Fasting Insulin Test: What It Shows and Its Limits
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[Glossary · How doses act in the body]

Fasting insulin

Also called insulin in blood test, serum insulin, fasting insulin level · Reviewed

Definition: Fasting insulin is a blood test that measures insulin after you have not eaten for a set time, usually 8 to 12 hours. Insulin is the hormone from the pancreas that moves sugar from the blood into cells. MedlinePlus says the test is usually read together with blood glucose, because insulin rises and falls with it.

What it can and cannot tell you

MedlinePlus says the main reason for the test is to find the cause of low blood sugar. It is also used, with other tests, to look at insulin resistance, where the body needs more insulin to keep glucose in range.

It is not usually used to diagnose diabetes. MedlinePlus says diabetes is diagnosed with tests that measure blood glucose, such as fasting glucose or A1C, because insulin in people with diabetes can vary widely.

MedlinePlus says reference ranges vary between labs, and an insulin result is usually not enough on its own.

Why it comes up when you track doses

Several labels warn about blood sugar, but they ask for glucose checks rather than insulin. The Genotropin (somatropin) label says growth hormone may lower insulin sensitivity and says to monitor glucose periodically. The Egrifta SV label says to check glucose before and during treatment.

The Depo-Testosterone label says androgens may lower blood glucose and insulin needs in people with diabetes.

Things that can skew it

  • Biotin. MedlinePlus says to stop biotin supplements at least a day before the test, as your provider directs.
  • Insulin medicines. Some people who take insulin form antibodies that interfere with the test, and some tests read insulin medicines differently.
  • Not fasting long enough raises the number, since insulin rises after eating.

Read more on Dose Track: Blood sugar · Logging lab results in Dose Track

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
HbA1c (hemoglobin A1C)HbA1c , or A1C, is a blood test that shows your average blood sugar over the past two to three months.… IGF-1 (insulin-like growth factor 1)IGF-1 (insulin-like growth factor 1) is a hormone that carries out many of the effects of growth hormone… GLP-1 receptor agonistA GLP-1 receptor agonist is a drug that switches on the same receptor as GLP-1 (glucagon-like peptide-1),… Growth hormone secretagogueA growth hormone secretagogue is a compound that makes your pituitary gland release more of its own growth…

Frequently asked questions

Can fasting insulin diagnose diabetes?+
Not usually. MedlinePlus says diabetes is diagnosed with glucose tests such as fasting glucose or A1C, and insulin is read alongside glucose.
How long do I fast before an insulin test?+
MedlinePlus says you will probably need to fast for 8 to 12 hours. Follow the instructions your provider gives you.

Sources

  1. MedlinePlus (NIH): Insulin in Blood Test
  2. Genotropin (somatropin) US prescribing information, sections 2 and 5, DailyMed
  3. Egrifta SV (tesamorelin) US prescribing information, section 5, DailyMed
  4. Depo-Testosterone (testosterone cypionate) US prescribing information, DailyMed

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