Contribution of thick ascending limb and distal convoluted tubule to glucose-induced hypercalciuria in healthy controls.

Carbohydrates increase kidney stone risk and increase urine calcium and magnesium. We hypothesize that the effects of glucose as an allosteric modulator of calcium-sensing receptors may mediate this effect. Six healthy subjects were on a low-sodium diet before consuming 100 g of glucose beverage. Timed fasting (3) and postglucose (6) urine and blood samples were collected every 30 min. Urine composition and serum markers were measured and microvesicular abundance of tubular transport proteins (NHE3, NKCC2, NCC, and TRPV5) were quantified. Postglucose, serum glucose, and insulin rose rapidly with a parallel increase in calcium and magnesium excretion and no change in fractional excretion of sodium. Both serum parathyroid hormone (PTH) and urine TRPV5 fell in the postglucose periods. The rise in the calcium and magnesium excretion likely occurred primarily in the thick ascending limb where they are both under control of the calcium-sensing receptor. The fall in PTH and TRPV5 support the role of glucose as an allosteric modulator of calcium-sensing receptor.NEW & NOTEWORTHY Sugar increases urine calcium and magnesium as well as kidney stone and bone disease risk. Our study provided new insights into the underlying mechanism as we gave healthy subjects an oral glucose load and used newer tools such as fractional excretion of lithium, serum parathyroid hormone, and microvesicular abundance of tubular transport proteins to characterize the mechanism and identify the thick ascending limb with possible calcium-sensing receptor mediation as a likely contributor to this mechanism.

A spatially anchored transcriptomic atlas of the human kidney papilla identifies significant immune injury in patients with stone disease.

Kidney stone disease causes significant morbidity and increases health care utilization. In this work, we decipher the cellular and molecular niche of the human renal papilla in patients with calcium oxalate (CaOx) stone disease and healthy subjects. In addition to identifying cell types important in papillary physiology, we characterize collecting duct cell subtypes and an undifferentiated epithelial cell type that was more prevalent in stone patients. Despite the focal nature of mineral deposition in nephrolithiasis, we uncover a global injury signature characterized by immune activation, oxidative stress and extracellular matrix remodeling. We also identify the association of MMP7 and MMP9 expression with stone disease and mineral deposition, respectively. MMP7 and MMP9 are significantly increased in the urine of patients with CaOx stone disease, and their levels correlate with disease activity. Our results define the spatial molecular landscape and specific pathways contributing to stone-mediated injury in the human papilla and identify associated urinary biomarkers.

Water Loading and Uromodulin Secretion in Healthy Individuals and Idiopathic Calcium Stone Formers.

BACKGROUND: Uromodulin is a protein made only by the kidney and released in urine, circulating in polymerizing and nonpolymerizing forms. This protein's multiple functions include inhibition of stone formation in the urine. The physiological determinants of uromodulin production are incompletely understood. METHODS: We investigated changes in uromodulin levels and key factors governing its production and release in urine and serum. We performed an experiment to determine whether water loading, a common intervention to prevent stone formation, will alter the rate of uromodulin production. During a 2-day period, 17 stone forming participants and 14 control participants were subjected to water loading (day 1) and normal fluid intake (day 2). Uromodulin levels were measured on timed hourly collections in urine and plasma during the period of the study. RESULTS: Water loading increased urinary uromodulin secretion (33±4 versus 10±4 µ g/min at baseline, P < 0.0001) in stone formers and control participants. Despite high urine volumes, most participants maintained relatively stable urinary uromodulin concentrations. Native Western blots for polymerizing and nonpolymerizing uromodulin suggest that polymerizing uromodulin was the predominant form at higher urinary flow volumes. Urine flow rates and sodium excretion were significant correlates of urinary uromodulin production. Water loading did not affect serum uromodulin levels, which were also not associated with urinary uromodulin. CONCLUSIONS: Water loading increases the secretion of polymerizing urinary uromodulin. This increased secretion reduces the variability of urinary uromodulin concentrations despite high urine volumes. Serum uromodulin levels were not affected by this treatment.

Clinical Effectiveness of Calcium Oxalate Stone Treatments.

