Best Practices & Benign Disease
Introduction & Objective :
Maintaining urine volume above 2.5 liters is a common recommendation made by clinicians to help prevent kidney stone formation as adequate urine volume is a potential inhibitor of stone formation. Our goal was to identify the prevalence of low urine volume amongst stone formers as well as identify if low urine volume was associated with specific 24-hour urine abnormalities for patients in an underserved community.
A retrospective chart review was performed of patients treated at a single center between August 2014 and January 2018. Patients who submitted 24-hour urine samples were identified. Patients were divided into two groups based on whether they’re urine volume was less than or greater than 2 liters, as it was unlikely for patients to be above 2.5 liters on an initial collection. Frequency of 24-hour urine abnormalities were then identified for patients with urine volume less than 2 liters and greater than 2 liters. Fishers exact test and multivariate logistic regression was then used to identify if low urine volume was associated with specific 24-hour urine abnormalities.
Of 461 patients, 208 (45.1%) submitted an initial 24-hour urine collection. Low urine volume was the most prevalent 24-hour urine abnormality (133/208, 63.9%) followed by low urine citrate in females (68/109, 62.4%) and elevated supersaturation of uric acid (ssUA) (91/206, 44.2%). On bivariate analysis, low urine volume was associated with low urine pH (pH<5.8: 73.1% vs 26.9%; p=0.04), elevated supersaturation of calcium oxalate (ssCaOx) (ssCaOx>10: 100% vs 0%; p=0.001), elevated supersaturation of calcium phosphate (ssCaP>2: 80.6% vs 19.4%; p=0.02), elevated ssUA (ssUA>1: 75.8% vs 24.2%; p=0.002), and low urine citrate in females (Citrate<550: 85.3% vs 14.7%; p=0.01). Interestingly, patients with low urine volume on 24-hour urine had lower frequencies of elevated urine oxalate (oxalate>40: 42.3% vs 57.8%; p=0.001), elevated urine calcium in males (calcium >250: 32.3% vs 67.7%; p=0.03), and elevated urine uric acid in males (uric acid >0.8: 32.4% vs 67.6%; p=0.01). On multivariate logistic regression low urine volume was an independent predictor of elevated ssCaP, elevated ssUA, low urine citrate in females, and low urine oxalate.
More than half of the patients in an underserved community fell short of achieving 80% of the recommended fluid volume necessary to lessen the risk of kidney stone formation, and this has implications on other metabolic parameters. Optimizing urine volume is a simple, low cost treatment and should be the first education point when counseling patients on strategies for stone prevention.