BACKGROUND: Timely follow-up of abnormal laboratory results is important for high-quality care. We sought to identify risk factors, facilitators, and barriers to timely follow-up of an abnormal estimated glomerular filtration rate (eGFR) for the diagnosis of chronic kidney disease. STUDY DESIGN: Mixed-methods study: retrospective electronic health record (EHR) analyses, physician interviews. SETTING & PARTICIPANTS: Large integrated health care delivery system. Quantitative analyses included 244,540 patients 21 years or older with incident abnormal eGFRs from January 1, 2010, to December 31, 2015, ordered by 7,164 providers. Qualitative analyses included 15 physician interviews. EXPOSURES: Patient-, physician-, and system-level factors. OUTCOME: Timely follow-up of incident abnormal eGFRs, defined as repeat eGFR obtained within 60 to 150 days, follow-up testing before 60 days that indicated normal kidney function, or diagnosis before 60 days of chronic kidney disease or kidney cancer. ANALYTICAL APPROACH: Multivariable robust Poisson regression models accounting for clustering within provider were used to estimate risk ratios (RRs) and 95% CIs for lack of timely follow-up. Team coding was used to identify themes from physician interviews. RESULTS: 58% of patients lacked timely follow-up of their incident abnormal eGFRs (ie, had a care gap). An abnormal creatinine result flag in the EHR was associated with better follow-up (RR for care gap, 0.65; 95% CI, 0.64-0.66). Patient online portal use and physician panel size were weakly associated with follow-up. Patients seen by providers behind on managing their EHR message box were at higher risk for care gaps. Physician interviews identified system-level (eg, panel size and assistance in managing laboratory results) and provider-level (eg, proficiency using EHR tools) factors that influence laboratory result management. LIMITATIONS: Unable to capture intentional delays in follow-up testing. CONCLUSIONS: Timely follow-up of abnormal results remains challenging in an EHR-based integrated health care delivery system. Strategies improving provider EHR message box management and leveraging health information technology (eg, flagging abnormal eGFR results), making organizational/staffing changes (eg, increasing the role of nurses in managing laboratory results), and boosting patient engagement through better patient portals may improve test follow-up.