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Fasting insulin (revision 12)

Old revision·15:09, 24 Feb 2025·B12_Bettina

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Fasting insulinLaboratory medicine
SpecimenFasting plasma or serum
InterpretableOnly alongside a simultaneous glucose
StandardisationPoor between assays
Topic infobox · conventions

Fasting insulin is the concentration of insulin measured after an overnight fast. It is widely available and widely over-interpreted: an insulin value alone answers no question, because the same concentration means different things at different glucose concentrations.[1]

Insulin immunoassays are poorly standardised between manufacturers, differing in cross-reactivity with proinsulin and its split products. Values from different laboratories are not interchangeable, and derived indices inherit the discrepancy.[1]

Its principal use is as an input to derived indices, chiefly HOMA-IR and HOMA-B, which combine it with fasting glucose to estimate insulin resistance and secretory capacity respectively.[2]

Why it needs glucose alongside it

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Insulin secretion is a response to glucose. A high fasting insulin with a normal fasting glucose indicates that normal glucose is being maintained at the cost of high insulin — the signature of insulin resistance with preserved secretion. The same insulin with a high glucose indicates resistance with inadequate compensation.[1]

A low insulin with a high glucose indicates secretory failure. A low insulin with a normal glucose is unremarkable. The four combinations mean four different things, and only two values together distinguish them.

This is why laboratory reports that give an insulin reference interval without reference to glucose are of limited use, and why the derived indices exist at all.[2]

Standardisation

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Insulin assays differ in their response to proinsulin and to split proinsulin products, which circulate at higher relative concentrations in insulin-resistant states. An assay with more proinsulin cross-reactivity reads higher in exactly the population most likely to be tested.[1]

ConsequenceEffect
Between-assay differencesValues not interchangeable
Proinsulin cross-reactivityOverestimation in resistant states
Derived indicesInherit both problems
Published cut-offsAssay-specific in origin

See also

References

  1. ^ a b c d Wallace TM, Levy JC, Matthews DR. "Use and abuse of HOMA modeling." Diabetes Care 27(6):1487–1495 (2004). DOI:10.2337/diacare.27.6.1487. PMID 15161807.
  2. ^ a b Matthews DR, Hosker JP, Rudenski AS, et al. "Homeostasis model assessment." Diabetologia 28(7):412–419 (1985). PMID 3899825.
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