Baseline proinsulin levels should be collected after a 12-hour fast

Fasting for a blood test

Gold serum separator (SST) tube

Immediately following collection, thoroughly mix sample by gently inverting 5 times

1. Allow sample to clot for a minimum of 30 minutes
2. Spin within two (2) hours of sample collection
3. Transfer serum to a Screw-cap polypropylene frozen transport vial/tube - 4mL (LabCorp), labelled as serum
4. Freeze

Screw-cap polypropylene frozen transport vial/tube - 4mL (LabCorp)

Red serum vial/tube, 5 mL

1. Allow sample to clot 
2. Spin 
3. Transfer serum to a Screw-cap polypropylene frozen transport vial/tube - 4mL (LabCorp), labelled as serum, within two (2) hours of sample collection
4. Freeze

Frozen (strict) - 12 days

Freeze/thaw cycles - stable x3

Refrigerated - NO

Ambient - NO

• Non-frozen specimen
• Non-serum specimen
• Gross hemolysis
• Gross lipemia

Enzyme immunoassay (EIA)

0.0 - 10.0 pmol/L

Proinsulin is synthesized in the pancreatic beta cells as a 9390 mw polypeptide of 86 amino acids.1-3 Proinsulin is subsequently cleaved enzymatically, releasing insulin into the circulation along with a residual 3000 mw fragment called C-peptide, so-named because it connects the A and B chains of insulin within the proinsulin molecule.

Proinsulin, which has relatively low biological activity (approximately 10% of insulin potency), is the major storage form of insulin. Normally, only small amounts (∼3% of the amount of insulin, on a molar basis) of proinsulin enter the circulation. Because the hepatic clearance of proinsulin is only 25% of insulin clearance, the half-life of proinsulin is two- to threefold longer and concentrations in the fasting state are approximately 10% to 15% of insulin concentrations.

High proinsulin concentrations have been associated with benign or malignant β-cell tumors of the pancreas4 and endocrine pancreatic tumors associated with MEN-1.5 Elevated proinsulin levels have been observed in individuals with impaired glucose tolerance even in the absence of abnormal glucose or C-peptide levels.6 Elevated proinsulin levels have been found to be a positive risk factor for the development on NIDDM.7,8 Most patients with β-cell tumors have increased insulin, C-peptide, and proinsulin concentrations, but occasionally only proinsulin is elevated. Despite its low biological activity, proinsulin may be increased sufficiently to produce hypoglycemia.9 In addition, a rare form of familial hyperproinsulinemia, due to impaired conversion to insulin, has been described. Increased proinsulin concentrations may also be detected in patients with chronic renal failure, cirrhosis, or hyperthyroidism.