Answer
Insulin testing is used to assist in identifying causes of hypoglycemia (plasma glucose levels < 55 mg/dL), especially upon signs and symptoms of hypoglycemia (neurohypoglycopenic and autonomic symptoms). In this scenario, a 72-hour fasting test is performed. [2]
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Insulinoma: High insulin and C-peptide levels
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Non–beta cell tumors: Low insulin and C-peptide levels and high insulinlike growth factor 2 level [3]
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Excessive insulin administration: High insulin levels and low C-peptide levels
-
Insulin secretagogue administration (sulfonylurea and glinides): High insulin and C-peptide levels
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Congenital hyperinsulinism (mutation in insulin-secreting gene): High insulin and C-peptide levels
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Autoimmunity to insulin or insulin receptor (common in patients receiving insulin or those who have autoimmune diseases such as systemic lupus erythematosus [SLE] or Hashimoto thyroiditis): Postprandial insulin is bound to antibodies and dissociated 1 hour later, resulting in an extremely elevated insulin level and high insulin–to–C-peptide ratio [4]
Table 2. Interpretation of 72-hour Fasting Test Results [2] (Open Table in a new window)
Condition |
Insulin |
C-Peptide |
Proinsulin |
Insulinlike Growth Factor 2 |
Sulfonylurea |
Glucose Level After Administration of Glucagon |
Insulinoma |
↑ |
↑ |
↑ |
↓ |
↓ |
↑ |
Non–beta cell tumors |
↓ |
↓ |
↓ |
↑ |
↓ |
↑ |
Insulin injection |
↑ |
↓ |
↓ |
↓ |
↓ |
↑ |
Sulfonylurea-induced |
↑ |
↑ |
↑ |
↓ |
↑ |
↑ |