Analysis of urinary c-peptide is also available, for specimen requirements see c-peptide (urine).
The measurement of plasma insulin and C-peptide concentrations, in the presence of hypoglycaemia, is a useful test in identifying the cause of hypoglycaemia.
Raised plasma insulin and C-peptide concentrations (i.e. not suppressed) in the presence of hypoglycaemia indicate endogenous hyperinsulinaemia which can be caused by islet-cell tumours, automimmune insulin syndrome or drugs. Inappropriately raised plasma insulin concentrations in the presence of low or suppressed plasma C-peptide will identify patients with exogenous insulin administration. Suppressed plasma insulin and C-peptide concentrations in the presence of hypoglycaemia can be caused by chronic kidney disease, liver disease, alcohol and drugs.
When a patient has newly diagnosed type 1 or type 2 diabetes, C-peptide can be used to help determine how much insulin the patient's pancreas is still producing (residual beta cell function) and whether or not that insulin is being used effectively.
C-peptide is cleared by the kidneys, therefore impairment in renal function can make C-peptide results difficult to interpret.
Hypoglycaemia, spontaneous or whilst fasting, must first be established by regular monitoring of blood glucose levels. Blood samples should be collected only when patient is hypoglycaemic.
If patient is not hypoglycaemic (i.e. glucose is above 3.0 mmol/L), C-peptide analysis is not performed.
Transfer the blood to a plain tube. Separate the serum preferably within 30 min of collection and freeze at -20C.
Larger volumes of sample (5 mL) are required for additional studies such as IGF-1 measurements or sulphonylurea identification.
Small blood samples from neonates should be transferred to a heparin tube so that the maximum volume of plasma may be collected.
A minimum volume of 0.5 mL should be stored at -20C.
Visible haemolysis may invalidate the result.
Ensure the samples remain frozen during transport. Record the blood glucose concentration on the request form.
Interpretation provided by referral laboratory.
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The Trust Laboratories at Heartlands Hospital, Good Hope Hospital and Solihull Hospital were awarded UKAS (United Kingdom Accreditation Service) accreditation to the internationally recognised ISO 15189 standard in May 2015. For a list of accredited tests and other information please visit the test database http://www.heftpathology.com/frontpage/test-database.html.
Tests not appearing on this scope are either under consideration or in the process of accreditation and so currently remain outside of our scope of accreditation. However, these tests have been validated to the same high standard as accredited tests and are performed by the same trained and competent staff.
For further information contact Louise Fallon, Quality Manager, 0121 424 1235