Assays / Hormones / C-peptide and Insulin (plasma, serum)

Clinical use
Differential diagnosis of documented spontaneous hypoglycaemia.

Application
The measurement of both insulin and C-peptide is advisable in these investigations since in some cases of insulinoma, insulin levels may be low or marginally inappropriate whereas C-peptide is almost always unequivocally raised. Both measurements are essential for the identification of factitious insulin administration. Further tests, such as those for ketones, proinsulin, sulphonylureas, growth hormone and the insulin-like growth factors may also be required to complete the diagnostic process.

Patient preparation
Hypoglycaemia, spontaneous or whilst fasting, must first be established by regular monitoring of blood glucose levels. Blood (10 mL) collected in a plain vacutainer or syringe, when the blood glucose concentration (confirmed by laboratory analysis) is less than 2.2 mmol/L (or less than 2.5 mmol/L in patients over 60 years), should be used for the tests. Vacutainers (7 mL, red top) may be used for blood collection.

Sample preparation
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.

Send samples to the SAS laboratory. Ensure the samples remain frozen during transport. Record the blood glucose concentration on the request form.

Reference range
An interpretation of the result(s) will be provided where sufficient information accompanies the request. For further clarification please contact the SAS laboratory.

Centres offering this assay
Cardiff, Guildford, London (Hammersmith).

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