DISPARITIES IN KIDNEY STONE DISEASE: A SCOPING REVIEW

Abstract

Purpose:

To review the available evidence regarding health disparities in kidney stone disease and identify knowledge gaps in this area.

Materials and Methods:

A literature search was conducted using PubMed, Embase, and Scopus, limited to articles published in English, from 1971 to 2020. Articles were selected based on their relevance to disparities in kidney stone disease among adults in the United States.

Results:

Several large epidemiologic studies suggest disproportionate increases in incidence and prevalence of kidney stone disease among women as well as Black and Hispanic individuals in the United States, whereas other studies of comparable size do not report racial and ethnic demographics. Numerous articles describe disparities in imaging utilization, metabolic workup completion, analgesia, surgical intervention, stone burden at presentation, surgical complications, follow-up, and quality of life based on race, ethnicity, socioeconomic status, and place of residence. Differences in urinary parameters based on race, ethnicity, and socioeconomic status may be explained by both dietary and physiologic factors. All articles assessed had substantial risk of selection bias and confounding.

Conclusions:

Health disparities are present in many aspects of kidney stone disease. Further research should focus not only on characterization of these disparities but also on interventions to reduce or eliminate them.

INTRODUCTION

Kidney stones impose a significant economic burden, costing several billion dollars annually in the United States (US)¹. They are also associated with reduced quality of life², and in rare instances threaten life³. With rising incidence and prevalence of kidney stone disease⁴, it is increasingly important to recognize obstacles to optimal health outcomes among stone formers, and how to overcome them. Health disparities are potentially avoidable differences in health and health risks between disadvantaged social groups and the general population^5,6. Although health disparities often refer to unjust health differences based on race or ethnicity⁷, disparities exist across many other dimensions, including socioeconomic status and geographic location⁸. In this scoping review, we examine differences and disparities that have been identified among groups afflicted with kidney stones, point out knowledge gaps, and outline future research directions.

EVIDENCE ACQUISITION

This review was conducted according to the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR)⁹. We performed a literature search using the PubMed, Embase, and Scopus databases. Articles were selected based on their relevance to disparities in kidney stone disease among adults in the US. One author (JJC) performed the literature search through all three databases on November 2, 2020. Eligible studies were relevant to (1) kidney stone disease; (2) disparities/differences defined by race/ethnicity, socioeconomic status, or rural/urban environment; and (3) incidence/prevalence, evaluation (including imaging and metabolic workup utilization), management (including pain control and surgical procedures), disease burden and outcomes (including surgical complications), access to care and quality of life, or stone composition/metabolic factors.

The specific search protocol was as follows: (stones OR nephrolithiasis OR urolithiasis OR calculi) AND (race OR ethnicity OR Black OR "African American" OR Hispanic OR Latino OR Asian OR "American Indian" OR "Native American" OR "Alaska Native" OR "Native Hawaiian" OR "Pacific Islander" OR socioeconomic OR rural OR urban) AND (disparities OR differences) AND (incidence OR prevalence OR metabolic OR urinary OR composition OR evaluation OR assessment OR workup OR treatment OR surgery OR shockwave OR ureteroscopy OR percutaneous OR access OR "quality of life" OR outcomes OR recurrence OR infection OR sepsis).

Only articles published in English, from 1971 to 2020 were evaluated. Pertinent cited references from selected publications were also retrieved for evaluation. We accepted all study designs except for case reports. Studies with cohorts not residing in the United States or pediatric-only cohorts were excluded. Review articles, meeting abstracts, editorials, and commentary were excluded as well. The review protocol was not prospectively registered.

All authors participated in the design of the literature search and inclusion criteria. The Supplementary Figure shows the number of items evaluated at each stage of the review process. One author (JJC) tabulated and screened search results for relevance based on title and abstract, and all authors reviewed full-text articles. The senior author (DGA) directed the review process and resolved any discrepancies. All authors agreed on the final list of articles to be included. Selected articles describing differences or disparities in kidney stone disease between groups were divided into categories: (1) Incidence and prevalence, (2) evaluation, (3) management, (4) disease burden and surgical outcomes, (5) access to care and quality of life, and (6) stone composition and metabolic factors.

