Understanding disutilities in cost-effectiveness analysis
Why the size and duration of a quality-of-life loss both matter when calculating QALYs.
Updated
A disutility is a decrement in health-state utility. It can represent a loss in health-related quality of life associated with a symptom, adverse event, treatment burden or diagnostic procedure.
In a model, the decrement may be temporary or sustained. Its effect on QALYs depends on both its size and its duration.
An illustrative example
Suppose a patient has a baseline utility of 0.7. An adverse event reduces utility by 0.1 for three months, then utility returns to baseline.
During the event, utility is 0.6. With a three-month duration represented as 0.25 years, the QALY loss is:
0.1 × 0.25 = 0.025 QALYs
This is an illustrative calculation before discounting. The same decrement lasting a full year would produce a loss of 0.1 QALYs.
Model the event consistently
Specify whether the loss applies to everyone or only to patients experiencing the event. A per-event decrement and a population-average decrement answer different questions.
Check whether baseline utilities already include the symptoms or adverse events being modelled. Adding a separate decrement for the same burden can double count the impact. Overlapping events also need an explicit approach rather than automatic subtraction.
Disutilities are not necessarily small, and utility values are not always restricted to positive numbers. The instrument and valuation method determine the relevant scale.
What to document
Record the source, event probability, utility decrement, duration and combination rule. Explore uncertainty where evidence is limited. That makes the quality-of-life assumptions easier to understand and challenge.