BACKGROUND: Uncoded chronic kidney disease (CKD) is associated with poorer quality of care. AIM: To ascertain the proportion and determinants of CKD, which have not been formally recorded (Read coded), and identify differences in management and quality-of-care measures for patients with coded and uncoded CKD. DESIGN AND SETTING: Cross-sectional survey undertaken in an ethnically diverse adult population using primary care electronic health records (EHRs) from GP clinics in Lambeth, South London, UK. METHOD: Multivariable logistic regression analysis examined the association of demographic factors, selected comorbidities, deprivation, and cardiovascular disease risk management in CKD, with coding status as outcome. RESULTS: In total, the survey involved 286 162 adults, of whom 9325 (3.3%) were identified with CKD stage 3-5 (assigned as CKD based on estimated glomerular filtration rate [eGFR] values). Of those identified with CKD, 4239 (45.5%) were Read coded, and 5086 (54.5%) were uncoded. Of those identified with CKD stage 3-5, individuals aged ≥50 years were more likely to be coded for CKD, compared with those aged <50 years. Lower levels of coding were independently associated with deprivation and black Caribbean, black African, South Asian, and non-stated ethnicities, compared with white ethnicity. Prescribed statin and angiotensin-converting enzyme inhibitor/angiotensin receptor blocker medications were associated with increased odds of coded CKD. CONCLUSION: This study found that >50% of CKD was uncoded and, for those patients, quality of care was lower compared with those with coded CKD. Future research and practices should focus on areas of greater deprivation and targeted initiatives for those aged <50 years and of black African, black Caribbean, South Asian, or non-stated ethnic groups. Possible areas for improvement include diagnostic coding support, automated CKD recording, and clinical decision support (based on adjusted eGFR results) in the GP clinical records.