Two authors (JJC and DGA) conducted a level of evidence assessment based on the 2009 Centre for Evidence-Based Medicine categorization¹⁰, and assessed the risk of selection bias and confounding (either low risk or high risk). These assessments and other key study features are summarized in Table 1 and listed for each individual study in the Supplementary Table. Based on the articles selected, it was deemed appropriate to include additional tables summarizing results specific to race/ethnicity and kidney stone incidence and prevalence (Table 2) and urinary biochemistry (Table 3). Methodologies and results reporting of studies in the other four categories were more heterogeneous and thus appropriate for narrative description. Additional references were cited when appropriate to supplement the discussion. Findings from the 2018 Urologic Diseases in America (UDA) narrative report on kidney stones¹¹ were also referenced when appropriate, though this was not part of the published literature retrievable through our search.

Table 1.

Methodologic summary of selected studies; level of evidence was determined based on the 2009 Centre for Evidence-Based Medicine categorization¹⁰.

Theme (number of articles)	Study setting	Data collection	Level of evidence	Risk of selection bias	Risk of confounding
Incidence and prevalence (n=8)	Countywide: 1 Multicenter: 3 Nationwide: 2 Statewide: 1	Prospective, self-reported: 4 Prospective, claim/code: 1 Retrospective, claim/code: 3	1b: 5 2b: 3	Low: 0 High: 8	Low: 0 High: 8
Evaluation (n=4)	Nationwide: 1 Single center: 3	Retrospective, claim/code: 2 Retrospective, clinic attendance: 2	2b: 2 3b: 2	Low: 0 High: 4	Low: 0 High: 4
Management (n=7)	Multicenter: 1 Multistate: 1 Nationwide: 4 Statewide: 1	Retrospective, claim/code: 6 Retrospective, trial enrollment: 1	2b: 7	Low: 0 High: 7	Low: 0 High: 7
Disease burden and surgical outcomes (n=5)	Multistate: 1 Nationwide: 1 Single center: 2 Statewide: 1	Prospective, claim/code: 1 Retrospective, claim/code: 2 Retrospective, clinic attendance: 1 Retrospective, registry: 1	1b: 1 2b: 3 3b: 1	Low: 0 High: 5	Low: 0 High: 5
Access to care and quality of life (n=5)	Multicenter: 3 Single center: 2	Prospective, self-reported: 2 Retrospective, claim/code: 1 Retrospective, clinic attendance: 1 Retrospective, surgical series: 1	1b: 2 2b: 3	Low: 0 High: 5	Low: 0 High: 5
Stone composition and metabolic factors (n=9)	Nationwide: 1 Multicenter: 2 Single center: 6	Prospective, self-reported: 1 Retrospective, claim/code: 1 Retrospective, clinic attendance: 7	1b: 1 3b: 1 4: 7	Low: 0 High: 9	Low: 0 High: 9

Table 2.

Selected epidemiologic studies describing kidney stone disease (KSD) incidence and prevalence in the United States (US).

Ref.	Years of study	Study design/setting	Cohort description	Major findings	Comment
12	2007-2018	Cross-sectional (National Health and Nutrition Examination Survey)	554 individuals with KSDa; 62% non-Hispanic White, 16% Hispanic, 11% non-Hispanic Black, 11% otherb	KSD prevalence in US remains higher in men, but is increasing only in women	KSD was self-reported
13	1994-2010	Cross-sectional (National Health and Nutrition Examination Survey)	1,017 individuals with KSDc	KSD prevalence has increased among Blacks and Hispanics	KSD was self-reported
14	1997-2012	Cross-sectional (South Carolina Medical Encounter data)	152,925 individuals receiving care for KSD; 85% White, 12% Black, 0.3% Asian, 0.1% American Indian, 2.4% other	KSD incidence has increased among young patients, women, and Blacks	Hispanic ethnicity not reported
15	1999-2014	Cohort of individuals residing in 12 southeastern states (Southern Community Cohort Study)	1,233 incident KSD cases; 47% White, 53% Black	White men had highest risk of KSD, while no difference in risk observed between Black men and women	Analysis restricted to Black and non-Hispanic White individuals
16	2000-2012	Multisite cohort (Multi-Ethnic Study of Atherosclerosis)	6,814 individuals aged 45-84; 38% White, 28% Black, 22% Hispanic, 12% Asian	Recurrent KSD is associated with subclinical coronary atherosclerosis	Analyses adjusted for age, gender, and race/ethnicity
17	1992-2008	Cohort spanning 24 rural Wisconsin ZIP codes (Marshfield Epidemiologic Study Area)	9,642 incident and recurrent KSD cases	KSD incidence, prevalence, and recurrence have increased, with greater increases noted among women	Race/ethnicity not reported; in 2000, population of study region was 97% non-Hispanic White
18	1984-2003	Sample of Olmstead County, Minnesota residents	1,633 individuals with incident symptomatic stones	Older individuals presenting with urolithiasis less likely to have typical renal colic	Race/ethnicity not reported; in 2000, population of study region was 90% White
19	1980-2002	Cohort of healthcare workers (Health Professionals Follow-up Study and Nurses’ Health Study I-II)	4,827 incident KSD cases	Obesity and weight gain increased risk of KSD	Race/ethnicity not reported

