Insulin, Plasma

Diagnostic Use

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 a hypoglycaemic episode and/or after a carefully supervised 48-72 hour fast.

The secretion of Pro-insulin and C-Peptide may also be increased in patients with insulinoma.
Tests of insulin suppressibility 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

Endocrinology 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)

Preferred if West Coast

• SST II (Gold Cap)

Acceptable

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

Preferred Sample Volume (ml)

2

Minimum Sample Volume (ml)

0.5

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 (Light Green Cap)

Additional Specimen Information

Pre-Testing Requirements:
Fasted overnight.
Samples must not be haemolysed.
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 of little value.
Biguanides, sulphonylurease and β blocking drugs should be discontinued prior to testing.

Serum or plasma stability;
Ambient 4 hrs
Chilled (2-8deg) 2 days
Frozen 6 months

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. No haemolysis. Use gel separating tubes if sample cannot be separated within 4 hours.
Transport plasma or serum frozen.

West Coast: If specimen is unable to be separated, take blood on SST and send chilled to Greymouth Lab. At Greymouth lab freeze serum/plasma for transport to CHL.

Interpretation

Raised values (i.e. 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 (i.e. decreased insulin/glucose ratio):

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

Reference Intervals

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

Test Method

Roche - Cobas

Non-Funded Test Cost

$40.16 (Exclusive of GST)

Billing Code

6533

Insulin is very sensitive to even slight haemolysis and the effect increases with temperature and time. For example, haemolysis 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 Insulin lispro, aspart or glargine.

Contact Information

Enquiry Form