Fig. 1

Directed acyclic graph depicting the assumed relationship among leukocyte mitochondrial DNA copy number, serum high-sensitive C-reactive protein, risk of death, and the potential confounders. Panel A displayed the relationships among leukocyte mitochondrial DNA copy number (exposure, represented by the green circle), serum high-sensitive C-reactive protein (outcome, represented by the blue circle), and covariates associated with both the leukocyte mitochondrial DNA copy number and serum high-sensitive C-reactive protein (ancestors of both the exposure and outcome, represented by the red circles). Panel B displayed the relationships among serum high-sensitive C-reactive protein (exposure, represented by the green circle), risk of death (outcome, represented by the blue circle), and the covariates associated with both serum high-sensitive C-reactive protein and risk of death (ancestors of both the exposure and outcome, represented by the red circles). The arrow from variable X to variable Y represents a direct effect of X on Y (an effect not mediated by other observed variables in the graph) that may have been present. The hypothesized causal pathway is presented in green. The biasing pathway is presented in red [34]