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 cross-reactivity between C-peptide and proinsulin based on data generated in our own laboratory (>20%). 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 highly unlikely to occur in patients, but if individuals are suspected of having serum levels >180 ng/mL, the laboratory should be alerted in order to allow dilution of the specimen prior to testing.
This assay uses 2 mouse-derived monoclonal antibodies and may, therefore, be prone to interference by heterophile anti-mouse antibodies (HAMA). The lab should be alerted to suspected or known HAMA-positive specimens in order to allow the use of heterophile antibody blocking tubes for such patients.
In the assessment of hypoglycemia, neither C-peptide nor insulin measurements are useful or indicated if serum blood sugar 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 basal metabolic index (BMI) >25 may have elevated fasting C-peptide levels.