Economic analysis and assessment of net clinical benefit often requires
estimation of absolute risk difference (ARD) for binary outcomes (e.g. survival,
response, disease progression) given baseline epidemiological risk in a
jurisdiction of interest and trial evidence of treatment effects. Typically, the
assumption is made that relative treatment effects are constant across baseline
risk, in which case relative risk (RR) or odds ratios (OR) could be applied
to estimate ARD. The objective of this article is to establish whether such use
of RR or OR allows consistent estimates of ARD.
ARD is calculated from alternative framing of effects (e.g. mortality vs
survival) applying standard methods for translating evidence with RR and
OR. For RR, the RR is applied to baseline risk in the jurisdiction to estimate
treatment risk; for OR, the baseline risk is converted to odds, the OR applied
and the resulting treatment odds converted back to risk.
ARD is shown to be consistently estimated with OR but changes with
framing of effects using RR wherever there is a treatment effect and epidemiological
risk differs from trial risk. Additionally, in indirect comparisons,
ARD is shown to be consistently estimated with OR, while calculation with
RR allows inconsistency, with alternative framing of effects in the direction,
let alone the extent, of ARD.
OR ensures consistent calculation of ARD in translating evidence from
trial settings and across trials in direct and indirect comparisons, avoiding
inconsistencies from RR with alternative outcome framing and associated
biases. These findings are critical for consistently translating evidence to inform
economic analysis and assessment of net clinical benefit, as translation
of evidence is proposed precisely where the advantages of OR over RR arise.