Why a Cost-Effectiveness Threshold Is Really an Opportunity-Cost Question

A cost-per-QALY threshold is often read as the price a health system is willing to pay for a year of good-quality life. That reading is incomplete. In a system with a fixed budget, the threshold is better understood as a measure of the health that gets displaced somewhere else when a payer funds something new. Every pound committed to a new therapy is a pound that was already doing work elsewhere in the system, and the benefit given up is the opportunity cost. Seen this way, a threshold is not a valuation of health so much as an estimate of what the next-best use of the same money would have produced.

This article explains the economic logic behind that idea, how a range like the one used by the National Institute for Health and Care Excellence is applied, and why the empirically estimated value of the threshold remains genuinely contested. It is educational and not medical advice.

The fixed-budget starting point

The whole argument rests on one assumption: the budget is fixed, at least in the short and medium term. A health system with a set annual allocation cannot fund a new intervention by conjuring extra money. It funds the new thing by spending money that would otherwise have gone to something else. That something else was also producing health, whether through a different drug, a diagnostic service, a nursing role, or a community program.

The quality-adjusted life year, or QALY, gives a common unit for that health. One QALY is roughly a year of life in full health, with years in worse health counted as fractions. Because the QALY lets very different interventions be compared on one scale, it also lets you ask a sharper question than whether a treatment is worth it. The sharper question is whether the treatment produces more health per pound than the care it will displace.

What the incremental cost-effectiveness ratio measures

The standard tool for this comparison is the incremental cost-effectiveness ratio, or ICER. The NICE manual defines it as the ratio of the expected additional cost of a technology to its expected additional QALYs, compared with the relevant alternative. If a new drug costs an extra 200,000 pounds and yields an extra 10 QALYs against current care, its ICER is 20,000 pounds per QALY.

That number, on its own, means nothing until it is set against a benchmark. The benchmark is where opportunity cost enters. Suppose the care being displaced elsewhere in the system produces one QALY for every 15,000 pounds spent. Funding the new drug at 20,000 pounds per QALY then buys less health per pound than the care it pushes out. The system gains 10 QALYs from the new drug but loses more from whatever those pounds were previously buying. Net, the population is worse off in health terms, even though the new drug clearly helps the patients who receive it. That counterintuitive result is the entire reason the threshold matters.

How the range is applied

NICE has long described technologies costing between 20,000 and 30,000 pounds per QALY gained as generally representing an acceptable use of NHS resources. The figure is a range, not a single line, and the manual describes it as a set of maximum acceptable ratios applied with judgment rather than as an automatic cutoff.

Toward the lower end of the range, an intervention is usually considered good value without much further argument. As the ratio rises within the range, the manual states that the influence of other factors on the decision grows, and the appraisal committee weighs additional considerations. The manual also allows structured modifiers, for example greater weight for treatments addressing more severe conditions, which can effectively raise the acceptable ratio in defined circumstances. The core discipline stays the same throughout: a higher ICER implies more health displaced elsewhere, so a higher ICER calls for a stronger reason to accept that displacement.

Why the true threshold is debated

Here the logic runs into a hard empirical problem. If the threshold is supposed to reflect the health actually displaced when the budget is stretched, then the correct value is not a policy choice at all. It is a fact about the system, one that has to be measured: how much does the health system currently spend, at the margin, to generate one QALY?

Claxton and colleagues took on exactly this measurement in a 2015 study for the Health Technology Assessment programme. Using variation in NHS spending and health outcomes across programmes of care, they estimated the marginal productivity of NHS spending and derived a central threshold of roughly 13,000 pounds per QALY, below NICE's stated 20,000 to 30,000 pound range. The implication drew wide attention: if that lower figure is right, approving technologies at the top of the range would remove more health from other NHS patients than it adds.

That work did not settle the question. A later editorial reviewing the field described opportunity cost in this setting as contentious and ambiguous across theoretical, empirical, and public-opinion dimensions. It noted analysis by Zamora and Towse arguing that once structural uncertainty in the underlying models is taken seriously, the claim that the true threshold sits far below NICE's range is weakened, and that plausible modelling assumptions can be consistent with the 20,000 to 30,000 pound range being a legitimate estimate. The disagreement is not really about the economic logic, which both sides accept. It is about how confidently the marginal productivity of a whole health system can be measured from observational data, and how much weight the resulting numbers should carry.

Why the framing matters

None of this points to a particular number, a particular country, or a particular policy stance, and it is not meant to. The value of the opportunity-cost framing is that it reframes a familiar objection. When a threshold blocks a beneficial treatment, the intuitive complaint is that the system is putting a price on life. The framing says something different and more uncomfortable. In a fixed budget, the choice is never between a treatment and nothing. It is between one group of identifiable patients and another, often invisible, group who lose the care that the money would otherwise have funded. A threshold is the system's attempt to make that second group visible. Whether any specific threshold succeeds is exactly what the evidence, and the debate around it, is still working out.