^aEstimate based on reported 2017-2018 data

^bPercentages based on overall 2017-2018 cohort (n=5,222)

^cEstimate based on reported 2007-2010 data, overall race/ethnicity distribution not reported

Table 3.

Selected studies describing racial/ethnic differences in urinary biochemistry among individuals in the United States.

Ref.	Years of study	Study design/setting	Study groups	Major findings	Comment
47	2008-2017	Cross-sectional (University of Chicago)	Age-matched Black (n=117) and White (n=172) stone formers	Black participants: lower UVol, UCa UCit, UK, UMg, UPhos, USO4 in univariate analyses	Electrolyte-free water clearance was significantly lower in Black stone formers
50	1994-2005	Cross-sectional, randomly selected postmenopausal nurses (Nurses’ Health Study)	Black (n=146) and White (n=330) non-stone formers	Black participants: lower UVol, UCa, UK, UMg, UPhos, USO4; higher UpH	Differences in UCa, UpH persisted after multivariate adjustment
51	2004-2015	Cross-sectional (University of Alabama at Birmingham)	Black (n=61) and White (n=528) stone formers	Black participants: lower UVol, UCa, UOx, UCit, UK, UMg, UPhos, USO4 in univariate analyses	Lower UVol, UCa, UOx, UCit, UK, UMg, UPhos, USO4 persisted after multivariate adjustment
52	1994-2002	Cross-sectional (Duke University)	Black (n=44), Asian (n=8), Hispanic (n=8), and sex- and age-matched White stone formers (n=66)	Compared to White participants: lower UCa in Black participants, lower UCit and higher UVol in Asian participants	Fewer Asian and Hispanic participants
55	2010a	Cross-sectional (University of California, San Francisco)	Asian/Pacific Islander (n=91) and White (n=391) stone formers	Asian/PI participants: higher UUA, lower UCit, UPhos, UCr in univariate analyses	Higher UUA, lower UCit, UPhos, UCr persisted after multivariate adjustment

^a2010 was year of publication, years of data collection not reported; U_Vol=urinary volume, U_Ca=urinary calcium, U_Cit=urinary citrate, U_K=urinary potassium, U_Mg=urinary magnesium, U_Phos=urinary phosphorus, U_SO4=urinary sulfate U_pH=urinary pH, U_Ox=urinary oxalate, U_UA=urinary uric acid, U_Cr=urinary creatinine

DIFFERENCES IN INCIDENCE AND PREVALENCE

Data on kidney stone incidence and prevalence reflect differences based on gender, race, and ethnicity.

Several large epidemiologic studies have reported distributions of stone formers by race and ethnicity^12-16, although other reports do not incorporate these demographic characteristics^17-19 (Table 2). The gender, racial, and ethnic differences in the incidence and prevalence of kidney stones are evolving. In a recent analysis of National Health and Nutrition Examination Survey (NHANES) 2007-2018 data, Abufaraj et al. reported that, while the estimated prevalence was stable in men, it significantly increased in women¹². In another NHANES analysis comparing 1994 to 2007-2010 data, substantial temporal increases in stone prevalence were noted among Blacks and Hispanics¹³. Increasing incidence of kidney stones among women and Black individuals was observed between 1997-2012 in the South Carolina Medical Examination dataset, although White men were still at highest risk¹⁴. In the recent Southern Community Cohort Study, White men had the highest risk of kidney stones (hazard ratio [HR]=1.45 relative to White women, p<0.001), but there was no difference in risk between Black men and women (HR=0.9, p=0.2)¹⁵. The aforementioned studies suggest that while kidney stone disease is more common in men, the gender ratio is decreasing. The rising incidence and prevalence of kidney stone disease among Black women could be contributing significantly to this trend.

DISPARITIES IN EVALUATION

Imaging.

