Relation of Urine Cell-Free Mitochondrial DNA Quantified by Droplet Digital Polymerase Chain Reaction With Tubule Injury, Dysfunction, and Ambulatory Acute Kidney Injury

<h2>Abstract</h2><h3>Rationale & Objective</h3> The detectability and clinical significance of urine cell free mitochondrial DNA (cf-mtDNA) in ambulatory individuals is under explored. Moreover, traditional methods of cf-mtDNA quantification have lacked reproducibility. In this study, we utilized droplet digital PCR as a sensitive and specific method of determining urine cf-mtDNA concentrations in an ambulatory population of chronic kidney disease (CKD) patients. We then correlate baseline urinary cf-mtDNA with biomarkers of kidney tubule dysfunction and ambulatory acute kidney injury (AKI). <h3>Study Design</h3> Nested case-control longitudinal analysis of clinical trial participants. <h3>Setting & Participants</h3> 103 participants in the Systolic blood PRessure Intervention Trial (SPRINT) who had chronic kidney disease (CKD) at baseline and developed ambulatory AKI (defined as ≥ 0.3 mg/dL increase in serum creatinine at 1 or 2 year follow up visit) and 103 matched controls. <h3>Exposure</h3> Cf-mtDNA measured in stored urine at baseline and 12 or 24 month follow up study visit. <h3>Outcome</h3> ≥ 0.3 mg/dL increase in serum creatinine from baseline to visits at 12 or 24 months. <h3>Analytical Approach</h3> Spearman correlations and multivariable conditional logistic regression. <h3>Results</h3> Mean age was 70 ±10 years, baseline eGFR was 39 ml/min/1.73m². Urine cf-mtDNA was measurable in all patients and had a positive and significant correlation with seven urine tubule health biomarkers. Doubling of urine cf-mtDNA was also significantly associated with increased odds of developing ambulatory AKI (odds ratio 1.17, 95% confidence interval 1.01, 1.35). <h3>Limitations</h3> "Ambulatory AKI" was defined as changes in serum creatinine over 1 to 2 years as opposed to over 48 hours, a definition utilized in prior publications for outpatients. <h3>Conclusions</h3> Measurement of cf-mtDNA in the urine by droplet digital PCR is feasible in an ambulatory population. Additionally, levels of cf-mtDNA in the urine correlate with urine biomarkers related to proximal tubule dysfunction and subsequent risk of ambulatory AKI.