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- Immunoreactive Insulin
This insulin assay is performed using a 2-site electrochemiluminescent immunoassay on the Roche automated platform.1 This test could be effectively used for those patients not receiving exogenous insulin or those receiving unmodified human insulin. Test 004333, Insulin is highly specific for human insulin, as shown in the table below. This test does not react, for instance, with several of the insulin analogs used to treat diabetes as indicated in Table 1 below. The efficacy of each test depends on the patient treatment and goal of the ordering physician. It is important that the physician ordering and interpreting the results understands the specificity of the test.
Table 1: Cross reactivity to insulin analogs1-3
• Human insulin: 100%
• Insulin aspart: <1%
• Insulin glargine: <1%
• Insulin lispro: <1%
• Insulin glulisine: <1%
• Insulin detemir: <1%
• Circulating antibodies to insulin, both autoantibodies and antibodies to therapeutic insulin, can interfere with the immunoassays for insulin.4,5
• As with all tests containing monoclonal mouse antibodies, erroneous findings may be obtained from samples taken from patients who have been treated with monoclonal mouse antibodies or who have received them for diagnostic purposes.1 In rare cases, interference due to extremely high titers of antibodies to streptavidin and ruthenium can occur.1 The test contains additives that minimize these effects.
Electrochemiluminescence immunoassay (ECLIA)
Insulin is a peptide hormone with a molecular weight of approximately 6000 daltons. Serum insulin determinations may be performed on patients with symptoms of hypoglycemia4,5 where insulinoma is a possible etiology. Patterns of secretion by insulinomas may be sporadic or constant. Insulinoma is a rare functional tumor of pancreatic origin that secretes insulin inappropriately, causing hypoglycemia. Whipple originally described the tumor and classic diagnostic criteria called "Whipple's Triad". The criteria included:
1) neurologic symptoms of hypoglycemia
2) low glucose (<50 mg/dL) which are corrected immediately by giving glucose or carbohydrate.
Today, as described above, we would add inappropriately elevated insulin levels during an episode of hypoglycemia. Usually hypoglycemia is induced with a fast or exercise, and the diagnostic criteria may include a 48 to 72 hour fast with insulin levels greater than 5 μU/mL during a hypoglycemic episode.4
Hypoglycemia can, however, also be facilitated intentionally or unintentionally (factitious hypoglycemia),4,5 C-peptide levels along with insulin levels may be helpful in ruling out factitious insulin use, however note the limited reactivity of various insulin analogs. Accordingly, one use of the insulin test is to screen patients with low glucose for insulinoma if other factors such as glucose-lowering drugs or reactive hypoglycemia are excluded.
Insulin levels can be also useful in predicting susceptibility to the development of type II diabetes.4 However, the American Diabetes Association recommendations for the diagnosis of diabetes do not include the measurement of insulin levels.6
Insulin levels may also be measured to estimate the patient's capacity for insulin secretion (eg, glucagon test) or in the evaluation of insulin sensitivity (eg, oral glucose tolerance test). Although the adequacy of pancreatic insulin synthesis is frequently assessed via the determination of C-peptide, fasting insulin is also measured to assess insulin resistance.
In treated patients therapeutic administration of insulin can lead to the formation of insulin binding antibodies. Insulin binding antibodies invariably interfere with insulin measurement methods and results are unreliable. For patients with known autoantibodies to insulin should measure free and total insulin to assess biologically active fraction of insulin (free insulin) and total insulin. Measurements of free and total insulin, C-peptide, insulin antibodies and hemoglobin A1C are often tested in conjunction to clarify the contribution of endogenous and exogenous insulin to overall diabetic management. See Test 501561 for Free and Total Insulin.
0.3 mL (Note: This volume does not allow for repeat testing.)
If a red-top tube is used, transfer separated serum to a plastic transport tube. Avoid hemolysis.
Causes for Rejection
Citrate plasma specimen; hemolyzed specimen
This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.
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