OBJECTIVES: The Recurrent Genital Herpes Quality of Life questionnaire (RGHQoL) is a needs-based quality of life (QoL) instrument specific to recurrent genital herpes (RGH), with excellent psychometric properties. However, it cannot be used to directly derive utility. Existing utility instruments cover symptoms/functioning, rather than QoL, raising concerns over their relevance for RGH and ability to show differences between interventions. The project investigated the feasibility of deriving relative and absolute utility values from the RGHQoL.
METHODS: A discrete choice conjoint analysis (CA) task (for eliciting relative utility) and time trade off (TTO) exercise (absolute utility) were designed using QoL states generated from a subset of six RGHQoL items. RGH patients completed tasks via interview. CA data were analysed using the random effects probit model. For TTO, a random effects tobit regression was employed to estimate absolute utility values for combinations of attribute levels not presented in the valuation exercise.
RESULTS: One hundred and ninety-two interviews were conducted (79 male; 113 female). For CA, all attributes were statistically significantly influential in determining preferences. An equation containing preference weights to be applied to the six RGHQoL items was derived. Inserting values for relevant levels of each attribute yields an index of relative utility based on QoL outcomes. Scores generated are ordinal. A second equation was derived for calculating absolute utilities using TTO, allowing absolute utility values to be calculated for each individual. As the TTO technique possesses cardinal properties, it is possible to derive QALY scores.
CONCLUSIONS: It is feasible to generate both relative and absolute utility values for responses to the RGHQoL questionnaire, allowing utility to be based on true QoL. The formulae derived can be applied to both existing RGHQoL data and those derived from future studies.
