimulated C‑peptide (r = 0.91), improving to r = 0.96 when concurrent glucose is ≥8 mmol/L (≥144 mg/dL). 2

Interpretation Thresholds

Diabetes Classification

• C‑peptide <200 pmol/L (<0.6 ng/mL) is consistent with type 1 diabetes and indicates substantial β‑cell loss, prompting initiation or continuation of insulin therapy. 1, 3
• C‑peptide 200–600 pmol/L (0.6–1.8 ng/mL) may indicate type 1 diabetes, maturity‑onset diabetes of the young (MODY), or insulin‑treated type 2 diabetes with long duration, requiring further testing (autoantibodies, genetic testing) for definitive diagnosis. 1
• C‑peptide >600 pmol/L (>1.8 ng/mL) strongly suggests type 2 diabetes and preserved β‑cell function, guiding consideration of oral antihyperglycemic agents rather than insulin. 1, 4

Severe Insulin Deficiency

• Very low C‑peptide (<80 pmol/L or <0.24 ng/mL) indicates absolute insulin deficiency and is diagnostic of type 1 diabetes, requiring insulin therapy for survival. 1
• Results showing very low levels (<80 pmol/L) do not need to be repeated. 1

Concurrent Glucose Considerations

• If C‑peptide is <600 pmol/L and concurrent glucose is <4 mmol/L (<70 mg/dL), consider repeating the test, as low glucose may suppress C‑peptide secretion and yield a falsely low result. 1
• Given an A1c ≈13%, concurrent hypoglycemia at the time of sampling is extremely unlikely, supporting the reliability of the C‑peptide result. 4

Additional Laboratory Tests

Autoantibody Testing

• The American Diabetes Association recommends measuring islet autoantibodies first in patients with ambiguous presentation, and if antibody‑negative, proceed to C‑peptide testing. 1
• Test for GAD, IA‑2, and ZnT8 autoantibodies to confirm suspected type 1 diabetes in patients with low C‑peptide levels. 1

Insulin Resistance Assessment (When Indicated)

• The American Diabetes Association recommends against routine testing for insulin or proinsulin in most people with diabetes or at risk for diabetes, stating these assays are useful primarily for research purposes (Grade B, moderate evidence). 5
• Clinical assessment using BMI and acanthosis nigricans is recommended over laboratory testing for insulin resistance screening. 5
• HOMA‑IR (calculated as [fasting glucose (mmol/L) × fasting insulin (mU/mL)] ÷ 22.5) may be used in non‑diabetic individuals to evaluate metabolic dysfunction‑associated steatotic liver disease (MASLD), but its validity is questionable in overt diabetes. 5
• HOMA‑IR values >2.5 consistently indicate pathological insulin resistance, though no universal threshold exists. 5

Hypoglycemia Work‑Up

• Confirm insulinoma when inappropriately elevated plasma insulin (>3 µIU/mL, often >6 µIU/mL) is observed alongside low glucose (<40–45 mg/dL) with an insulin‑to‑glucose ratio ≥0.3 and elevated C‑peptide. 1, 4
• A fasting C‑peptide ≥0.6 ng/mL (≥200 pmol/L) in the setting of glucose <55 mg/dL supports the diagnosis of insulinoma. 4
• Check urine sulfonylurea to rule out factitious hypoglycemia in patients with elevated C‑peptide and low glucose. 4

Monitoring Strategy

Glycemic Control

• Monitor HbA1c every 3 months and intensify treatment if goals are not met. 4
• Target HbA1c <7.0% in most patients to reduce microvascular disease risk. 4
• Finger‑stick glucose monitoring is indicated when initiating or changing treatment regimens or if treatment goals are not met. 4

β‑Cell Function Over Time

• C‑peptide measurement is most useful after 3–5 years from diagnosis in insulin‑treated patients, when persistence of substantial insulin secretion suggests type 2 or monogenic diabetes rather than type 1. 6
• Absent C‑peptide at any time confirms absolute insulin requirement and the appropriateness of type 1 diabetes management strategies regardless of apparent etiology. 6

Clinical Pitfalls

• Do not use C‑peptide routinely in the general diabetic population or in individuals at risk for diabetes; its primary role is research‑oriented rather than for everyday clinical management. 1
• Avoid C‑peptide testing for assessment of polycystic ovary syndrome, as it provides no advantage over standard clinical markers of insulin resistance (e.g., BMI, acanthosis nigricans). 1
• For insulin‑treated patients, C‑peptide must be measured prior to insulin discontinuation to exclude severe insulin deficiency. 1