Disparities in the utilization of imaging for the diagnosis of kidney stones have been reported. The use of computed tomography (CT), the most sensitive modality for establishing a kidney stone diagnosis, was assessed using data from the Nationwide Emergency Department Sample between 2005-2015²⁰. CT for suspected stone disease was performed more frequently among patients from higher-income ZIP codes, with private insurance, and at urban and non-teaching hospitals²⁰. In a smaller cohort (40% Hispanic, 15% non-Hispanic Black) presenting to an emergency department (ED) in the Bronx, New York with a presumed diagnosis of ureteral stone, patients in the lowest quintile of ZIP code household income received less imaging than patients in the highest quintile (odds ratio=0.50, p=0.02)²¹. In the Medicare cohort of the 2018 UDA report, White stone formers used imaging more frequently than Black stone formers, even though Black stone formers utilized the ED to a greater extent than White stone formers¹¹.

Metabolic workup completion.

We identified two studies assessing the completion of 24-hour urine collections for the identification of metabolic stone risk factors in stone formers: a retrospective chart review from a single clinic²² and an interventional study to improve compliance with collections²³. Both studies reported that younger patients had the lowest compliance with collection, but no significant differences were observed based on race or ethnicity^22,23. The interventional study described better compliance among patients who were adequately insured vs. un/underinsured (58% vs. 37%, p=0.017)²³.

DISPARITIES IN MANAGEMENT

Pain control.

Renal colic can be excruciatingly painful, making effective analgesia an essential element of kidney stone management. Disparities in pain management exist, including for renal colic. In one study of patients presenting to the ED for pain ascribed to various causes, Whites were significantly more likely to receive an opioid than Blacks, Hispanics, and Asians²⁴. Other studies have demonstrated similar findings for patients presenting to the ED with generalized back or abdominal pain²⁵. Non-Hispanic White patients presenting to the ED with suspected kidney/ureteral stones received opioids more frequently and in greater quantity than Hispanics^26,27. Black patients with renal colic received opioids less frequently than Whites; they were also less likely to receive ketorolac²⁷. In addition, patients with higher education and health insurance coverage were more likely to receive opioids²⁶.

Surgery.

Gender, racial, ethnic, and socioeconomic disparities exist with regard to kidney stone removing procedures. Racial/ethnic minority patients with ureteral stones underwent stone removing procedures less frequently compared to non-minority patients²⁸. In one study, individuals with Medicaid, Medicare, or self-pay coverage had a significantly longer interval from diagnosis to kidney stone surgery than patients with private insurance²⁹. This delay in care was also reported for Black and Hispanic patients, but not Whites²⁹. Consistent with the these findings, uninsured or minority patients hospitalized for renal colic have significantly lower odds of undergoing upfront ureteroscopic stone removal (URS) during the admission³⁰. Furthermore, in the Medicare cohort of the 2018 UDA report, it was noted that White stone formers were slightly more likely to undergo a stone removing procedure than Blacks¹¹.

DISPARITIES IN DISEASE BURDEN AND SURGICAL OUTCOMES

Stone burden.

Stone burden can influence kidney stone management, with a need for more invasive procedures with larger calculi. Associations of lower socioeconomic status and other demographic factors with larger stone burden have been reported. Quarrier et al. performed an analysis of 3,939 patients undergoing stone removing procedures at a single center between 2009-2019³¹. The authors used the Distressed Communities Index (DCI), a composite of 7 socioeconomic variables, to define severely distressed communities (SDC). SDC status was associated with larger stones in men but not women. The generally accepted threshold for undertaking a more invasive stone removing procedure such as percutaneous nephrolithotomy (PCNL) is stone size >20 mm. Thus, the finding of a higher rate of PCNL in men residing in SDC was expected (17.5% vs. 10.2%, p=0.019). The authors noted that patients classified as Latinx ethnicity were more likely to reside in an SDC. However, approximately 95% of the cohort were White, precluding a generalizable analysis of the impact of race and ethnicity. Bayne et al. performed a study of 650 patients with unilateral stone size >20 mm receiving care at a single center³². This cohort was more diverse, being 36.3% non-White (including 3.5% Black, 10% Hispanic, and 12.3% Asian). In a univariate analysis, lower density of urologists in area of residence, lower income, primary language other than English, greater distance from referral center, and lower educational level were associated with stones >20 mm. However, only lower educational level and increased distance from center remained significant in multivariate analysis³².

Surgical outcomes.

