Is C-peptide testing necessary for a patient with poor glycemic control, hypertriglyceridemia, and impaired glucose regulation, who is currently on Rosuvastatin (rosuvastatin) 5 mg?

C-Peptide Testing in Poor Glycemic Control with Hypertriglyceridemia

Yes, C-peptide testing is indicated in this patient to determine the degree of insulin deficiency and guide appropriate treatment intensification. Given the poor glycemic control and hypertriglyceridemia, measuring C-peptide will distinguish between insulin-deficient diabetes requiring insulin therapy versus insulin-resistant type 2 diabetes that may respond to oral agents or GLP-1 receptor agonists.

Primary Indication for C-Peptide Testing

This patient meets criteria for C-peptide testing based on poor glycemic control requiring treatment escalation decisions. 1 The American Diabetes Association recommends C-peptide measurement to guide treatment decisions when diabetes classification is uncertain or when determining whether a patient has sufficient endogenous insulin production to respond to non-insulin therapies. 1

• For patients with poor glycemic control (blood glucose >15 mmol/L persistently or HbA1c >9%), C-peptide testing helps determine if insulin therapy is necessary versus intensification of oral agents. 2
• The test should be performed as a random sample within 5 hours of eating, which can replace formal stimulation testing for diabetes classification purposes. 1

Interpretation Framework for Treatment Decisions

The C-peptide result will directly determine your treatment approach:

Very Low C-peptide (<200 pmol/L or <0.6 ng/mL)

• Indicates absolute insulin deficiency requiring immediate insulin therapy. 2, 1
• This finding would suggest either undiagnosed type 1 diabetes, latent autoimmune diabetes in adults (LADA), or type 3c diabetes with pancreatic insufficiency. 2
• If C-peptide is <0.4 nmol/L, all patients should be managed with insulin similar to type 1 diabetes. 2

Intermediate C-peptide (200-600 pmol/L or 0.6-1.8 ng/mL)

• May indicate type 1 diabetes, MODY, or insulin-treated type 2 diabetes with declining beta-cell function. 1
• Consider checking islet autoantibodies (GAD, IA-2, ZnT8) to distinguish autoimmune from non-autoimmune causes. 1, 3
• Treatment approach depends on autoantibody results and clinical context. 1

Normal/High C-peptide (>600 pmol/L or >1.8 ng/mL)

• Suggests type 2 diabetes with preserved beta-cell function and insulin resistance as the primary problem. 1
• This patient can be managed with oral agents, GLP-1 receptor agonists, or combination therapy rather than insulin. 1
• The hypertriglyceridemia and poor control suggest significant insulin resistance that may respond to metformin intensification or addition of GLP-1 receptor agonist. 2

Critical Testing Considerations

Ensure the following conditions are met before ordering C-peptide:

• Do not test within 2 weeks of any hyperglycemic emergency (DKA or hyperosmolar state). 1
• If the patient is already on insulin, C-peptide must be measured prior to insulin discontinuation. 1
• If concurrent glucose is <4 mmol/L (<70 mg/dL) when C-peptide is drawn, consider repeating the test as low glucose suppresses C-peptide secretion. 1
• Very low results (<80 pmol/L) do not need to be repeated. 1

Additional Diagnostic Workup

Concurrent with C-peptide, obtain:

• Lipase level to assess for pancreatic pathology (type 3c diabetes). 2
• Electrolytes and renal function to assess for complications and medication safety. 2
• Consider islet autoantibodies if C-peptide is low or intermediate to distinguish autoimmune from non-autoimmune causes. 1, 3

Rosuvastatin Considerations

The current rosuvastatin 5 mg dose is appropriate and should be continued despite the poor glycemic control. 4 While rosuvastatin has been associated with dose-dependent increases in insulin resistance in some studies (particularly at higher doses of 20-40 mg), 5 the cardiovascular benefits of statin therapy in diabetic patients far outweigh any modest effects on glucose metabolism. 4 The 5 mg dose is relatively low and unlikely to be a major contributor to the poor glycemic control. 5

Common Pitfalls to Avoid

• Do not delay C-peptide testing in patients with persistent hyperglycemia >15 mmol/L, as this represents high-risk diabetes requiring urgent treatment decisions. 2
• Do not assume all patients with "type 2 diabetes" phenotype have adequate insulin production—approximately 5-10% may have LADA with progressive beta-cell failure. 3
• Do not attribute poor glycemic control solely to medication non-adherence without assessing insulin secretory capacity. 1
• Do not stop rosuvastatin based on poor glycemic control, as the cardiovascular benefits are critical in diabetic patients with dyslipidemia. 4