INTRODUCTION: Lowering kidney stone risk and urine calcium oxalate supersaturation is a primary clinical focus for kidney stone prevention and can be achieved with multiple strategies. Common strategies include advice to increase fluid intake, restrict dietary sodium, or prescribing a thiazide-type diuretic. We investigated how physicians make these decisions in real-world practice and evaluate their efficacy based on 24-h urine collections. METHODS: We reviewed medical charts for 203 kidney stone formers with idiopathic calcium stones from University of Chicago Kidney Stone Clinic between 2005 and 2020. Patients had three 24-h urines before an initial pre-treatment clinic visit and one follow-up 24-h urine. We analyzed changes in urine composition based on treatment advice using t tests and ANOVA. RESULTS: Patients who received advice to increase fluid intake had lower urine volume at baseline (1.5 vs. 2.5 L/day, p < 0.001) and larger increase in urine volume at follow-up (0.6 vs. 0.1 L/day, p < 0.001) compared to those who did not receive the advice. Patients who were advised to restrict dietary sodium had a higher urine sodium at baseline (208 vs. 139 mEq/day, p < 0.001), a larger reduction in urine sodium (-28 vs. 13 mEq/day, p = 0.002), and larger reduction in urine calcium (-74 vs. -28 mg/day, p = 0.005) compared with those not advised to restrict dietary sodium. Patients started on a thiazide had a higher baseline urine calcium (281 vs. 213 mg/day) and larger reduction in urine calcium (-83 vs. -9 mg/day, p < 0.001) compared with patients not started on a thiazide. In combination, thiazide prescriptions with dietary sodium restriction reduced urine calcium by 99 mg/day and reduced calcium oxalate supersaturation from 8.0 to 5.5 and calcium phosphate supersaturation from 1.4 to 1.0. CONCLUSION: Providers use 24-h urine data to guide treatment strategy decisions. These strategies achieved the intended effects on urine composition and lowered kidney stone risk.

What treatments reduce kidney stone risk in patients with bowel disease?

We examined how physicians made therapeutic choices to decrease stone risk in patients with bowel disease without colon resection, many of whom have enteric hyperoxaluria (EH), at a single clinic. We analyzed clinic records and 24-h urine collections before and after the first clinic visit, among 100 stone formers with bowel disease. We used multivariate linear regression and t tests to compare effects of fluid intake, alkali supplementation, and oxalate-focused interventions on urine characteristics. Patients advised to increase fluid intake had lower initial urine volumes (L/day; 1.3 ± 0.5 vs. 1.7 ± 0.7) and increased volume more than those not so advised (0.7 ± 0.6 vs. 0.3 ± 0.6 p = 0.03; intervention vs. non-intervention). Calcium oxalate supersaturation (CaOx SS) fell (95% CI -4.3 to -0.8). Alkali supplementation increased urine pH (0.34 ± 0.53 vs. 0.22 ± 0.55, p = 0.26) and urine citrate (mg/d; 83 ± 256 vs. 98 ± 166, p = 0.74). Patients advised to reduce oxalate (mg/day) absorption had higher urine oxalate at baseline (88 ± 44 vs. 50 ± 26) which was unchanged on follow-up (88 (baseline) vs. 91 (follow-up), p = 0.90). Neither alkali (95% CI -1.4 to 2.1) nor oxalate-focused advice (95% CI -1.2 to 2.3) lowered CaOx SS. Physicians chose treatments based on baseline urine characteristics. Advice to increase fluid intake increased urine volume and decreased CaOx SS. Alkali and oxalate interventions were ineffective.

Agreement of Food Records and 24-Hour Urine Studies in Clinical Practice.

OBJECTIVE: The objective of this study was to determine the level of agreement between 3-day food records obtained as part of clinical care with 24-hour urine collections specifically assessing sodium, potassium, phosphorus, calcium, protein, and fluid intake. DESIGN AND METHODS: Data were collected from patients at a nephrology clinic in a metropolitan, academic medical center. Patients who completed both a 3-day food record and a 24-hour urine collection were analyzed. Food record and urine collection measurements were compared using a simple ratio, Pearson's correlation, and general linear models. RESULTS: Patients (n = 85) were 47.9 ± 15.2 years of age, 54% were female, with a mean serum creatinine of 1.3 ± 0.7 mg/dL and estimated glomerular filtration rate of 64.2 ± 25.6 mL/min. Patients had autosomal-dominant polycystic kidney disease (48.2%), nephrolithiasis (31.1%), chronic kidney disease (4.7%), or other genetic or cystic conditions impacting the kidney (12.9%). Nutrient intake was measured utilizing a 3-day food record. Food records and urine collections were compared using the values, correlations, and general linear models. Fluid intake demonstrated the highest agreement (ratio 1.01) and calcium demonstrated the least agreement (ratio 6.30). Significant correlations were demonstrated for phosphorus (r = 0.321, P = .003), magnesium (r = 0.256, P = .018), protein (r = 0.555, P < .000), and fluid (r = 0.277, P = .010) intake. Food record intake of potassium (P = .046), protein (P = .004), and fluid (P = .010) were significant predictors of 24-hour urine excretion. CONCLUSION: 3-day food records are useful tools to determine patient dietary patterns, but should be used with caution when assessing specific nutrient intake in clinical settings.