The most common procedure for stone removal in the US and many other areas of the world is URS followed by shock wave lithotripsy (SWL) and PCNL³³. These procedures are associated with complications and side effects that may result in ED visits or a postoperative hospital admission, which are reported to be influenced by sociodemographic factors. Khanna et al. analyzed the Agency for Healthcare research and Quality databases for Florida, Iowa, California, and New York and found that URS and PCNL were associated with higher rates of ED visits than SWL³⁴. Such ED visits were more common in un/underinsured and Hispanic patients, whereas those with private insurance and highest income had significantly fewer post-procedural ED visits³⁴. Mittakanti et al. undertook an analysis of 16,060 patients captured in the California Office of State-wide Health Planning and Development database and reported that postoperative ED visits and hospital admissions were higher in the Medi-cal (California Medicaid) cohort as compared to those with private insurance³⁵. Finally, Tyson and Humphreys queried the National Inpatient Sample for patients undergoing PCNL and found that, among healthy subjects undergoing this procedure, complication rates were significantly higher in women and Black patients³⁶.

DISPARITIES IN ACCESS TO CARE AND QUALITY OF LIFE

Access to care and follow-up.

There are examples of disparities in utilization of health care communication technologies and compliance with follow-up. Jhamb et al. reported that Black patients, those with public health insurance, and those living in lower household income areas were less likely to use an Electronic Health Record portal for nephrology care³⁷. Race and insurance status have been shown to influence follow-up after ureteral stent placement, as well as return appointments after stent removal. In a retrospective study adjusted for age, sex, race, surgical urgency (elective vs. emergent), and insurance status, Javier-DesLoges et al. reported that Black race was an independent predictor of missed follow-up after placement of a ureteral stent³⁸. Moses et al. found that government-assisted insurance was the only predictor of missed follow-up after stent removal³⁹. The latter study was adjusted for age, sex, education, procedure, season, distance to clinic, and price of gas, but not for race/ethnicity.

Health-related quality of life (HRQoL).

Kidney stones are associated with significant morbidity, including pain, that negatively impacts HRQoL², or the subjective evaluation of the impact an illness has on one’s physical, psychological, and social functioning⁴⁰. Gender, race, ethnicity, and socioeconomic status have implications for HRQoL. Surveying more than 2,000 patients from 11 stone centers across the U.S. with the Wisconsin Stone Quality of Life questionnaire, Ahmad et al. found that lower socioeconomic status (lower residential ZIP code income and being unemployed) and non-White race were associated with lower HRQoL independent of clinical characteristics such as obesity and stone disease severity⁴¹. Similarly, Stern et al. documented lower HRQoL among non-White patients and women in this cohort⁴².

DIFFERENCES IN STONE COMPOSITION AND METABOLIC FACTORS

When interpreting studies reporting racial or ethnic differences in stone composition or stone metabolism, it is important to recognize that race and ethnicity are social constructs, and not biological constructs^43-45. In fact race and ethnicity have significant limitations even as social variables, with concepts such as social determinants of health being more pertinent to observed racial/ethnic health disparities⁴⁶. In light of these limitations, as well as the low level of evidence of most of the studies discussed further (predominantly case series, see Table 1), we can only infer loose associations between race/ethnicity and biologic variables (e.g., urine chemistry).

Stone composition.

There are limited reports on differences in stone analysis across races and ethnicities in the US. A recent study by Zisman et al. reported that Black and White patients had comparable distributions of stone composition⁴⁷. However, the analysis was based on a small sample size. The Hmong, an ethnically distinct refugee population from Laos, exhibit a substantially higher incidence of uric acid kidney stones⁴⁸. In fact, uric acid was present in 50% of the analyzed stones from Hmong living in metropolitan Minneapolis-St. Paul, Minnesota, representing the highest proportion of uric acid stone disease reported to date in any population worldwide⁴⁸. The association between socioeconomic status and kidney stone formation was retrospectively analyzed in two regional stone clinics. In one analysis, 55 stone formers with state-assisted insurance (SAI) were compared with 291 stone formers with private insurance⁴⁹. In multivariate analysis restricted to patients with calcium stones, calcium phosphate stones were ten times more common among patients with SAI.

Metabolic parameters.

