The Cost-Effectiveness of Metformin in Diabetes Prevention in Singapore
Revisited, Revised and Reversed
Abstract
The Diabetes Prevention Program trial compared metformin, lifestyle
modification and placebo (the latter, as with metformin, including basic
lifestyle advice). Both metformin and lifestyle alternatives reduced
diabetes versus placebo. A published cost-effectiveness analysis (CEA)
of the DPP as if it were implemented in Singapore concluded that both
metformin and lifestyle were cost-effective. The original work
miscalculated the key metric in economic evaluations – the Incremental
Cost-Effectiveness Ratio (ICER) - by comparing metformin and lifestyle
each to a common alternative (here, placebo), a violation of
long-established economic methods. We revisited that analysis, using the
identical data, but with appropriate methods, to calculate appropriate
ICERs. We showed that with correctly calculated ICERs, metformin was not
even technically efficient and thereby incapable of being the
more-restrictive economically efficient (cost-effective) treatment.
These data show that only lifestyle was cost-effective. We also expanded
the analysis, using three additional methods (Incremental
Cost-Effectiveness Plane, Net Monetary Benefits and Net Loss Curves).
These methods confirm the results from the correct ICERs. Had any of
these been used in the original CEA, errors may have been identified by
the authors. Importantly, our Net Loss calculations also show that there
may be significant health and cost consequences to patients in a system
that may have implemented policy based on the error that incorrectly
implied metformin’s cost-effectiveness. Policymakers may easily be
misled by peer-reviewed published economic evaluations that fail to
follow appropriate economic methods and lead them to implement policies
that are harmful and costly relative to using cost-effective treatments.