Abstract
BackgroundThe Time Trade-Off (TTO) is a widely used instrument for valuing preference-based health-related quality of life (HRQoL). The TTO reveals preferences for own current health (‘utilities’) on a scale anchored between death (0) and perfect health (1). Limited information on the external validity of the TTO is available. Aim of this pilot study was to examine the construct validity of both an interview TTO and a computer-based TTO in patients with rheumatoid arthritis (RA).MethodsThirty patients visiting the outpatient rheumatology clinic participated. Construct validity was assessed by measuring convergent and discriminative validity. Convergent validity was assessed by calculating Spearman’s correlations between the utilities obtained from the TTOs and pain, general health (rating scales), health-related quality of life (SF-36 and SF-6D) and functional status (HAQ-DI). Discriminative power of both TTO measures was determined by comparing median utilities between worse and better health outcomes.ResultsCorrelations of both TTO measures with HRQoL, general health, pain and functional status were poor (absolute values ranging from .05 to .26). Both TTOs appeared to have no discriminative value among groups of RA patients who had a worse or better health status defined by six health outcome measures. About one-third of respondents were zero-traders on each of the TTO measures. After excluding zero-traders from analysis, the correlations improved considerably.ConclusionsBoth the interview TTO and computer TTO showed poor construct validity in RA patients when using measures of HRQol, general health, pain and functional status as reference measures. Possibly, the validity of the TTO improves when using an anchor that is more realistic to RA patients than the anchor ‘death’.
Highlights
The Time Trade-Off (TTO) is a widely used instrument for valuing preference-based health-related quality of life (HRQoL)
We report on preliminary results with respect to the construct validity of the TTO assessed in patients with rheumatoid arthritis (RA) using an interview TTO as well as a computer TTO, and using a standardised procedure for both TTOs
General health, health-related quality of life and functional status were administered at the first TTO assessment
Summary
The Time Trade-Off (TTO) is a widely used instrument for valuing preference-based health-related quality of life (HRQoL). The TTO reveals preferences for own current health (‘utilities’) on a scale anchored between death (0) and perfect health (1). The Time Trade-Off (TTO) [1] is an instrument developed to assess effects of treatments in cost-utility analyses (CUAs) by measuring changes in health-related quality of life (HRQoL) directly by patients. The TTO reveals preferences for own current health (‘utilities’) on a scale anchored between death (0) and perfect health (1) by asking people how many life years they are willing to give up to become perfectly healthy. The mode of administration (interview/ computer/questionnaire) or the way the TTO question is formulated can influence utilities. The size of time frame that is used (e.g. fixed time period, life expectancy) has a great impact, since utilities are calculated as the proportion of the remaining lifetime sacrificed [2]
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