Several studies have reported differences in urine biochemistry between racial and ethnic groups in the US (Table 3). In a subset of postmenopausal non–stone forming women enrolled in the Nurses’ Health Study, compared with White women, Black women exhibited significantly higher urine pH and 24-hour oxalate excretion, as well as significantly lower 24-hour volume and urinary excretion of calcium, potassium, magnesium, phosphate, and sulfate. Only differences in urinary calcium and pH persisted after multivariate adjustment⁵⁰. Consistent findings in stone formers have included a significantly lower 24-hour urine calcium and phosphorus excretion in Black vs. White patients^47,51,52, which has been ascribed to differences in intake, vitamin D stores, and genetic factors^53,54. Other shared findings include lower urine citrate, potassium, and magnesium in Black vs. White stone formers^47,51, potentially representing lower intake of alkali-rich foods. This may reflect a disadvantage such as low food access. Moreover, a lower urine volume in Black stone formers has been ascribed to a higher circulating vasopressin, limiting the ability to increase free water clearance in the face of a free water load⁴⁷. In one report, stone formers of Asian/Pacific Islander race excreted significantly more uric acid and significantly less citrate, phosphate, and creatinine than White stone formers on both univariate and multivariate analyses, despite all patients receiving similar dietary counseling ≥30 days before the urine collection⁵⁵.

Of note, most of the aforementioned studies controlled for age and gender, but adjustment for additional conditions that may differentially affect certain races/ethnicities (including obesity, type 2 diabetes and glycemic control, gout, hypertension, etc., as well as socioeconomic factors and associated exposures) was not uniform across studies. This was highlighted in the Medicare cohort from the 2018 UDA report; diabetes and hypertension being more prevalent amongst Black stone formers as comparted to White stone formers¹¹. Furthermore, in one cohort with a predominance of Hispanic and Black patients, race/ethnicity was not a significant determinant of urinary parameters in multivariate analysis adjusting for age, gender, body mass index (BMI), hypertension, insulin use, and hemoglobin A1C⁵⁶.

With respect to socioeconomic status, stone formers with SAI were more likely to be younger females and to exhibit higher urine sodium and pH⁴⁹, potentially reflecting dietary or other unmeasured differences. A separate analysis from the same clinics found that increasing poverty and lower education level were both associated with greater calcium excretion in multivariate analysis⁵⁷. Furthermore, a lower education level was also associated with increased supersaturation of calcium oxalate and calcium phosphate⁵⁷. In a separate single-center study, a higher DCI was associated with lower 24-hour urine citrate and potassium excretion but similar urine calcium, oxalate, and pH³¹. Lower citrate and potassium excretion persisted after multivariate adjustment (for age, gender, race, ethnicity, BMI, diabetes, hypertension, gout, chronic urinary tract infection, and chronic kidney disease) and potentially reflects lower dietary intake of alkali-rich foods³¹, again potentially reflecting a disparity such as low food access.

CONCLUSIONS AND FUTURE DIRECTIONS

Health disparities by race, ethnicity, socioeconomic status, and geography are evident in many aspects of kidney stone disease. The incidence and prevalence of kidney stone disease appear to be increasing among women, Blacks, and Hispanics; however, some epidemiologic studies do not report on the racial and ethnic distributions of stone formers or on their socioeconomic characteristics. We also identified numerous studies reporting sociodemographic differences in time to surgery, HRQoL, and stone composition/urine biochemistry. Additionally, reports of racial and ethnic disparities in analgesia for patients presenting to the ED with renal colic may reflect implicit bias⁵⁸.

A limitation of this review is the heterogeneity of the objectives and methods of the studies identified through the literature search, which precluded a systematic review or meta-analysis, and rendered a scoping review a more appropriate format. We found that all of these studies had substantial risk of both selection bias and confounding. An important example of selection bias is patient cohorts receiving specialist care for kidney stone disease, which may not represent the care received by the broader stone-forming population. Unmeasured potential contributors to racial/ethnic differences or disparities, such as socioeconomic status or medical comorbidities, were frequent confounding variables.

It is also important to note that nearly all of the identified studies were descriptive rather than interventional. While it is critical to raise awareness of existing health disparities, interventions to reduce or even eliminate them are the ultimate goal⁵⁹. Thus, future research on the specific mechanisms by which minority race/ethnicity and low socioeconomic status impact kidney stone formation and care delivery, including patient evaluation, management, and outcomes, is necessary but not sufficient. Initiatives to improve access to care for vulnerable populations with stone disease need to be enacted. Perhaps enhanced access to outpatient services for disadvantaged groups could prevent unneeded ED visits and the disparities in imaging and analgesia associated with this setting, while improving availability of stone removing procedures and metabolic evaluation. Community- and population-level interventions to improve diet and other risk factors pertinent to kidney stone risk (e.g., obesity and diabetes, which are known to be more prevalent in disparate cohorts⁶⁰) should be prioritized. Furthermore, studies to determine if changes in the social environment, such as improvements in food security and housing, will favorably alter biological responses are imperative.