HOMA-IR (revision 8)

HOMA-IR is an index of insulin resistance derived from a single fasting sample. In its original linear approximation it is the product of fasting glucose and fasting insulin divided by a constant, and it correlates moderately with clamp-derived measures of insulin sensitivity across populations.^[1]

Its appeal is that it requires one blood draw rather than a several-hour clamp. Its weakness is that it is a model-derived approximation from two values, inherits the poor standardisation of insulin assays, and describes hepatic rather than peripheral insulin resistance most directly.^[2]

The authors of the model have themselves written about its misuse, and the practical conclusion is that it tracks change within an individual on a consistent assay better than it classifies individuals against published thresholds.^[2]

The original linear approximation is

\text{HOMA-IR} = (\text{glucose}_\text{mmol/L} \times \text{insulin}_\text{mU/L}) / 22.5

A revised non-linear computer model, HOMA2, was published later and gives different values, particularly at the extremes. The two are not interchangeable, and a figure quoted without saying which was used is ambiguous.^[2]

Unit conventions add a second ambiguity: glucose in mg/dL requires a different divisor, and insulin in pmol/L a further conversion. A published cut-off carries its unit convention with it.^[1]

Both forms assume steady-state fasting conditions, and neither has a meaning in anyone receiving exogenous insulin, since the insulin measured is then not the beta cell's output.