Stone Morphology Distinguishes Two Pathways of Idiopathic Calcium Oxalate Stone Pathogenesis.

Introduction: About 1 in 11 Americans will experience a kidney stone, but underlying causes remain obscure. The objective of the present study was to separate idiopathic calcium oxalate stone formers by whether or not they showed positive evidence of forming a stone on Randall's plaque (RP). Materials and Methods: In patients undergoing either percutaneous or ureteroscopic procedures for kidney stone removal, all stone material was extracted and analyzed using micro-CT imaging to identify those attached to RP. Twenty-four-hour urine samples were collected weeks after the stone removal procedure and patients were off of medications that would affect urine composition. The endoscopic video was analyzed for papillary pathology (RP, pitting, plugging, dilated ducts, and loss of papillary shape) by an observer blinded to the data on stone type. The percent papillary area occupied by RP and ductal plugging was quantified using image analysis software. Results: Patients having even one stone on RP (N = 36) did not differ from non-RP patients (N = 37) in age, sex, BMI, or other clinical characteristics. Compared with the non-RP group, RP stone formers had more numerous, but smaller, stones, more abundant papillary RP formation, and fewer ductal plugs, both by quantitative measurement of surface area (on average, three times more plaque area, but only 41% as much plug area as in non-RP patients) and by semiquantitative visual grading. Serum and blood values did not differ between RP and non-RP stone formers by any measure. Conclusions: Growth of many small stones on plaque seems the pathogenetic scheme for the RP stone-forming phenotype, whereas the non-RP phenotype stone pathogenesis pathway is less obvious. Higher papillary plugging in non-RP patients suggests that plugs play a role in stone formation and that these patients have a greater degree of papillary damage. Underlying mechanisms that create these distinctive phenotypes are presently unknown.

Evidence for abnormal linkage between urine oxalate and citrate excretion in human kidney stone formers.

BACKGROUND: Animal models have demonstrated an interactive relationship between the epithelial anion exchanger SLC26A6 and transporter NaDC-1 that regulates citrate and oxalate homeostasis. This relationship is a potential mechanism to protect against kidney stones as higher urine oxalate is accompanied by higher urine citrate but it has not been explored in humans. METHODS: We examined 24-h urine data on 13,155 kidney stone forming patients (SF) from separate datasets at the University of Chicago and Litholink, a national laboratory, and 143 non-kidney stone forming participants (NSF) to examine this relationship in humans. We used multivariate linear regression models to examine the association between oxalate and citrate in all study participants and separately in SF and NSF. RESULTS: Higher urinary oxalate was associated with higher urinary citrate in both SF and NSF. In NSF, the multivariate adjusted urine citrate excretion was 3.0 (1.5-4.6) (mmol)/creatinine (mmol) per oxalate (mmol)/creatinine (mmol). In SF, the multivariate adjusted urine citrate excretion was 0.3 (0.2-0.4) (mmol)/creatinine (mmol) per oxalate (mmol)/creatinine (mmol). CONCLUSIONS: Higher urinary oxalate excretion was associated with higher urinary citrate excretion and this effect was larger in non-kidney stone forming participants compared with those who form kidney stones.

Relative contributions of urine sulfate, titratable urine anion, and GI anion to net acid load and effects of age.

