Background: Cohort studies are among the most robust of observational studies but have issues with external validity. This study assesses threats to external validity (generalisability) in the European QUALity (EQUAL) study, a cohort study of people over 65 years with stage 4/5 chronic kidney disease. Methods: Patients meeting the EQUAL inclusion criteria were identified in The Health Improvement Network database and stratified into those attending renal units (secondary care cohort-SCC) and not (primary care cohort-PCC). Survival, progression to renal replacement therapy (RRT), and hospitalisation were compared. Results: The analysis included 250, 633, and 2,464 patients in EQUAL, PCC, and SCC. EQUAL had a higher proportion of men in comparison to PCC and SCC (60.0% vs. 34.8% vs. 51.4%). Increasing age (≥85 years odds ratio (OR) 0.25 (95% confidence interval (CI) 0.15-0.40)) and comorbidity (Charlson Comorbidity Index ≥ 4 OR 0.69 (CI 0.52-0.91)) were associated with non-participation in EQUAL. EQUAL had a higher proportion of patients starting RRT at 1 year compared to SCC (8.1% vs. 2.1%%, p <0.001). Patients in the PCC and SCC had increased risk of Hospitalisation (incidence rate ratio=1.76 (95% CI 1.27-2.47) & 2.13 (95% CI 1.59-2.86)) and mortality at one year (hazard ratio=3.48 (95% CI 2.1-5.7) & 1.7 (95% CI 1.1-2.7)) compared to EQUAL. Conclusions: This study provides evidence of how participants in a cohort study can differ from the broader population of patients, which is essential when considering external validity and applying to local practice.