Signature
INMB = lambda * Delta_E - Delta_C
| Inputs | Definition | Unit |
|---|---|---|
lambda | Monetary value placed on one unit of health effect | currency per unit of health effect, for example £ per QALY |
Delta_E | Expected health effect of the intervention minus that of the comparator | health-outcome unit per defined population or person |
Delta_C | Expected cost of the intervention minus that of the comparator | currency per defined population or person |
INMB | Net monetary benefit of the intervention minus that of the comparator at threshold lambda | currency per defined population or person |
|---|
Function
Net monetary benefit function
Values an option's expected health effect in money at a stated cost-effectiveness threshold and subtracts its expected cost, placing health and cost on one monetary scale.
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Implementations
Excel
Calculate incremental NMB
Excel values the incremental effect at the threshold and subtracts the incremental cost.
=Threshold*IncrementalEffect-IncrementalCost
Assumptions
Consistent subtraction direction for INMB
Delta_E and Delta_C both take the intervention minus the comparator, so INMB equals the intervention's NMB minus the comparator's NMB.
Equivalence with the ICER rule
When Delta_E is greater than zero, INMB is positive exactly when the ICER is below lambda. When Delta_E is negative the ICER rule reverses while the INMB rule does not, and when Delta_E equals zero only INMB is defined.
Worked examples
Positive INMB at £20,000 per QALY
Incremental cost is £4,000 and incremental effect is 0.30 QALYs. At £20,000 per QALY the INMB is £2,000, the difference between the option-level NMBs of £82,000 and £80,000. The ICER of about £13,333 per QALY is below £20,000, so both rules favour the intervention. The figures are illustrative.
lambda = 20000; Delta_E = 0.30; Delta_C = 4000; INMB = 2000
Negative INMB at a lower threshold
At an illustrative threshold of £10,000 per QALY the same comparison gives an INMB of minus £1,000, so current care has the higher NMB. The INMB changes sign at about £13,333 per QALY, the ICER.
lambda = 10000; Delta_E = 0.30; Delta_C = 4000; INMB = -1000
Common errors
Applying the ICER rule when Delta_E is negative
In the south-west quadrant an ICER below lambda means that the savings per QALY forgone are too small, so reading it as favourable reverses the conclusion that INMB gives directly.
Reading a positive INMB as affordability
INMB compares value at a threshold and says nothing about budget impact or the final adoption decision.
Sources
Net benefit expressed in money
Tambour M, Zethraeus N, Johannesson M. A note on confidence intervals in cost-effectiveness analysis. International Journal of Technology Assessment in Health Care. 1998;14(3):467-471.
Net health benefit framework
Stinnett AA, Mullahy J. Net health benefits: a new framework for the analysis of uncertainty in cost-effectiveness analysis. Medical Decision Making. 1998;18(2 Suppl):S68-S80.
Canonical Identity
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