Quantifying risk of injury from usual alcohol consumption: An instrumental variable analysis

Numerous studies of roadside accidents among emergency room patients show elevated risk of injury from acute alcohol consumption, i.e., recent drinking precedes the injury event. The observed effects are large and show a dose-response relationship. In contrast, studies quantifying the association between injury risk and chronic consumption, such as past-year average volume, show lower relative risk estimates than those from acute consumption.

Combining data from 4 waves of US National Alcohol Surveys (NAS) for years 2000-2015 (N = 29,571, 53% overall cooperation rate), we estimated the risk of any past-year injury from past-year volume using logistic regression. This was contrasted with an instrumental variable (IV) analysis utilizing a 2-stage residual inclusion (2SRI) approach to estimate injury risk from volume, which adjusted for unobserved confounders using state beer and spirits tax rates, zip code-level outlet and bar density, and control state status as instruments.

Based on the combined US population surveys and controlling for sociodemographics, using conventional logistic regression, the odds ratios of injury from an average volume of 1, 2, and 5 drinks per day were 1.12 [95% confidence interval: 1.02, 1.24], 1.10 [1.00, 1.22], and 1.04 [0.88, 1.22], respectively. These compared with 1.67 [1.00, 2.78], 2.38 [0.87, 6.54], and 6.98 [0.57, 85.89] using the IV method. The proportion of injury attributed to alcohol also increased in magnitude, from 6.2% [0.3%, 11.9%] using the conventional approach to 17.9% [8.2%, 27.7%] using the IV method.

The association between injury and chronic alcohol consumption may be confounded by unobserved factors, resulting in a possible downward bias of the risk estimate.