Epidemiology of Acute Kidney Injury in England - 1998 to 2013

Subject Selection and Withdrawal We will identify all cases of AKI by using validated International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) codes in any of he diagnoses codes, in keeping with the objective of the study. Patients with any of the following codes will be included: N17.0 for acute renal failure with tubular necrosis, N17.1 for acute renal failure with acute cortical necrosis, N17.2 for acute renal failure with medullary necrosis, N17.8 for other acute renal failure and N17.9 for acute renal failure, unspecified. Acute renal failure has been replaced by new terminology, acute kidney injury, but due to lack of ICD-10-CM codes for AKI, we will use the ICD-10-CM codes for acute renal failure and henceforth, will be referred to as AKI in this protocol.

Subject Recruitment and Screening We will extract 1998 to 2013 data from the Hospital Episode Statistics (HES), a data warehouse containing details of all admissions, outpatient appointments and A&E attendances at National Health Service (NHS) hospitals in England. HES collects a detailed record for each 'episode' of admitted patient care delivered in England, either by NHS hospitals, primary care trusts, mental health trusts or delivered in the independent sector but commissioned by the NHS. These data are collected during a patient's time in hospital and stored as a large collection of separate records, one for each period of care, in a secure data warehouse. We will extract secondary diagnosis codes for up to 14 diagnoses available till 2006-07 and up to 20 additional diagnoses from 2007 till 2013. In addition to demographic data, we will also extract up to 15 procedure codes. To identify AKI patients requiring dialysis, we will include procedure code of X40.3 for hemodialysis or X40.4 for hemofiltration in any of the 15 procedures. To exclude patients with chronic kidney disease starting dialysis, we will exclude patients who had chronic kidney disease (CKD) stage 5 (N18.5) and end stage renal disease (N18.6) and procedure codes for arteriovenous fistula (L74.2) or arteriovenous shunt (L74.3) during the inpatient admission (figure 1). This algorithm has been shown to be sensitive and specific, with a high positive and negative predictive value (all >90%. Patients who were admitted, but were not discharged during the study period will not be included in the study.

We will obtain data on patient demographics, hospital characteristics, in-hospital mortality, disposition, length of stay (LOS), and up to 20 diagnosis and 15 procedure codes that are based on the ICD-10-CM and Office of Population Censuses and Surveys Classification of Surgical Operations and Procedures, 4th revision (OPCS-4) from the HES database. To obtain population incidence of AKI requiring dialysis, we will obtain mid-year populations of England in each year from 1998 to 2013 from the Office of National Statistics (ONS). We will also obtain length of stay and mortality for all patients who did not have acute kidney injury coded as N17 in any of the diagnoses codes, in England over the same time period.

Patients will not be contacted. This study will not obtain informed consent from the patients' as we will be working with anonymous patient data. No screening requirements (laboratory or diagnostic testing) are necessary to meet the inclusion or exclusion criteria. Unique Subject Identification numbers are already allocated to the patients and the HES database does not include any patient's name or hospital number.

Sample Size Determination The data from HES will be extracted for the period between April 1998 and March 2013. HES data is collected during a patients' time at hospital and is submitted to allow hospitals to be paid for the care they deliver. As this is a retrospective population cohort study using a national database, all cases of AKI as defined by the inclusion criteria, from the HES database will be used.