The biochemical tests will be conducted two months after study enrollment. Research staff will contact patients via phone and/or email to schedule visits to complete these tests. Investigators will ask all control group members to take the test and compensate them for doing so, regardless of test result ("measurement incentive"). Only treatment group members who self-report not smoking during the last week, or from whom investigators were unable to procure a self-report, will be asked to take the test. This group will not receive a measurement incentive.