Proinsulin
Label Mnemonic: PINS
Epic Lab Code: LAB3911
Downtime Form: A-1a Miscellaneous Request
Commercial Mail-out Laboratory
5231 RCP
356-8593
Specimen(s):
Plasma
Collection Medium:
Lavender top tube 3 mL (EDTA)
Minimum:
Preferred Minimum: 1.5 mL of EDTA plasma. Draw blood in an ice-cooled lavender-top (EDTA) tube(s) from a fasting patient.

Absolute Minimum: 0.65 mL of EDTA plasma. Draw blood in an ice-cooled lavender-top (EDTA) tube(s) from a fasting patient.
Delivery Instructions:
Keep on ice and deliver to laboratory immediately.
Testing Schedule:
Weekly
Turn Around Time:
1 week upon receipt at reference laboratory
Reference Range:
3-20 pmol/L
Interpretive Data:
Normal individuals will have proinsulin concentrations below the upper limit of the normal fasting reference range (20 pmol/L) when hypoglycemic (blood glucose <45-60 mg/dL). Conversely, most (>80%) insulinoma patients will have proinsulin concentrations above the upper limit of the reference range. The sensitivity and specificity for a diagnosis of insulinoma during hypoglycemia are approximately 75% and near 100%, respectively, at the 20 pmol/L cutoff. A higher sensitivity (>95%) can be achieved using a 5 pmol/L cutoff, and this is the cutoff recommended by the reference laboratory. However, the lower cutoff results in a reduced specificity (approximately 40%), emphasizing the need for a combination of different tests to assure accurate biochemical diagnosis.

Patients with PC1/3 deficiency have low, or sometimes undetectable, insulin levels and substantially elevated proinsulin levels, exceeding the upper limit of the reference range substantially in the fasting state and rising even higher after food intake. Many other hormonal abnormalities are also present, including cortisol deficiency (because of lack of processing of pro-opiomelanocortin to adrenocorticotropic hormone and other peptides), infertility and, often, morbid obesity.

This assay demonstrates no cross-reactivity with insulin or C-peptide.
Comments:
Useful:
As part of the diagnostic workup of suspected insulinoma.

As part of the diagnostic workup of patients with suspected PC1/3 deficiency.

As part of the diagnostic workup of patients with suspected proinsulin mutations.

Cautions:
To avoid misdiagnoses, all proinsulin measurements used in the diagnostic workup of patients with hypoglycemia must be interpreted in the context of coexisting illnesses, the blood glucose concentration at the time of sampling, and other test results (ie, insulin, C-peptide, beta-hydroxybutyrate, and sulfonylurea drug screen). For example, patients with chronic renal failure or type 2 diabetes mellitus can have increased proinsulin, C-peptide and insulin values, but usually without suppressed (<45 g/dL) blood glucose.
Methodology:
Immunochemiluminescent Assay
CPT Code:
84206