Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (849 page)

   Acute stress (including hospitalization and surgery), alcoholism, depression, and many drugs (e.g., exogenous cortisones, anticonvulsants) can obliterate normal diurnal variation, affect response to suppression/stimulation tests, and cause elevated baseline levels.

   Patients taking prednisone may have falsely increased cortisol levels because prednisone is converted to prednisolone after ingestion, and prednisolone has a 41% cross-reactivity.

   Cortisol levels may be increased in pregnancy and with exogenous estrogens.

   Some patients with depressive disorders have a hyperactive hypothalamic– pituitary–adrenal axis, similar to Cushing syndrome.

C-PEPTIDE

   Definition

   Human C-peptide is a 31-amino-acid chain with a molecular mass of approximately 3,020 Da. Metabolically inert, it originates in the pancreatic B cells as a by-product of the enzymatic cleavage of proinsulin to insulin. In this process, insulin and C-peptide are split from the prohormone and secreted into the portal circulation in equimolar concentrations. Within limits, C-peptide levels can serve as a valuable index to insulin secretion. Therefore, low C-peptide levels are to be expected where insulin secretion is diminished, as in insulin-dependent diabetes, or suppressed, as a normal response to exogenous insulin, whereas elevated C-peptide levels may result from the increased B-cell activity observed in insulinomas.

   
Normal range:
0.9–7.1 ng/mL.

   Use

   For estimating insulin levels in the presence of antibodies to exogenous insulin

   Diagnosis of factitious hypoglycemia due to surreptitious administration of insulin in which high serum insulin levels occur with low C-peptide levels

   Evaluation of insulinoma

   Monitoring pancreatic and islet cell transplant function

   Interpretation