OBJECTIVES: Cost-effectiveness analyses require the measurement of health-related quality of life (HRQoL) to estimate quality–adjusted life years (QALYs). Challenges with measuring HRQoL arise in the context of episodic conditions such as epilepsy, because symptoms may not have occurred within the recall period of the HRQoL instrument. The aim of this paper is to explore whether HRQoL measured at regular time intervals correctly reflects the HRQoL of patients with episodic conditions, using the example of refractory epilepsy.

METHODS: Follow-up data from the first year of the EPISODE-study on the (cost-)effectiveness of seizure dogs were used. In the stepped-wedge randomised controlled trial HRQoL is collected in 23 respondents at month 3, 6 and 9 with the EQ-5D and seizure frequency is recorded continuously using a seizure diary. Descriptive statistics, Student’s t-tests and graphical displays were used to compare the average seizure count on the day of completion of the EQ-5D with the average seizure counts over the preceding 7, 14 and 28 days.

RESULTS: The average seizure count on the day of EQ-5D completion was lower than the average seizure count for the preceding period (2.88±5.10 vs. respectively 3.76±5.62, 3.81±5.69 and 3.75±5.39, p < 0.01). The same pattern was seen at the individual measurements. Graphical displays confirm the hypothesis that patients appear to complete the questionnaire on a day that seizure count is relatively low.

CONCLUSIONS: When HRQoL is measured at regular time intervals, people with an episodic condition such as epilepsy appear to complete these measurements on relatively good days. Consequently, the HRQoL is potentially overestimated in such populations, especially for bad days. When treatment is targeted at reducing the frequency or the intensity of such bad days, measuring HRQoL at regular time intervals might lead to underestimation of the effectiveness of treatment, and therefore to biased estimates of cost-effectiveness.