Models of acid-base balance include acid production from (1) oxidation of sulfur atoms on amino acids and (2) metabolically produced organic acid anions. Acid load is balanced by alkali from metabolism of GI anions; thus, net acid production is equivalent to the sum of urine sulfate and organic anion (measured by titration in urine), minus GI anion. However, the relative contributions of these three sources of acid production in people eating free choice diets, and presumably in acid-base balance, have not been well studied. We collected 26 urines from 18 normal subjects (10 male) and 43 urine samples from 34 stone formers (17 male) and measured sulfate, organic anion, and components of GI anion and acid excretion in each; values were expressed as mEq/mmol creatinine. Mean values of the urine components, except creatinine and pH, did not differ between the sexes or groups. Urine organic acid and acid production varied directly with age (p ≤ 0.03). In a general linear model of acid excretion, the coefficients for sulfate, organic anion, and GI anion were 0.34 ± 0.09, 0.49 ± 0.12, and -0.51 ± 0.06, respectively, p ≤ 0.005, and the model accounted for 54% of the variance. A model for urine ammonia gave similar results. Urine organic anion is a significant contributor to total acid production and may be responsible for an increase in acid production with age.

Racial Differences in Risk Factors for Kidney Stone Formation.

BACKGROUND AND OBJECTIVES: Incidence of kidney stone disease is rising. It is not known whether mechanisms of stone formation differ across racial groups. Our objective was to identify differing lithogenic risk factors across racial groups in idiopathic nephrolithiasis. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: We conducted a retrospective cohort study evaluating metabolic risk factors in black and age-matched white idiopathic stone formers at our tertiary referral center. We compared serum and urine metabolic risk factors pre- and post-treatment across racial groups using analysis of covariance. Generalized linear modeling was used to build regression models for risk of stone formation in both groups. RESULTS: Among 117 black and 172 white stone formers, urine volume was lower in black stone formers (1.4±0.8 versus 2.0±0.8 L/d, P<0.001). Urine calcium was lower in black stone formers (116±70 versus 217±115 mg/d, P<0.001). Supersaturations for calcium oxalate were similar among the groups, whereas calcium phosphate supersaturation was higher in white stone formers, and uric acid supersaturation was higher in black stone formers. Electrolyte free water clearance was significantly lower in black stone formers (207±780 versus 435±759 ml/d, P=0.02). In the subgroup of 77 black patients and 107 white patients with post-treatment evaluations, urine volume rose significantly and similarly in both groups. Urine sodium was unchanged in whites but increased in blacks by 40 mmol/d (95% confidence interval, 32 to 48 mmol/d). Electrolyte free water clearance remained lower in black stone formers (385±891 versus 706±893 ml/d, P=0.02). Post-treatment supersaturations were similar across the groups except for calcium phosphate, which improved with treatment in whites. CONCLUSIONS: Black stone formers have lower 24-hour urine calcium excretion and urine volume. Increases in urine volume with treatment were associated with increased solute, but not free water, excretion in black stone formers.

A Precision Medicine Approach Uncovers a Unique Signature of Neutrophils in Patients With Brushite Kidney Stones.

INTRODUCTION: We have previously found that papillary histopathology differs greatly between calcium oxalate and brushite stone formers (SF); the latter have much more papillary mineral deposition, tubular cell injury, and tissue fibrosis. METHODS: In this study, we applied unbiased orthogonal omics approaches on biopsied renal papillae and extracted stones from patients with brushite or calcium oxalate (CaOx) stones. Our goal was to discover stone type-specific molecular signatures to advance our understanding of the underlying pathogenesis. RESULTS: Brushite SF did not differ from CaOx SF with respect to metabolic risk factors for stones but did exhibit increased tubule plugging in their papillae. Brushite SF had upregulation of inflammatory pathways in papillary tissue and increased neutrophil markers in stone matrix compared with those with CaOx stones. Large-scale 3-dimensional tissue cytometry on renal papillary biopsies showed an increase in the number and density of neutrophils in the papillae of patients with brushite versus CaOx, thereby linking the observed inflammatory signatures to the neutrophils in the tissue. To explain how neutrophil proteins appear in the stone matrix, we measured neutrophil extracellular trap (NET) formation-NETosis-and found it significantly increased in the papillae of patients with brushite stones compared with CaOx stones. CONCLUSION: We show that increased neutrophil infiltration and NETosis is an unrecognized factor that differentiates brushite and CaOx SF and may explain the markedly increased scarring and inflammation seen in the papillae of patients with brushite stones. Given the increasing prevalence of brushite stones, the role of neutrophil activation in brushite stone formation requires further study.

Evidence for disordered acid-base handling in calcium stone-forming patients.

