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Summary  The value of plasma insulin, human C-peptide and proinsulin estimation in the diagnosis of 15 insulinomas has been investigated. Measurement of plasma proinsulin in an overnight fasting sample diagnosed all the insulinomas studied, irrespective of the plasma glucose. Patients with insulinomas had plasma proinsulin in the range 0.04–4.2 pmol/l and normal values were less than 0.01 pmol/ml. If hypoglycaemia was present, an inappropriately raised plasma immunoreactive insulin (including proinsulin) was diagnostic, but this assay was of little assistance if the plasma glucose was normal. Hypoglycaemia was induced with fish insulin in twelve patients with insulinomas and eight normal subjects. Using an antiserum which did not detect fish insulin, but cross-reacted with human proinsulin, the endogenous immunoreactive insulin was suppressed in the normal subjects, but all insulinoma patients had impaired suppression. Assay of plasma human C-peptide, or of the combined immunoreactive C-peptide and proinsulin, discriminated less well and did not clearly diagnose three insulinomas which secreted proinsulin rather than insulin and C-peptide. Plasma human proinsulin values during induced hypoglycaemia gave excellent discrimination and should detect insulinomas irrespective of their degree of histological differentiation. The assay of plasma human proinsulin allows a suppression test to be performed with hypoglycaemia induced by any type of insulin. A raised plasma proinsulin in proportion to C-peptide suggests an undifferentiated insulinoma, which may be more likely to be malignant.

Plasma proinsulin, C-peptide and insulin in diagnostic suppression tests for insulinomas