Procedure:
(1) Perform an intravenous injection of 1 mg glucagon.
(2) Collect a serum sample 6 minutes after injection and measure C-peptide levels.
Interpretation:
• The C-peptide level used to discriminate between those diabetic patients needing insulin and those who do not need insulin is dependent on the assay method used.
• Laakso et al found that insulin was seldom successfully stopped if the postglucagon C-peptide value was < 1 nmol/L, but that insulin therapy could be successfully stopped with values > 1 nmol/L.
• Patients with C-peptide levels > 0.60 nmol/L can sometimes be maintained without insulin, but often need to have insulin therapy restarted.
Procedural notes:
(1) In newly diagnosed obese diabetics, improvement in C-peptide response can be seen after weight loss to ideal body weight.
(2) C-peptide will also increase about 90 minutes following a post-fasting meal (postprandial) in patients with residual beta-cell function. This is due to the glucagon release induced by the meal.
(3) Since meals can affect C-peptide levels, the timing of glucagon stimulation should be standardized relative to pre-test meals, especially after an overnight fast.
(4) Low glucose levels (< 3.5 mmol/L, < 63 mg/dL) were found to suppress the stimulating action of glucagon on the pancreas almost entirely, so can result in false-negative response.
