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BackgroundSelection bias and non-participation bias are major methodological concerns which impact external validity. Cluster-randomized controlled trials are especially prone to selection bias as it is impractical to blind clusters to their allocation into intervention or control. This study assessed the impact of selection bias in a large cluster-randomized controlled trial.MethodsThe Improved Cardiovascular Risk Reduction to Enhance Rural Primary Care (ICARE) study examined the impact of a remote pharmacist-led intervention in twelve medical offices. To assess eligibility, a standardized form containing patient demographics and medical information was completed for each screened patient. Eligible patients were approached by the study coordinator for recruitment. Both the study coordinator and the patient were aware of the site’s allocation prior to consent. Patients who consented or declined to participate were compared across control and intervention arms for differing characteristics. Statistical significance was determined using a two-tailed, equal variance t-test and a chi-square test with adjusted Bonferroni p-values. Results were adjusted for random cluster variation.ResultsThere were 2749 completed screening forms returned to research staff with 461 subjects who had either consented or declined participation. Patients with poorly controlled diabetes were found to be significantly more likely to decline participation in intervention sites compared to those in control sites. A higher mean diastolic blood pressure was seen in patients with uncontrolled hypertension who declined in the control sites compared to those who declined in the intervention sites. However, these findings were no longer significant after adjustment for random variation among the sites. After this adjustment, females were now found to be significantly more likely to consent than males (odds ratio = 1.41; 95% confidence interval = 1.03, 1.92).ConclusionsThough there appeared to be a higher consent rate for females than for males, the overall impact of potential selection bias and refusal to participate was minimal. Without rigorous methodology, selection bias may be a threat to external validity in cluster-randomized trials.Trial registration NCT01983813. Date of registration: Oct. 28, 2013.Electronic supplementary materialThe online version of this article (doi:10.1186/s12874-017-0368-7) contains supplementary material, which is available to authorized users.

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BACKGROUND: Concurrent use of opioids and benzodiazepines (COB) can lead to additive respiratory and central nervous system effects, putting patients at increased risk of fatal overdose. In 2016, the Centers for Disease Control and Prevention released an opioid-prescribing guideline recommending against COB, and the Pharmacy Quality Alliance (PQA) endorsed a COB measure in its core opioid set. From May 1, 2017, to December 4, 2017, a California Medicaid plan launched a COB-focused prescriber outreach intervention for members receiving recent opioid and benzodiazepine claims with the intent of decreasing concurrent use.