Insulin, Plasma

Diagnostic Use

This test can only measure native human insulin.

Insulinoma – The concentration of insulin is disproportionately high in relation to the concurrent low, (<2.5mmol/L) fasting plasma glucose level. See Turner et al. Ideally insulin levels are measured during an hypoglycaemic episode and/or after a carefully supervised 48-72 hour fast.

The secretion of ProInsulin and C-Peptide may also be increased in patients with insulinoma. These assays can be carried out by special arrangement with Endolab.
Tests of insulin suppressability may be undertaken using infusion of insulin and measurement of C-Peptide levels when hypoglycaemic.

Hepatic vein insulin levels can be measured to help localise site of pancreatic islet cell adenoma after selective injections of calcium via branches of the celiac axis artery.
Early diagnosis of diabetes – the first phase of insulin release which occurs 3 to 5 minutes after the intravenous injection of 25g glucose, is absent or blunted. Subsequent insulin secretion may be normal or increased.

A raised fasting insulin value or an excessive response to glucose may suggest insulin resistance such as occurs in obesity, acromegaly, Cushing’s Syndrome, etc.
Note: For clinical purposes the conventional 75g oral glucose tolerance test or 25g IV glucose tolerance test gives adequate information and plasma insulin determinations are generally unnecessary.

R C Turner, N W Oakley and J D N Nabarro, BMJ. 1971: Vol 2, p 132 - 135

Department

Endo/Steroid Lab

Delphic Registration Code

INS

Synonyms

Free Insulin

Turnaround Time

4 days

Test Code

6533

Specimen Type

• Serum
• Plasma

Specimen Requirements

Tube/Container Type:

Preferred

• EDTA (Lavender Cap)

Acceptable

• Serum (Red Cap)
• SST II (Gold Cap)
• Lithium Heparin - Non-gel (Green Cap)
• Lithium Heparin - Gel (Green Cap)

Preferred Sample Volume (ml)

2

No of Samples Required

1

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

• EDTA (Lavender Cap)

Acceptable

• Serum (Red Cap)
• SST II (Gold Cap)
• Lithium Heparin - Non-gel (Green Cap)
• Lithium Heparin - Gel (Green Cap)

Additional Specimen Information

Pre-Testing Requirements:
Fasted overnight.

Simultaneous glucose levels are needed to interpret results.

Patients with suspected insulinoma should be fasted for 15 – 18 hours prior to sampling for plasma insulin and glucose, provided blood sugar values allow this. If possible, blood should be drawn for insulin and C-peptide as well as glucose when spontaneous hypoglycaemia occurs – since such measurements can be diagnostic in insulinoma.
Note:
The results of concurrent plasma blood glucose determinations must be sent with the samples for insulin assay, as isolated plasma insulin values are meaningless.
Biguanides, sulphonylurease and β blocking drugs should be discontinued prior to testing.
Does not measure Lispro insulin.

Samples must not be haemolysed
Samples should be separated and stored frozen within 4 hours if stored at room temperature.

Aliquot Instructions

Sample Type

• Serum
• Plasma

Minimum Volume (ml)

0.3

Transport to Lab

Frozen
If overnight - Frozen

Additional Referral/Aliquot Information

Minimum 0.3 mL Plasma or Serum. Preferably >0.5 mL Plasma or Serum. No haemolysis! Freeze.

Interpretation

Raised values (ie increased insulin/glucose ratio):

Obesity, insulinoma, Cushing’s Syndrome, Acromegaly, sulphonylurease, sex steroids, amino acids, glucagon and gut hormones, β blocking drugs, insulin receptor abnormalities.

In the presence of normal or elevated plasma glucose, raised fasting insulin suggests insulin resistance.
If patient hypoglycaemic, an elevated fasting insulin is suggestive of inappropriate insulin excess secondary to endogenous hypersecretion, sulphonylurea use or exogenous insulin administration.

Low values (ie decreased insulin/glucose ratio):

Diabetes mellitus, hypokalaemia, phaeochromocytoma, restricted carbohydrate intake, β-blocking drugs, hypoglycaemia, streptozotocin, somatostatin.

Reference Intervals

10 – 80 pmol/L (after overnight fast, adult, BMI < 25)
Non-fasting higher.

Test Method

Roche Cobas e411

Non-Funded Test Cost

$37.88 (Exclusive of GST)

Billing Code

6533

Measure glucose at the same time.

In the complete absence of hemolysis, insulin in EDTA plasma or serum is stable for 24 hours at room temperature. However since hemolysis is common in routine clinical samples we recommend transporting samples at 0o (in ice/water mixture) or frozen.

Insulin is very sensitive to even slight hemolysis and the effect increases with temperature and time. For example, hemolysis at a level of 1.3g Hb/L immediately lowers insulin by 10%. And at 0.4g Hb/L, 1 hour at room temperature lowers insulin by 10%.

This test can only measure native human insulin. Animal samples, including dogs, are not acceptable as there is little cross-reaction with the differing amino-acid sequences. Similarly, we cannot measure modified human insulins (insulin analogues) such as lispro insulin.

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