Estimation of a Multiattribute Utility Function for the Spanish Version of the TooL Questionnaire

Abstract

Objectives To estimate and assess the psychometric properties of a multiattribute utility function (MAUF) for the Spanish version of the Tolerability and Quality of Life (TooL questionnaire). Methods Balanced data on 243 patients diagnosed with schizophrenia or bipolar disorder were gathered. In addition to the demographic and clinical variables and the usual generic health-related quality of life (HRQoL) questionnaires (EuroQol-5D [EQ-5D] and Short Form-6D [SF-6D]), instruments considered included the Spanish versions of the Positive and Negative Symptoms of Schizophrenia Scale (PANSS), Young Mania Rating Scale (YMRS), Montgomery-Asberg Depression Rating Scale (MADRS), Udvalg for Kliniske Undersogelser (UKU), and Clinical Global Impression Severity (CGIS) scale. MAUF parameters estimation involved a number of visual analogue scale (VAS) and time trade-off (TTO) ratings that proved difficult to be performed by the patients. After checking for inconsistencies in patient responses, the original sample was reduced to a still balanced subsample of 70 individuals. A multiplicative-form MAUF was estimated following the standard methodology. Results Good convergent validity was demonstrated because utility estimates from the MAUF presented strong correlations with utilities from the generic HRQoL instruments included: SF-6D (0.66, P < 0.01), EQ-5D (0.69, P < 0.01), and moderate correlations with the rest of instruments considered: PANSS (−0.27, P = 0.10), YMRS (−0.30, P = 0.08), MADRS (−0.48, P < 0.01), UKU (−0.35, P < 0.01). Criterion validity was also met because differences in mean utilities by clinical severity were found ( P < 0.01). Utilities from the MAUF covered a wider range of health states [0.04,1.00] than those from the SF-6D [0.53,1.00] and EQ-5D [0.23,0.96]. Conclusions Utilities from the MAUF showed good psychometric properties, serving as a complement to generic health utilities. If misapplied, however, utilities from this instrument might favor the positive evaluation of drugs showing fewer associated side effects.