Have we been underestimating modifiable dementia risk? An alternative approach for calculating the combined population attributable fraction (PAF) for modifiable dementia risk factors.

Estimating the fraction of dementia cases in a population attributable to a risk factor or combination of risk factors (population attributable fraction, or PAF) informs the design and choice of dementia risk-reduction activities. It is directly relevant to dementia prevention policy and practice. Current methods employed widely in the dementia literature to combine PAFs for multiple dementia risk factors assume a multiplicative relationship between factors and rely on subjective criteria to develop weightings for risk factors. This paper presents an alternative approach to calculating the PAF based on sums of individual risk. It incorporates individual risk factor inter-relationships and enables a range of assumptions about the way in which multiple risk factors will combine to impact dementia. Applying this method to global data demonstrates that the previous estimate of 40% is potentially too conservative an estimate of modifiable dementia risk and would necessitate sub-additive interaction between risk factors. We calculate a plausible conservative estimate of 55.7% (95%CI: 55.2-56.1) based on additive risk factor interaction.