Obesity is becoming more common in the UK and already affects 25% of the population. Common health problems associated with obesity include heart disease, diabetes, kidney problems and mobility difficulties, all of which can significantly impact on the quality of life of those affected. Surgical treatments for obesity are now being used more frequently, but some of the long-term (i.e. over several years) effects of treatment are not known. We have previously done a study in the CPRD to compare several long-term outcomes in people having surgery for weight loss with similar people who have not had weight loss surgery but are also over weight. Now we plan to use the same study population to see whether people who have weight loss surgery are less likely to have future kidney problems. We will compare the occurrence of these outcomes in obese people who undergo weight loss surgery with obese people who do not undergo surgery. The results of this study will improve the evidence base available to doctors and patients trying to decide the best treatment options for people with obesity.
We will investigate the association between bariatric surgery and renal outcomes in obese people in the United Kingdom, focusing on: i) reaching a threshold level of renal function; ii) incidence of chronic kidney disease; and iii) incidence of acute kidney injury. To achieve this we will look at people who receive bariatric surgery, based on their CPRD record, and compare them with people who are also obese but did not receive bariatric surgery. Outcomes will be defined using estimated glomerular filtration rate (calculated using CPRD serum creatinine results) and evidence of hospitalisation due to acute kidney injury using Hospital Episode Statistics (HES). Propensity score matching will be used to achieve comparability between the surgery and non-surgery group to help us deal with potential confounding. Cox regression will then be used to compare how often the outcomes occur in each group, resulting in a hazard ratio for each outcome.
Health Outcomes to be Measured:
Baseline Renal Function - will be defined as: i) best of two: the highest eGFR from the most recent two serum creatinine results recorded in the 12 months prior to index and separated by a minimum of three months (three month timeframe chosen to correspond to the requirement for eGFR to remain at a consistent level of impairment for at least three months in order for a patient to be diagnosed at a specific CKD stage); or ii) if only one suitable creatinine result is available, the single most recent serum creatinine recorded prior to baseline.
eGFR threshold - Renal function will be defined using serum creatinine test results to calculate estimated glomerular filtration rate (eGFR) using the CKD-EPI equation.
CKD - As for eGFR threshold, but also requiring a second measure of eGFR <60 at least three months later.
Acute kidney injury - will be defined as the first episode of AKI identified within 28 days of the start of a hospital admission identified using ICD-10 morbidity coding in HES, to capture cases of AKI that were present at hospital admission but may have not been immediately diagnosed, without excluding cases that resulted in a prolonged admission. We will use a previously validated code list.