Valuing Treatment With Infliximab for Ankylosing Spondylitis Using a Willingness-to-Pay Approach

Abstract

Objective: To investigate willingness to pay (WTP) for treatment with infliximab by patients with ankylosing spondylitis (AS) and explore factors associated with WTP.

Methods: Data from 85 patients participating in the European AS Infliximab Cohort (EASIC) open-label extension of the AS Study for the Evaluation of Recombinant Infliximab Therapy (ASSERT) were used. WTP was included at baseline in EASIC and comprised a hypothetical scenario exploring whether the patient would be willing to pay for beneficial effects of infliximab and, if so, what amount they would be willing to pay per administration. Factors associated with WTP were explored using zero-inflated negative binomial (ZINB) regressions.

Results: Of the 85 patients, 63 (74.1%) were willing to pay, and among these, the mean amount they were willing to pay per administration was €275 (median €100 [interquartile range €50-200]). Multivariable ZINB analysis showed that Assessment of SpondyloArthritis international Society criteria for 20% improvement (ASAS20) response was associated with a 7-fold lower likelihood to pay 0 euros (odds ratio [OR] 0.14 [95% confidence interval (95% CI) 0.03-0.71]) and a 3-fold increase in the amount willing to pay (exp(β) = 3.32 [95% CI 1.44-7.69]). In addition, the country of residence was associated with a lower likelihood to pay 0 euros (OR 0.07 [95% CI 0.02-0.36]), while increased age was associated with the amount willing to pay (exp(β) = 1.05 [95% CI 1.01-1.09]).

Conclusion: In a hypothetical scenario, three-quarters of patients with AS receiving long-term infliximab stated that they were willing to pay an out-of-pocket contribution for this treatment. Treatment response contributed to the willingness as well as to the amount patients were willing to pay.

Trial registration: ClinicalTrials.gov NCT00237419.

© 2017, The Authors. Arthritis Care & Research published by Wiley Periodicals, Inc. on behalf of American College of Rheumatology.

Figures

Figure 1

Predicted probabilities of being a certain zero (i.e., unwilling to pay) as well as the predicted amount patients are willing to pay for 1 administration of infliximab. A, Predicted probability not willing to pay for patients residing in The Netherlands; B, predicted amount willing to pay for patients residing in The Netherlands; C, predicted probability not willing to pay for patients residing in other participating countries; D, predicted amount willing to pay for patients residing in other participating countries. ASAS = Assessment of SpondyloArthritis international Society. Note that the amount patients are willing to pay (B and D) is predicted using the ASAS20 response zero‐inflated negative binomial regression model shown in Table 2, taking into account the predicted probability that a patient is certainly not willing to pay (being a “certain” zero). For example, for a male patient of 45 years not residing in The Netherlands and who is an ASAS responder, the model predicts a probability of willingness to pay for infliximab of 0.92. If he is willing to pay, the predicted amount that he is willing to pay for 1 administration is €343. Therefore, taking into account the probability of 0.08 (1.00 − 0.92 = 0.08) that he was not willing to pay anything, this patient would be willing to pay 0.92 × €343 + 0.08 × €0 = €316. If the same male patient was an ASAS nonresponder, our model predicts a lower probability of willingness to pay for infliximab (0.62), with a predicted amount willing to pay of €103, if he is willing to pay. Again, taking into account the probability of not willing to pay (1.00 − 0.62 = 0.38), the average willing‐to‐pay amount is €64 (0.62 × €103 + 0.38 × €0). Because ASAS responder status is associated with both parts of the model (willingness to pay as well as willing‐to‐pay amount), ASAS response in this patient leads to an almost 5‐fold increase in the willing‐to‐pay amount.