Measurement of C-peptide, which is accomplished through radioimmunoassay techniques, provides an index of -cell activity that is unaffected by the administration of exogenous insulin. As the cells release insulin, they also release equimolar amounts of metabolically inactive C-peptide. Injectable insulin preparations are purified to remove C-peptide. Furthermore, injected insulin elevates immunoreactive serum insulin levels and suppresses pancreatic secretion of endogenous insulin and C-peptide. That is, although exogenous insulin elevates serum insulin levels, C-peptide levels are either unaffected or decreased. C-peptide determinations may be carried out to augment or confirm results of serum insulin measurements.⁵⁹

• Suspected excessive insulin administration in either diabetic or nondiabetic individuals, as indicated by low C-peptide and elevated serum insulin levels
• Determination of -cell function when insulin antibodies preclude accurate measurement of serum insulin production (Insulin antibodies are most common in diabetic clients receiving exogenous insulin prepared from animal extracts.)
• Support for diagnosing insulinoma, especially when the tumor secretes more proinsulin than active hormone, because the normal correlation between insulin and C-peptide will be altered

Nursing Care Before the Procedure

Client preparation is the same as that for any test involving the collection of a peripheral blood sample (see Appendix I).

• Some laboratories may require that the client fast from food for 8 hours before the test.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and sent promptly to the laboratory. The sample also can be tested for insulin measurement.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume usual diet.