Insulin testing measures the level of insulin in blood, providing information about pancreatic insulin production and tissue insulin sensitivity. The insulin test is less commonly ordered than glucose tests but is useful in specific clinical situations including evaluation of hypoglycemia, suspected insulin resistance, and rare insulinomas. This guide covers when insulin testing is appropriate, normal ranges, and what abnormal results mean.
What insulin testing measures
Standard insulin testing measures fasting insulin levels. Insulin is produced by pancreatic beta cells in response to glucose. The relationship between insulin and glucose provides information about beta cell function and insulin resistance.
The HOMA-IR calculation (Homeostatic Model Assessment of Insulin Resistance) uses fasting glucose and insulin to estimate insulin resistance:
HOMA-IR = (fasting glucose × fasting insulin) / 405
HOMA-IR above 2.5 suggests insulin resistance.
Reference ranges
- Fasting insulin: 2-25 μU/mL (varies by lab)
- HOMA-IR: under 2.5 (insulin resistance threshold debated)
When insulin testing is ordered
Hypoglycemia evaluation: Insulin levels during a hypoglycemic episode help diagnose insulinoma (rare pancreatic tumor causing high insulin and low glucose). Inappropriately high insulin during hypoglycemia points toward insulinoma; appropriately low insulin suggests other causes.
Insulin resistance assessment: Fasting insulin or HOMA-IR can help characterize insulin resistance in patients with prediabetes, PCOS, or metabolic syndrome.
Type 1 vs type 2 diabetes: C-peptide testing is more useful, but insulin levels can sometimes contribute information about endogenous insulin production.
What insulin doesn’t tell you
Insulin testing is rarely useful for routine diabetes screening. Fasting glucose, A1c, and OGTT are the standard diagnostic tools. Insulin levels add information in specific clinical scenarios but don’t replace other testing.
Frequently Asked Questions
Do I need to fast for insulin testing?
Yes for fasting insulin. Random or non-fasting insulin levels are difficult to interpret.
What does high insulin mean?
High fasting insulin suggests insulin resistance — the body is producing more insulin to maintain normal glucose. Common in obesity, PCOS, type 2 diabetes, metabolic syndrome.
What does low insulin mean?
Low fasting insulin in a non-diabetic person is usually normal. In diabetic patients, low insulin with high glucose suggests beta cell dysfunction (advanced type 2 or type 1 diabetes).
Is insulin testing useful for routine screening?
No. Standard glucose tests are preferred for routine diabetes screening.
What’s HOMA-IR?
A calculated measure of insulin resistance using fasting glucose and insulin. HOMA-IR above 2.5 suggests insulin resistance.
The bottom line on insulin testing
Insulin testing is useful in specific clinical situations but isn’t routine. Hypoglycemia workup, insulin resistance assessment in some patients, and rare insulinoma diagnosis are the main uses. For routine diabetes screening, fasting glucose and A1c remain the standard tools.