Insulin, a hormone produced by the pancreatic β cells of the islets of Langerhans, regulates carbohydrate metabolism together with contributions from the liver, adipose tissue, and other target cells. Insulin is responsible for maintaining blood glucose levels at a constant level within a defined range. The rate of insulin secretion is primarily regulated by the level of blood glucose perfusing the pancreas; however, it can also be affected by hormones, the autonomic nervous system, and nutritional status.
Insulin levels are valuable for establishing the presence of an insulinoma (i.e., tumor of the islets of Langerhans). This test is also used for investigating the causes of fasting hypoglycemic states and neoplasm differentiation. The insulin study can be done in conjunction with a GTT or FBG test or a FPG test.
Immunoreactive:
Adults: 0–35 mcIU/mL or 0–243 pmol/L
Children: 0–10 mcIU/mL or 0–69 pmol/L
Free:
Adults: 0–17 mcIU/mL or 0–118 pmol/L
Children (prepubertal): 0–13 mcIU/mL or 0–90 pmol/L
Clinical Alert
Critical range: >35 µIU/mL or >243 pmol/L (fasting
Obtain a 5-mL blood sample (red-topped tube) from a fasting (8 hours) person; serum is preferred. Observe standard precautions. Heparinized blood may be used.
If done in conjunction with a GTT, draw the specimens before administering oral glucose, at ingestion, and 120 minutes after glucose ingestion (the same times as the GTT).
Increased insulin values are associated with the following conditions:
Insulinoma (pancreatic islet tumor). Diagnosis is based on the following findings:
Hyperinsulinemia with hypoglycemia (glucose <30 mg/dL or <1.66 mmol/L)
Persistent hypoglycemia together with hyperinsulinemia (>20 µU/mL or >139 pmol/L) after tolbutamide injection (rapid rise and rapid fall)
Failed C-peptide suppression with a PG level <30 mg/dL or <1.66 mmol/L and insulin/glucose ratio >0.3
T2D, untreated
Acromegaly
Cushing syndrome
Endogenous administration of insulin (factitious hypoglycemia)
Obesity (most common cause)
Pancreatic islet cell hyperplasia
Decreased insulin values are found in the following conditions:
T1D, severe
Hypopituitarism
Pretest Patient Care
Explain test purpose and procedure.
Ensure that the patient fasts from all food and fluid, except water, unless otherwise directed.
Insulin release from an insulinoma may be erratic and unpredictable; therefore, it may be necessary for the patient to fast for as long as 72 hours before the test.
Follow guidelines in Chapter 1 regarding safe, effective, informed pretest care.
Posttest Patient Care
Have patient resume normal activity and diet.
Review test results; report and record findings. Modify the nursing care plan as needed. Patients with obesity may have insulin resistance and unusually high fasting and postprandial (after eating) insulin levels. Explain possible need for further testing and treatment.
Follow guidelines in Chapter 1 regarding safe, effective, informed posttest care.
Clinical Alert
A potentially fatal situation may exist if the insulinoma secretes unpredictably high levels of insulin. In this case, the blood glucose may drop to such dangerously low levels as to render the person comatose and unable to self-administer oral glucose forms. Patients and their families must learn how to deal with such an emergency and to be vigilant until the problem is treated
Surreptitious insulin or oral hypoglycemic agent ingestion or injection causes elevated insulin levels (with low C-peptide values).
Oral contraceptives and other drugs cause falsely elevated values.
Recently administered radioisotopes affect test results.
In the second to third trimester of pregnancy, there is a relative insulin resistance with a progressive decrease of PG and immunoreactive insulin.