Willingness to pay and time trade-off: sensitive to changes of quality of life in psoriasis patients?

Abstract

Willingness to pay (WTP) and time trade-off (TTO) have been used successfully as quality of life (QOL) measurements in dermatology. However, until now there have been no studies available individually comparing these measures pre- and post-treatment. To check sensitivity to changes for WTP and TTO (i). pre- to post-treatment, and (ii). to a 6-month follow-up period. We performed a prospective multicentre study in outpatients with psoriasis vulgaris treated with synchronous balneo-phototherapy (simultaneous application of narrowband ultraviolet B and bathing in 10% Dead Sea salt solution). Besides WTP and TTO, the Psoriasis Disability Index (PDI) and the Psoriasis Area and Severity Index (PASI) were monitored. One hundred and ninety-four patients participated in the pretreatment survey, of whom 138 (71%; 84 men, 54 women; mean age 43.9 years) also returned the post-treatment questionnaire. WTP (percentage of monthly income) was shown to be independent of patients' income. During treatment, mean +/- SD WTP fell from 13.8 +/- 19.2% to 11.5 +/- 15.9% (relative improvement 16.7%; P < 0.05), TTO (h per day) from 2.7 +/- 3.8 to 2.3 +/- 3.6 (relative improvement 15%; P < 0.001), PDI improved from 29.8 +/- 18.6 to 23.5 +/- 18.9 (relative improvement 21.1%; P < 0.001) and PASI (available for 113 patients) from 14.9 +/- 7.7 to 5.6 +/- 5.0 (relative improvement 62.4%; P < 0.001). Changes in WTP, PDI and PASI were statistically significantly correlated. Ninety-one of 138 patients (66%) also completed a third survey after a follow-up period: no further changes in PDI, WTP and TTO were found, indicating a stable QOL post-treatment. Correlation analysis indicated that WTP, assessed as percentage of monthly income, seems to be an appropriate way to measure QOL, unbiased by income of patients. WTP, TTO and PDI were correlated and were sensitive to changes during treatment. WTP and TTO therefore also seem to be appropriate tools for assessment of QOL in interventional studies, especially for pharmacoeconomic analyses.