Insulinoma

Assays

Insulin-determination in serum samples
Insulin-Assay
The reference range resp. the normal level for insulin concentrations in serum samples in many laboratories is being given from 5 - 25 µU/mL.
This range is only valid for overnight fasted patients (12 hours) with normal weight, without diabetes mellitus, and during normal caloric nutrition and normal insulin sensitivity - which cannot be accurately measured by simple techniques rather than estimated.
This reference range cannot be used for the interpretation of the fasting test. It is irrelevant, since "normal" insulin concentrations during manipulations by means of diagnostic endocrinological function tests are dependent upon the prevailing blood glucose level.

During the fasting test the suppressibility of serum insulin close to or below biochemical detection limits is being examined.

Regular and valid quality control of the used assay system therefore is mandatory.
This is also valid for the C-peptide assay with a "normal" reference range between
1.5 - 3.5 ng/mL (0.5-1.15 nmol/L).
Parallel C-peptide determination may be used as a control of plausibility of the insulin assay when serum levels near the detection limit are seen (range < 5 µU/mL), which means dependency upon
LOD and LOQ ("limit of detection" and "limit of quantitation").
Physicians ordering insulin assays should be informed in detail about the used assay system in order to interprete the results appropriately.

Literature
1. Heinemann L, Sawicki PT, Niederau C & Starke AAR: Klinische Chemie. Immunologische Messung von Insulin und C-Peptid. In: Diabetes mellitus. Urban & Fischer; München Jena, 2nd ed.2000, p60-65
2. Service FJ. Hypoglycemic disorders. New Engl J Med 332, 1995, 1144-1152
3. Starke AAR, Saddig C. Hypoglykämien im Erwachsenenalter. In: Diabetes mellitus. Urban & Fischer, München Jena, 2nd. ed. 2000, p775-782 + Urban & Schwarzenberg; München Wien; 1st. ed. 1995, p643-650

Assay-Technology
for Insulin, C-Peptide, Proinsulin

In order to interprete diagnostic test data (fasting test, C-peptide suppression test, test meals) correctly and reliably, detailed knowledge of the used assay technology is mandatoy, especially due to the variety of commercially available kit systems.

This applies to
1. optimal measuring range of the assay system (linear calibration curve),
2. cross reactivities with insulin, C-peptide, proinsulin and split-proinsulins
3. quality control performed in the endocrine lab (precision, recovery, linearity).

Quality control procedures should be performed according to the "Guidelines for Validation of Insulin Immunoassays" of the American Diabetes Association (ADA - Committee on Insulin Assay Standardization).

INSULIN-ASSAYS

should have a linear measuring range from 5 - 100 µU/mL (= 30 - 600 pmol/L). Crossreactivity with proinsulin should be known. When specific insulin assays are used the determination of proinsulin is mandatory in order to avoid the escape of predominantly proinsulin-secreting insulinomas.

C-PEPTIDE-ASSAYS

are available with a measuring range from 0.5 - 15 ng/mL.
Values > 10 ng/mL may be relevant in the diagnostics of impaired glucose tolerance and insulin resistance. Thus, a easuring range from < 0.3 to 5 ng/mL is essential in the diagnostics of hypoglycemia. Such sensitive assays are used in the evaluation of the secretory reserve in type I diabetes.

The PROINSULIN-ASSAY

should not crossreact with insulin and C-peptide.
Due to physiologically low concentrations in healthy people in the low pmol-range and rather high levels of some hundred pmols in patients with insulinoma dilution of samples is normally necessary. The optimal measuring range reaches from 5 to 100 pmol/L.