Fasting insulin test: why it is done and how to prepare

The main reasons a clinician orders fasting insulin:

• Suspected insulin resistance. Fasting insulin rises when the pancreas compensates for insulin resistance, often years before glucose changes [3] [4]. A fasting insulin above 9.0 µIU/mL identified 80% of prediabetes in one clinic population [5].
• Calculating HOMA-IR. Paired with fasting glucose, insulin gives HOMA-IR, which predicted coronary heart disease better than either marker alone [6]. See how to calculate HOMA-IR.
• Conditions linked to insulin resistance. The ADA lists polycystic ovary syndrome as a diabetes risk factor, and severe obesity, acanthosis nigricans and fatty liver disease as conditions associated with insulin resistance [1]. Some clinicians use fasting insulin to follow these.
• Low blood sugar. When the body may be making too much insulin, the Endocrine Society recommends measuring glucose, insulin, C-peptide and proinsulin during a documented low blood sugar episode [7]. That is a different, supervised test.

Fasting insulin is not part of routine screening. Guidelines diagnose diabetes and prediabetes with glucose and HbA1c [1].

• Fast at least 8 hours. The ADA defines fasting as no caloric intake for at least 8 hours [1], and labs require the patient to be fasting for insulin [8]. Most people fast overnight and test in the morning.
• Water is fine. Avoid anything with calories, including milk in coffee, juice and sweetened drinks.
• Keep the day before ordinary. Unusual meals, a very short night or a hard training session can shift insulin sensitivity. Short sleep reduces insulin sensitivity in trials [9].
• Medications. Do not stop a prescribed medication without asking your clinician. If you inject insulin, the test cannot separate injected from natural insulin, and insulin antibodies interfere with the assay [8]; a C-peptide test is used instead [10].
• Same lab each time. Assays disagree by a median of 24% [2], so use the same lab for repeat tests.

Lab names vary. Labcorp lists it as Insulin (test 004333) with results in µIU/mL and the LOINC code 20448-7 [8]. Other labs call it serum insulin, fasting insulin or insulin, fasting, and some sell an insulin resistance panel that adds glucose and HOMA-IR.

Do not confuse it with:

• C-peptide, which measures the body's own insulin output and is used mostly in people treated with insulin [10] [1].
• An oral glucose tolerance test with insulin, which measures insulin at several time points after a glucose drink. Kraft-style versions sample for 3 to 5 hours [11].
• HbA1c or fasting glucose, which measure blood sugar, not insulin. See HbA1c.

In many countries fasting insulin is an inexpensive, widely available blood test, but insurers may not cover it without a clinical reason, because it is not a diagnostic test for diabetes [1]. Prices differ between countries, labs and plans, so ask your lab or insurer before ordering.

At-home kits. Several companies offer finger-prick or self-collected samples for insulin. The same assay problems apply: results depend on the method and calibration [12]. If you use a kit, stick with the same service so trends are comparable, and confirm a surprising result with a standard venous draw.

UK and elsewhere. Availability through public health services varies, and it is more often ordered by specialists than in primary care. Private labs in most countries offer it.

1. Check the unit. µIU/mL and mU/L are the same. pmol/L divided by 6.0 gives µIU/mL; some labs used 6.945 [13].
2. Check the lab range. One common range is 2.6 to 24.9 µIU/mL [8].
3. Look at glucose alongside it. A normal fasting glucose with higher insulin suggests the pancreas is compensating [3]. Calculate HOMA-IR.
4. Compare with research thresholds, not targets. Values above 9.0 µIU/mL [5] or about 12 to 12.5 µIU/mL [14] [15] have been linked with prediabetes or insulin resistance.
5. Repeat before acting. Single samples can vary 25% to 50% [11].

A very low insulin with high glucose needs prompt medical review. See why fasting insulin can be low. For what each range means, see what is a good fasting insulin level.