Cautions

Significant hemolysis will result in artifactually lower C-peptide levels, and such specimens are usually rejected. However, even mild hemolysis can lead to modest decrements in C-peptide values.

There is significant (>20%) cross-reactivity between C-peptide and proinsulin. There is no significant cross-reactivity with other pancreatic islet cell peptides or neuroendocrine peptides.

Very high C-peptide levels (>180 ng/mL) may result in artifactually low measurements (hook effect). Such levels are very unlikely to occur in patients, but if individuals are suspected of having serum levels above 180 ng/mL, the laboratory should be alerted to allow dilution of the specimen prior to testing.

In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA] or heterophile antibodies), which may cause interference in some immunoassays. The presence of antibodies to streptavidin or ruthenium can also rarely occur and may also interfere in this assay. Caution should be used in interpretation of results and the laboratory should be alerted if the result does not correlate with the clinical presentation.

In the assessment of hypoglycemia, neither C-peptide nor insulin measurements are useful, or indicated, if serum blood glucose levels exceed 60 mg/dL.

In the diagnosis and management of diabetes mellitus, measurement of serum insulin levels usually provides superior information to that of serum C-peptide.

Patients with a body mass index above 25 may have elevated fasting C-peptide levels.

In rare cases, interference due to extremely high titers of antibodies to ruthenium or streptavidin can occur.