Insulinogenic Index Calculator
Insulinogenic Index Calculator
Measure early-phase insulin secretion during an OGTT — one of the earliest detectable defects on the path to type 2 diabetes.
Insulinogenic Index
Early-phase beta-cell functionFasting insulin 8 µU/mL, 30-minute insulin 45 µU/mL, fasting glucose 95 mg/dL, 30-minute glucose 160 mg/dL
Formula
- I₀, I₃₀
- insulin at fasting and at 30 minutes of a 75 g OGTT, µU/mL
- G₀, G₃₀
- glucose at fasting and at 30 minutes, mg/dL
- validity
- the index is invalid if G₃₀ is not higher than G₀ — the denominator then reaches zero or goes negative
Worked example
Fasting insulin 8 µU/mL, 30-minute insulin 45 µU/mL, fasting glucose 95 mg/dL, 30-minute glucose 160 mg/dL
45 − 8 = 37
160 − 95 = 65
37 ÷ 65 = 0.569 µU/mg
Insulinogenic index bands
| Index | Interpretation |
|---|---|
| Below 0.4 | Impaired early-phase response |
| 0.4 – 0.7 | Intermediate |
| Above 0.7 | Preserved |
Why the first 30 minutes matter
The insulinogenic index captures early-phase insulin secretion — the rapid burst of insulin release from beta cells in the first minutes after a glucose load, driven largely by insulin already stored and ready for release rather than by insulin synthesised in response to the stimulus. It is measured across the first 30 minutes of a standard 75 g OGTT because that window captures most of the early-phase response before the slower second phase takes over.
Loss of this early-phase response is one of the earliest detectable abnormalities on the path to type 2 diabetes, and it typically precedes fasting hyperglycaemia by years. Beta cells can still produce enough insulin overall to keep fasting glucose normal for a long time, compensating with a larger, more sustained second-phase response, even as their ability to release insulin promptly is already failing. This is precisely why a normal fasting glucose does not exclude an early beta-cell defect, and why an OGTT-based measure can flag a problem that fasting testing misses entirely.
The index is invalid if the 30-minute glucose is not higher than the fasting value. In that situation the denominator approaches zero or turns negative, and the resulting number is not physiologically meaningful — it does not indicate excellent secretion, it indicates that the calculation itself has broken down and the test should be reviewed rather than the number reported.
Frequently asked questions
What does the insulinogenic index measure?
Early-phase insulin secretion — the rapid initial release of stored insulin in the first 30 minutes of an oral glucose tolerance test.
Why does early-phase insulin secretion matter?
Its loss is one of the earliest detectable defects in the progression to type 2 diabetes, typically preceding fasting hyperglycaemia by years, because beta cells can compensate with a larger second-phase response for some time.
When is the insulinogenic index invalid?
When the 30-minute glucose is not higher than the fasting glucose. The denominator then reaches zero or becomes negative, and the result is not physiologically meaningful.
Can a normal fasting glucose rule out a beta-cell defect?
No. Fasting glucose can remain normal for years while early-phase insulin secretion is already impaired, because a larger second-phase response compensates. This is why OGTT-based indices can detect a defect that fasting testing misses.
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References
- Seltzer HS et al. Insulin secretion in response to glycemic stimulus: relation of delayed initial release to carbohydrate intolerance in mild diabetes mellitus. J Clin Invest. 1967;46(3):323–35.
- Kanauchi M. A new index of insulin sensitivity obtained from the oral glucose tolerance test applicable to advanced type 2 diabetes. Diabetes Care. 2002;25(11):1891–2.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.