Have We Been Underestimating Modifiable Dementia Risk? An Alternative Approach for Calculating the Combined Population Attributable Fraction for Modifiable Dementia Risk Factors

Abstract

Estimating the fraction of dementia cases in a population attributable to a risk factor or combination of risk factors (the population attributable fraction (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. In this paper we present an alternative approach to calculating the PAF based on sums of individual risk. It incorporates individual risk factor interrelationships and enables a range of assumptions about the way in which multiple risk factors will combine to affect dementia risk. 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 subadditive interaction between risk factors. We calculate a plausible conservative estimate of 55.7% (95% confidence interval: 55.2, 56.1) based on additive risk factor interaction.

Keywords: dementia; modifiable risk; population attributable fraction; risk factors.

Figure 1

Comparison between multiplicative approaches and additive approaches to estimating the combined population attributable fraction (PAF) for multiple modifiable dementia risk factors. The weighted multiplicative model (reported by Livingston et al. (4)) and the unweighted multiplicative model (as per Barnes and Yaffe (1)) calculate a combined total PAF and then partition the total PAF into risk factor contributions based on the proportional split for each risk factor: individual PAF/sum of individual PAFs. The additive model (equation 11) calculates risk factor contributions directly by taking the attributable risk associated with a risk factor and dividing by the overall risk in the population: . All risk factors are defined as per Livingston et al. (4).