In stone formers (SFs) with idiopathic hypercalciuria, urine pH governs the mineral phase of stones. Calcium phosphate (CaP) SFs have higher urine pH than calcium oxalate (CaOx) SFs. Normal women have higher urine pH than men on fixed diets, accompanied by greater absorption of food alkali. Female CaP and male CaOx SFs have similar urine pH as same sex normal individuals, but male CaP and female CaOx SFs may have abnormal acid-base handling. We studied 25 normal individuals (13 men and 12 women), 17 CaOx SFs (11 men and 6 women), and 15 CaP SFs (8 men and 7 women) on fixed diets. Urine and blood samples were collected under fasting and fed conditions. Female CaOx SFs had lower urine pH and lower alkali absorption, fed, compared with normal women; their urine NH4 was higher and urine citrate excretion lower than in normal women, consistent with their higher net acid excretion. Male CaOx SFs had higher urine citrate excretion and higher serum ultrafilterable citrate levels than normal men. Both male and female CaP SFs had higher urine pH fasting than same sex normal individuals, but only men were higher in the fed period, and there were no differences from normal in gut alkali absorption. CaP SFs of both sexes had higher urine NH4 and lower urine citrate than same sex normal individuals. The lower urine pH of female CaOx SFs seems related to decreased gut alkali absorption, while the higher pH of CaP SFs, accompanied by higher urine NH4 and lower urine citrate, suggests a proximal tubule disorder.

Discrepancy Between Stone and Tissue Mineral Type in Patients with Idiopathic Uric Acid Stones.

Objectives: To describe the papillary pathology found in uric acid (UA) stone formers, and to investigate the mineral form of tissue deposits. Materials and Methods: We studied eight UA stone formers treated with percutaneous nephrolithotomy. Papillae were imaged intraoperatively using digital endoscopy, and cortical and papillary biopsies were taken. Biopsies were analyzed by light microscopy, micro-CT, and microinfrared spectroscopy. Results: As expected, urine pH was generally low. UA supersaturation exceeded one in all but one case, compatible with the stone material. By intraoperative imaging, the renal papillae displayed a heterogeneous mixture of plaque and plugging, ranging from normal to severe. All patients had mineral in ducts of Bellini and inner medullary collecting ducts, mainly apatite with lesser amounts of urate and/or calcium oxalate in some specimens. Papillary and cortical interstitial tissue injury was modest despite the tubule plugging. No instance was found of a stone growing attached to either plaque or plugs. Conclusions: UA stone formers resemble those with ileostomy in having rather low urine pH while forming tubule plugs that contain crystals that can only form at pH values above those of their bulk urine. This discrepancy between tissue mineral deposits and stone type suggests that local tubular pH exceeds that of the bulk urine, perhaps because of localized tubule injury. The manner in which UA stones form and the discordance between tubule crystals and stone type remain open research questions.

In Vivo Renal Tubule pH in Stone-Forming Human Kidneys.

Introduction: There is evidence that patients with a history of ileostomies, who produce acidic urine and form uric acid or calcium oxalate stones, may plug some collecting ducts with calcium phosphate (CaP) and urate crystals. This is a paradoxical finding as such minerals should not form at an acid pH. One possible explanation is the presence of acidification defects due to focal damage to inner medullary collecting duct and Bellini duct (BD) cells. We sought to further investigate this hypothesis through direct measurement of ductal pH in dilated BDs in patients with ileostomies undergoing percutaneous nephrolithotomy (PCNL) for stone removal. Methods: After obtaining institutional review board approval, we used a fiber-optic pH microsensor with a 140-µm-diameter tip to measure intraluminal pH from the bladder, saline irrigant, and dilated BDs of patients undergoing PCNL. Results: Measurements were taken from three patients meeting inclusion criteria. Measured pH of bladder urine ranged from 4.97 to 5.58 and pH of saline irrigant used during surgery ranged from 5.17 to 5.75. BD measurements were achieved in 11 different BDs. Mean intraductal BD pH was more than 1 unit higher than bulk urine (6.43 ± 0.22 vs 5.31 ± 0.22, p < 0.01). Conclusions: This is the first evidence for focal acidification defects within injured/dilated BDs of human kidneys producing highly acidic bulk phase urine. These results may help explain the paradoxical finding of CaP and urate plugs in dilated ducts of patients with stone-forming diseases characterized by highly acidic urine.

A laboratory-based algorithm to predict future kidney function decline in older adults with reduced estimated glomerular filtration rate
.

