Background Comprehensive background check (CBC) policies are hypothesised to reduce firearm-related violence because they extend background checks to private party firearm sales, but no study has determined whether these policies actually increase background checks, an expected intermediary outcome. We evaluate the association between CBC policies and the rates of firearm background checks in three states that recently implemented these policies: Delaware (July 2013), Colorado (July 2013) and Washington (December 2014).

Methods We used the synthetic control group method to estimate the difference from estimated counterfactual postintervention trends in the monthly rate of background checks per 1 00 000 people for handguns, long guns and both types combined, using data for January 1999 through December 2016. Inference was based on results from permutation tests. We conducted multiple sensitivity analyses to assess the robustness of our results.

Results Background check rates increased in Delaware, by 22%–34% depending on the type of firearm, following enactment of its CBC law. No overall changes were observed in Washington and Colorado. Our results were robust to changes in the comparison group and statistical methods.

Conclusions The enactment of CBC policies was associated with an overall increase in firearm background checks only in Delaware. Data external to the study suggest that Washington experienced a modest, but consistent, increase in background checks for private party sales, and Colorado experienced a similar increase in checks for sales not at gun shows. Non-compliance may explain the lack of an overall increase in background checks in Washington and Colorado.