2. Insulin testing is used to assist in identifying causes of hypoglycemia
(plasma glucose levels < 55 mg/dL), especially upon signs and
symptoms of hypoglycemia (neurohypoglycopenic and autonomic
symptoms). In this scenario, a 72-hour fasting test is performed.
4. Collection and panels:
Method: Radioimmunoassay; enzyme-linked immunosorbent assay (ELISA)
Specifics for collection and panels are as follows:
● Specimen type: Blood serum
● Container: Vacutainer, red top
● Collection method: Venipuncture
● Specimen volume: 1 mL
● Measure blood glucose and C-peptide level in same sample
● An 8-hour fasting specimen required
5. Other instructions:
The patient is instructed to fast, and plasma glucose,
insulin, proinsulin, and C-peptide levels are measured every
6 hours until the plasma glucose level is less than 65
mg/dL, after which the testing frequency is increased to
every 1-2 hours. Fasting is ended when plasma glucose
levels are less than 45 mg/dL accompanied by signs and
symptoms of hypoglycemia. At the endpoint, a blood sample
is collected and tested for glucose, insulin, proinsulin,
C-peptide, beta-hydroxybutyrate, and sulfonylurea levels.
The patient is given 1 mg of intravenous glucagon, and the
response of the blood glucose level is measured.
Your fasting
will be ended
as soon as
possible
9. Insulin Secretion
Schematic representation of the
anatomic relationships in an islet of
Langerhans. The insulin-producing Β
cells (in blue) are in the center closest
to the blood supply and are surrounded
by the glucagon-producing alpha (α)
cells (in orange). On the outside are the
delta (δ) cells (in yellow), which make
somatostatin, and the PP cells (in
green), which make pancreatic
polypeptide. Uptodate 21.3
10. Processing of
insulin within the
beta cell
Insulin is synthesized as preproinsulin
in the ribosomes of the rough
endoplasmic reticulum. The
preproinsulin is then cleaved to
proinsulin, which is transported to the
Golgi apparatus where it is packaged
into secretory granules located close to
the cell membrane. Most of the
proinsulin is cleaved into equimolar
amounts of insulin and C peptide in the
secretory granules. Courtesy of David
K McCulloch, MD. Uptodate 21.3
16. Sulfonylurea Drugs:
Type 2 diabetics may be prescribed
drugs that increase insulin secretion in
order to correct this relative insulin
deficiency. Sulfonylureas and
meglitinides are drugs that increase
insulin secretion by binding to and
closing the potassium channels for
which ATP is the normal ligand.
From University of Washington web server - Courses -
Humoral Regulation of Hormone Release
19. Considerations:
Insulin levels may be falsely elevated by the
following:
● Amino acid (leucine, arginine, and lysine)
● Steroid
● Insulin secretagogue (sulfonylurea and
glinide)
● Estrogen
● Beta2 agonist
Insulin levels may be falsely decreased by the
following:
● Acarbose, metformin, octreotide, and
beta-blocker
● Hemolysis (insulin-degrading enzyme in red
blood cell released)
● Hemodialysis
20. Thanks For
Listening
References:
● Medscape 2017 - Insulin
● Uptodate 21.3 - Insulin secretion and
pancreatic beta-cell function
● University of Washington web server -
Courses - Humoral Regulation of
Hormone Release
