Risk of bias in non-randomized observational studies assessing the relationship between proton-pump inhibitors and adverse kidney outcomes: a systematic review

<jats:sec><jats:title>Background:</jats:title><jats:p> Proton-pump inhibitors (PPIs) are widely prescribed as acid-suppression therapy. Some observational studies suggest that long-term use of PPIs is potentially associated with certain adverse kidney outcomes. We conducted a systematic literature review to assess potential bias in non-randomized studies reporting on putative associations between PPIs and adverse kidney outcomes (acute kidney injury, acute interstitial nephritis, chronic interstitial nephritis, acute tubular necrosis, chronic kidney disease, and end-stage renal disease). </jats:p></jats:sec><jats:sec><jats:title>Methods:</jats:title><jats:p> We searched the medical literature within 10 years of 17 December 2020. Pre-specified criteria guided identification of relevant English language articles for assessment. Risk of bias on an outcome-specific basis was evaluated using the Risk of Bias in Non-randomized Studies of Interventions (ROBINS-I) tool by two independent reviewers. </jats:p></jats:sec><jats:sec><jats:title>Results:</jats:title><jats:p> Of 620 initially identified records, 26 studies met a priori eligibility criteria and underwent risk of bias assessment. Nineteen studies were judged as having a moderate risk of bias for reported adverse kidney outcomes, while six studies were judged as having a serious risk of bias (mainly due to inadequate control of confounders and selection bias). We were unable to determine the overall risk of bias in two studies (one of which was assessed as having a moderate risk of bias for a different adverse kidney outcome) due to insufficient information presented. Effect estimates for PPIs in relation to adverse kidney outcomes varied widely (0.24–7.34) but associations mostly showed increased risk. </jats:p></jats:sec><jats:sec><jats:title>Conclusion:</jats:title><jats:p> Using ROBINS-I, we found that non-randomized observational studies suggesting kidney harm by PPIs have moderate to serious risk of bias, making it challenging to establish causality. Additional high-quality, real-world evidence among generalizable populations are needed to better understand the relation between PPI treatment and acute and chronic kidney outcomes, accounting for the effects of varying durations of PPI treatment, self-treatment with over-the-counter PPIs, and potential critical confounders. </jats:p></jats:sec>