HOMA-B Index (Beta-Cell Function)

What HOMA-B estimates and when it is used

HOMA-B (or HOMA-%B) is an index that estimates the secretory capacity of pancreatic beta cells from the glucose and insulin of a single fasting sample. It is part of the same homeostasis model (Homeostasis Model Assessment) published by Matthews and colleagues in 1985.

The logic is that of a feedback loop. For a given fasting glucose, the model calculates how much insulin the pancreas is delivering. More insulin for the same glucose results in a higher HOMA-B; little insulin in the face of high glucose results in a lower HOMA-B.

In the original article, the beta-cell function estimate correlated at 0.61 with the hyperglycemic clamp and at 0.64 with the intravenous glucose tolerance test. The index appears in research on progression to type 2 diabetes and in laboratory requests together with the HOMA-IR insulin resistance index, calculated from the same two tests.

How to calculate HOMA-B in the calculator: inputs and units

The calculator asks for two values from the same fasting sample, both greater than zero:

1. Fasting glucose, in mg/dL.
2. Fasting insulin, in µU/mL or mU/L, numerically identical units.

The tool subtracts 63 from the glucose, multiplies the insulin by 360, divides one value by the other and shows the result as a percentage, with one decimal place.

There is a safeguard: if glucose is equal to or less than 63 mg/dL, the calculator does not calculate and displays a message stating that glucose must be greater than 63 mg/dL for this calculation, with the tip to multiply by 18 if the value is in mmol/L. The reason is mathematical: with glucose equal to 63, the denominator becomes zero; below that, the result would be negative and meaningless.

Before entering values, check that glucose is in mg/dL and not mmol/L, and that insulin is not in pmol/L. To assess glycemic control over recent months, which HOMA-B does not show, use the estimated average glucose from HbA1c calculator.

HOMA-B formula: 1985 version and symbol definitions

The calculator uses the linear approximation of the original HOMA model (HOMA1), by Matthews and colleagues (1985).

HOMA-B (%) = 360 × I_fasting (µU/mL) ÷ [G_fasting (mg/dL) − 63]
HOMA-B (%) = 20 × I_fasting (µU/mL) ÷ [G_fasting (mmol/L) − 3.5]

• I_fasting: fasting plasma insulin.
• G_fasting: fasting plasma glucose.
• 360 and 63: mg/dL equivalents of 20 and 3.5 mmol/L, because 1 mmol/L of glucose corresponds to about 18 mg/dL (20 × 18 = 360; 3.5 × 18 = 63).

The result is expressed as a percentage of a reference function. The model was calibrated so that young adults of normal weight are close to 100%.

HOMA2, a later computational version, recalculates beta-cell function with different modeling and does not use this formula. Levy, Matthews and Hermans (1998) recommended the computer program for correct assessment. HOMA1 and HOMA2 values should not be compared with each other.

Worked HOMA-B example, step by step

Take a hypothetical case, for teaching only: fasting glucose of 100 mg/dL and fasting insulin of 12 µU/mL, in the same sample.

1. Numerator: 360 × 12 = 4,320.
2. Denominator: 100 − 63 = 37.
3. Division: 4,320 ÷ 37 = 116.756…
4. Rounded to one decimal place: HOMA-B = 116.8%.

In the mmol/L version the number is the same. With 100 mg/dL ≈ 5.5556 mmol/L (100 ÷ 18), the calculation is 20 × 12 ÷ (5.5556 − 3.5) = 240 ÷ 2.0556 = 116.8%. Rounding the converted glucose to 5.56 would already change the last digit, which shows how sensitive the denominator is.

The calculator returns 116.8% for these data, without classifying the value into ranges, and notes that the model was calibrated to about 100% in young adults of normal weight. The same data give a HOMA-IR of 2.96, which illustrates an important point: preserved or high beta-cell function can accompany insulin resistance, because the pancreas compensates by producing more. For that reason the two indices are read together, and neither one establishes a diagnosis.

How to understand the HOMA-B result

HOMA-B indicates how much insulin the pancreas is secreting for the observed fasting glucose, compared with the model's reference. There is no published universal normal range for the linear formula.

For that reason the calculator shows only the number, with a neutral note: the Matthews model was calibrated so that young adults of normal weight are close to 100%. Values well below that suggest lower secretion for the observed glucose, and values well above suggest higher secretion, but there is no published cutoff separating normal from abnormal for this formula.

Reading HOMA-B on its own is misleading: a “normal” value may be insufficient for someone with high insulin resistance, and a high value may simply be compensation. Wallace, Levy and Matthews (2004) classified measuring beta-cell function in isolation, without considering insulin sensitivity, as inappropriate use. Interpretation is up to the healthcare professional, with the clinical picture and the other tests.

Common errors, units and verification checklist

In HOMA-B, a glucose unit error has a greater effect than in HOMA-IR, because glucose enters the denominator with a subtraction.

• Glucose in mmol/L entered in the mg/dL field: any value below 63 blocks the calculation. Convert first (mmol/L × 18 ≈ mg/dL).
• Glucose slightly above 63 mg/dL: the denominator becomes small and the result shoots up. Small analytical variations change the number a lot.
• Insulin in pmol/L: needs conversion to µU/mL using the assay factor provided by the laboratory.
• Non-fasting collection or patient on insulin: the basal steady-state premise no longer holds.

Checklist before recording the result:

1. Are glucose and insulin from the same fasting sample?
2. Glucose in mg/dL, above 63, and insulin in µU/mL or mU/L?
3. Is the formula HOMA1 (360 and 63), and not HOMA2?
4. Was the HOMA-IR from the same sample calculated for joint reading?
5. If any cutoff is used in interpretation, are its source and population recorded?

Limitations of HOMA-B and comparison with other methods

HOMA-B has low individual precision. In the 1985 study, the coefficient of variation of the beta-cell deficit estimate was 32%, a value that limits its use for classifying a person from a single sample.

The estimate is less reliable in these situations:

• use of exogenous insulin or secretagogues that alter measured insulin;
• type 1 diabetes or very reduced secretion;
• glucose values close to 63 mg/dL, where the denominator amplifies errors;
• comparison between laboratories with different insulin assays, without standardization between manufacturers.

Excess weight changes insulin demand; the BMI calculator helps put the result in context.

Frequently asked questions about the HOMA-B index

References

1. Matthews DR, Hosker JP, Rudenski AS, Naylor BA, Treacher DF, Turner RC. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. 1985;28(7):412-9. View
2. Wallace TM, Levy JC, Matthews DR. Use and abuse of HOMA modeling. Diabetes Care. 2004;27(6):1487-95. View
3. Levy JC, Matthews DR, Hermans MP. Correct homeostasis model assessment (HOMA) evaluation uses the computer program. Diabetes Care. 1998;21(12):2191-2. View