P0561ACUTE KIDNEY INJURY AS A RISK FACTOR FOR KIDNEY STONES: A PROPENSITY MATCHED ANALYSIS
Authors: Hicham Ibrahim Cheikh Hassan , Lonergan Maureen , Mullan Judy , McAlister Brendan , Lambert Kelly , Suesse Thomas 
Published: June 2020

Abstract

Background and Aims
Kidney stones affects 6-9% of the population, with an almost 30% risk of reoccurrence. Factors associated with kidney stone formation include male gender, ethnicity, family history and stone type. The underlying pathology of stone formation is complex and includes increased urine saturation, decreased urine stone inhibitors in addition to metabolic factors.
Acute kidney injury (AKI) refers to the abrupt decrease in kidney function, resulting in retention of urea and other waste products and the dysregulation of fluid and electrolyte balance. AKI affects 10-15% of the hospitalised population and is associated with long-term outcomes, such as chronic kidney disease, end-stage kidney disease (ESKD), cardiovascular disease, fractures and earlier mortality.
There is evidence that AKI may result in long-term renal damage and fibrosis. A potential effect is impairment in urine concentration which would limit kidney stone formation. However there has been no previous investigation about this potential association. We aim to investigate if there is an association between AKI and kidney stone formation.

Method
Retrospective clinical data available for all adult (≥18 years of age) hospital admissions to a local health district in Australia between January 2008 and December 2017 was used in the analysis. We excluded 1) non-residents, 2) stone diagnosis pre-AKI, 3) death within 1 month of an AKI episode, 4) dialysis dependent ESKD prior to an AKI episode and 5) incomplete patient information. AKI episode was diagnosed from ICD-10 coding and kidney stone from ICD-10 and SNOMED coding. Our outcome was a kidney stone episode. We examined the entire cohort in addition to propensity score matching (PSM) using 1:1 optimal matching, caliper 0.1 and without replacement based on covariates known to be associated with renal calculi and AKI. Balance before and after PSM was assessed between the groups to evaluate quality using standardised means. Baseline characteristics were compared with chi-square and Mann Whitney U. Multivariate analysis was compared using Logistic regression.

Results
For the cohort of 180,927 patients, after exclusions, 12,338 (6.8%) patients were diagnosed with an AKI and 4,495 (2.5%) patients with a kidney stone. Patients with an AKI (12,338), compared to patients with no AKI (168,523), were more likely to be older (75.0 vs 50.0 years, p&lt;0.001) with more comorbidities such as hypertension (31.8% vs 12.7%, p&lt;0.001), diabetes (21.5% vs 9.4%, p&lt;0.001), coronary artery disease (14.6% vs 7.0%, p&lt;0.001) and peripheral vascular disease (5.4% vs 1.8%, p&lt;0.001).
The risk of kidney stone formation for the entire cohort in patients with a history of AKI was lower when compared to the no AKI patients (1.1% vs 2.6%, Hazard ratio 0.42, 95% confidence interval 0.36- 0.52, p&lt;0.001). After PSM 12,336 patients with AKI were matched with 12,336 patients with no AKI with good balance of covariates. Patients with a history of AKI had a lower risk of kidney stone formation when compared to no AKI patients (Hazard ratio 0.57, 95% Confidence Interval 0.36- 0.51, p&lt;0.001)

Conclusion
Our findings suggest that patients with AKI appear to be at a significantly lower risk of developing subsequent kidney stone formation when compared to patients with no previous AKI episodes. Further analysis on a greater scale are required to confirm these findings and their implications.