BACKGROUND: Reduced estimated glomerular filtration rate (eGFR) in older adults is common and may reflect normal aging or significant kidney disease. Our objective was to develop a predictive model to better triage these individuals using routine laboratory data. MATERIALS AND METHODS: Using a large US laboratory data set, we calculated individual eGFR regression slopes for 43,523 individuals aged 60 - 75 years with baseline eGFRs between 30 and 59 mL/min/1.73m2. We developed general linear models to predict the eGFR regression slope using urine protein measurements and other routinely available laboratory data as dependent variables. We validated these models on a similar data set comprised of 11,979 individuals. RESULTS: In a model utilizing log10 urine albumin/creatinine (UACR), the variables that significantly predicted the eGFR regression slope were log10 UACR, initial eGFR, serum albumin, chloride, glucose, and aspartate aminotransferase (AST). In an otherwise identical model substituting log10 urine protein/creatinine (UPCR) for UACR, results were similar except that serum calcium was significant and AST was not. We analyzed the correspondence between actual eGFR regression slopes and those predicted by our models using receiver operator characteristic (ROC) statistics to calculate areas under the curves (AUC) for four eGFR slope cut points: -2, -3, -4, and -5 mL/min/year. AUCs using the UACR and UPCR models ranged from 0.716 to 0.900 and 0.751 to 0.868, respectively, for the training data set. Results were nearly identical for the validation data set. CONCLUSION: Use of a laboratory-based predictive model of eGFR decline for older adults with eGFR 30 - 59 mL/min/1.73m2 may help distinguish between individuals with and without risk for further decline in kidney function.

Mechanisms for falling urine pH with age in stone formers.

One of the main functions of the kidney is to excrete an acid load derived from both dietary and endogenous sources, thus maintaining the pH of other fluids in the body. Urine pH is also of particular interest in stone formers, since it determines the presence of either calcium phosphate or uric acid content in stones. Others have noted in epidemiological studies a rise in incidence of low pH-dependent uric acid stones with age, coinciding with a decrease in the incidence of high pH-dependent phosphate stones. Taken together, these trends are suggestive of a longitudinal decline in urine pH in stone-forming patients, and, if true, this could explain the observed trends in stone incidence. We studied 7,891 stone formers, all of whom collected a 24-h urine sample and matching serum. Multivariate modeling revealed that urine pH did indeed fall with age and particularly between the ages of 20 and 50 yr old in both men and women. We sought to explain this trend through the inclusion of traditionally understood determinants of urine pH such as urinary buffers, estimates of glomerular filtration, and dietary acid load, but these, taken together, accounted for but a small fraction of the pH fall. Gastrointestinal anion absorption was the strongest predictor of urine pH in all age groups, as we have previously reported in middle-aged normal men and women. However, we found that, despite a decreasing urine pH, gastrointestinal anion absorption increased monotonically with age. In fact, after adjustment for gastrointestinal anion absorption, urine pH declined more markedly, suggesting that bicarbonate-producing anion absorption is regulated in a manner that offsets the decline of urine pH.

Association Between Randall's Plaque Stone Anchors and Renal Papillary Pits.

INTRODUCTION: Renal papillary pits are commonly encountered during ureteroscopy. The mechanism by which such pits arise is unclear. One hypothesis is that pits represent sites where stones overgrowing Randall's plaque (RP) were dislodged. We sought to examine this theory by using digital ureteroscopy and stone µCT. MATERIALS AND METHODS: Patients undergoing endoscopic stone removal had procedures recorded and stones analyzed by using µCT. Stones with evidence of Randall's plaque anchors (RPAs) were identified in a blinded fashion. Surgical videos were reviewed independently by two urologists. RESULTS: Twenty-eight patients had µCT-confirmed stones with RPA. Among them, 93% were recurrent stone formers and 75% had had prior stone procedures. Metabolic abnormalities were present in 87%, with 79% classified as idiopathic calcium oxalate stone formers. A mean of 7.6 stones with RPA were identified per procedure. In each case, papillary pits were visualized before any stone manipulation and in several cases the active dislodgement of an attached stone led to immediate identification of an underlying pit. Such stones routinely demonstrated an RPA on µCT. The average depth of RPA was 302 ± 172 µm, consistent with the corresponding shallow pits visualized on the papillary surface. CONCLUSIONS: Stones overgrowing RP are capable of pulling away a piece of papilla when dislodged, resulting in a visible papillary pit. This process manifests as an RPA on the undersurface of the stone and a papillary pit on the corresponding area of attachment. Identification of pits may help identify patients who form stones primarily by the RP mechanism.