Incremental Cost Effectiveness Ratio (ICER)

The incremental cost effectiveness ratio (ICER) is the ratio of the difference in costs to the difference in health outcomes between two interventions, typically expressed as the additional cost per quality adjusted life year (QALY) gained. It is the primary metric used in health economic evaluations to determine whether a new intervention provides sufficient value relative to its price.

Health systems operate under budget constraints, and the ICER provides a structured way to compare the value of competing investments in health. Many HTA agencies use the ICER as a central decision criterion when recommending whether new treatments should be funded. Pharmaceutical companies design their pricing strategies around projected ICERs, and clinical guideline committees use ICERs to rank treatment options by value. For health economics and outcomes research (HEOR) professionals, calculating, presenting, and interpreting ICERs is a core competency.

The formula for the ICER is: (Cost of new intervention minus Cost of comparator) divided by (Effect of new intervention minus Effect of comparator). Costs include all relevant direct medical costs such as drug acquisition, administration, monitoring, hospitalization, and management of adverse events, and may also include indirect costs such as productivity losses depending on the analytic perspective. Effects are most commonly measured in QALYs, which combine survival duration with health related quality of life.

The ICER can be visualized on the cost effectiveness plane, where the horizontal axis represents incremental effectiveness and the vertical axis represents incremental cost. The plane is divided into four quadrants. An intervention that is more effective and more costly falls in the northeast quadrant, where the ICER determines whether the additional cost is justified. An intervention that is more effective and less costly (southeast quadrant) is dominant and should be adopted. An intervention that is less effective and more costly (northwest quadrant) is dominated and should be rejected. The southwest quadrant represents an intervention that is less effective but also less costly.

Because model inputs carry uncertainty, analysts conduct probabilistic sensitivity analysis (PSA). PSA draws thousands of random samples from the distributions of all input parameters and recalculates the incremental cost and incremental effect for each sample, producing a scatter plot of these pairs on the cost effectiveness plane. The cost effectiveness acceptability curve (CEAC) summarizes these results by showing the probability of cost effectiveness across a range of willingness to pay thresholds.

The ICER is the appropriate metric whenever you are comparing a new health intervention against an established comparator and wish to express the tradeoff between additional costs and additional health gains. It is expected in reimbursement submissions to HTA bodies such as NICE in England, Canada's Drug Agency (formerly CADTH) and PBAC in Australia. It is less appropriate when the comparison involves interventions with incomparable outcome measures, when a budget impact analysis is more relevant than an efficiency analysis, or when the new intervention dominates or is dominated by the comparator (in which case the ICER is either unnecessary or meaningless).

One frequent error is reporting a deterministic ICER without probabilistic sensitivity analysis, which understates the uncertainty around the estimate. Another mistake is calculating an ICER when the new intervention is dominated, which produces a negative ratio that is difficult to interpret. Analysts sometimes also fail to discount both costs and outcomes to present value, which biases the ICER when the time horizon extends beyond one year. Comparing ICERs calculated using different perspectives (societal versus payer) or different discount rates without adjustment can also lead to misleading conclusions.

An economic model compares a novel gene therapy costing $250,000 as a one time treatment against lifelong enzyme replacement therapy costing $150,000 per year for a rare metabolic disorder. Over a lifetime horizon, the gene therapy produces 12.5 QALYs compared to 10.8 QALYs with enzyme replacement, at total discounted costs of $280,000 and $2,400,000 respectively. The ICER is ($280,000 minus $2,400,000) divided by (12.5 minus 10.8), yielding negative $1,247,059 per QALY. Because the gene therapy is both less costly and more effective, it dominates enzyme replacement therapy, and the ICER is not needed; the gene therapy should be adopted.