The standard quality-adjusted life year (QALY) model (SQM) assumes time–utility independence within constant health states and additive independence when health varies over time. The validity of SQM has been challenged through reported violations of these assumptions. An alternative approach that relaxes these assumptions is to assign a single valuation to an entire health profile: an integral assessment of disease severity over time. Here, we compare SQM with the annual profile model (APM) and test SQM for additive independence.
Eighty-two respondents valued 6 episodic conditions, including 4 of short duration, with SQM and APM, using the time trade-off method. Inter-rater reliability was assessed using intraclass correlation coefficients. Face validity was tested by asking respondents how well they were able to imagine the health states under SQM and APM. We calculated SQM QALY values for a 1-year time period, allowing for a direct comparison with APM values. For the short-term conditions we expected higher QALY values for SQM, violating additive independence.
APM showed higher interrater reliability (intraclass correlation coefficient of 0.53 vs 0.18, respectively) and better face validity than SQM, with 6% (APM) vs 21% (SQM) of all respondents reporting difficulties. Additive independence of SQM was violated in 5 of the 6 conditions (including the 4 short duration health states), with higher QALY values under SQM (mean difference 0.04).
The impact of short-term conditions is systematically underestimated under SQM when compared to a health profile model. APM is a less restrictive model and demonstrates better validity.
Mathieu F. Janssen Erwin Birnie Gouke J. Bonsel