The cost effectiveness of two new antiepileptic therapies in the absence of direct comparative data: a first approximation.

Background: A number of new antiepileptic agents have been introduced within a short period of time. Direct comparisons are not available, and information about the balance between costs and effects for these new therapies is lacking.

Objective: To introduce a first approximation of the cost effectiveness of the new therapeutic agents (topiramate and lamotrigine) for epilepsy that have been assessed in clinical trials against placebo.

Methods: Without head to head comparative data no formal methods are available to assess the relative cost effectiveness of two products; therefore, a Bayesian approach was developed. The approach starts with the 'proportionality assumption' saying that the differences in healthcare expenditure (less the direct cost of therapy) are directly proportional to the differences in effectiveness. Given this assumption, a therapy that is x times as expensive as an alternative therapy has an equivalent cost-effectiveness profile if the acquisition cost is x times as high. Moreover, simple formulas can be derived to calculate the probabilities that a therapy is dominant (more effective and less expensive) and that it is weakly dominant (more effective and a better cost-effectiveness profile). The approach is applied to data from published fixed dosage, parallel-design studies comparing both topiramate and lamotrigine with placebo.

Results: Assuming that the 'proportionality assumption' holds for the medical treatment of epilepsy, and disregarding uncertainties, it is estimated that topiramate may be priced more than 2.2 times its current acquisition cost and still be more cost effective than lamotrigine. Taking uncertainties into account, it is estimated that lamotrigine 500 mg/day is dominated by topiramate 200 mg/day with a probability of 0.875 and by topiramate 400 mg/day with a probability of 0.986.

Conclusions: A simple method can be applied to assess the relative cost effectiveness of two therapies in the absence of direct comparative data. Applying this method to compare topiramate and lamotrigine leads to a strong preference for topiramate. However, to be able to draw this conclusion, some heroic assumptions need to be made. As such the method as developed here only reflects a first approximation. It needs to be used with care and is not intended to replace good comparative research.