Data source and study population

All practices in England and Wales who were current users of the Informatica Audit Plus software were invited to participate in NCKDA between March 2015 and July 2016. NCKDA round 1 data were collected from all GPs in 1039 GP practices representing an underlying population >18 years of age of 8.24 million in England and Wales. Coverage in England and Wales differed substantially as a result of technical difficulties and differential use of the software used to extract data for the NCKDA [1]; final coverage was ∼76% of practices in Wales and 9% of practices in England. All Welsh practices had Audit Plus installed (funded by the National Health Service in Wales) while in England, practices actively purchased Informatica Audit Plus software to support better disease management [1]. Data on CKD coding, eGFR test results and relating to CKD management were extracted for all patients with risk factor coding for CKD at least 1 year prior to data extraction. A full list of risk factor codes and full details regarding the study population are available elsewhere [1]. Practices received e-mail feedback about the prevalence of biochemical, coded and uncoded CKD suggesting that they might use the QI software to improve coding.

Round 2 data were collected from 948 of these practices, with a median of 8 months from round 1 (range 3–20 months). Figure ​Figure11 shows patient progress from round 1 to round 2 by coding status. A total of 65 661 patients with uncoded CKD at round 1 (i.e. no code for Stages 3–5 CKD, but with biochemical evidence for CKD) for whom round 2 data confirming biochemical CKD were available were included in an analysis of coding improvement.

Flow chart showing progress through round 1 (R1) and round 2 (R2).

Information about referrals to secondary care was available through extraction of outpatient referral codes collected at round 1 from the GP record and linkage to outpatient records from Hospital Episode Statistics (HES) for England (data collected for the period 1 April 2012–30 June 2016) and NHS Wales Informatics Statistics (NWIS) (1 January 2012–30 June 2016).