Emergency department visits, use of imaging, and drugs for urolithiasis have increased in the United States
Authors: Chyng-Wen Fwu, Paul W. Eggers, Paul L. Kimmel, John W. Kusek, Ziya Kirkali
Journal: Kidney International (2013)

The occurrence of urolithiasis in the United States has increased; however, information on long-term trends, including recurrence rates, is lacking. Here we describe national trends in rates of emergency department visits, use of imaging, and drug treatment, primarily using the National Hospital Ambulatory Medical Care Survey to describe trends and the National Health and Nutrition Examination Survey to determine the frequency of lifetime passage of kidney stones. Emergency department visit rates for urolithiasis increased from 178 to 340 visits per 100,000 individuals from 1992 to 2009. Increases in visit rates were greater in women, Caucasians, and in those aged 25–44 years. The use of computed tomography in urolithiasis patients more than tripled, from 21 to 71%. Medical expulsive therapy was used in 14% of the patients with a urolithiasis diagnosis in 2007–2009. Among National Health and Nutrition Examination Survey participants who reported a history of kidney stones, 22.4% had passed three or more stones. Hence, emergency department urolithiasis visit rates have increased significantly, as has the use of computed tomography in the United States. Further research is necessary to determine whether recurrent stone formers receive unnecessary radiation exposure during diagnostic evaluation in the emergency department and allow development of corresponding evidence-based guidelines. The occurrence of urolithiasis in the United States has increased; however, information on long-term trends, including recurrence rates, is lacking. Here we describe national trends in rates of emergency department visits, use of imaging, and drug treatment, primarily using the National Hospital Ambulatory Medical Care Survey to describe trends and the National Health and Nutrition Examination Survey to determine the frequency of lifetime passage of kidney stones. Emergency department visit rates for urolithiasis increased from 178 to 340 visits per 100,000 individuals from 1992 to 2009. Increases in visit rates were greater in women, Caucasians, and in those aged 25–44 years. The use of computed tomography in urolithiasis patients more than tripled, from 21 to 71%. Medical expulsive therapy was used in 14% of the patients with a urolithiasis diagnosis in 2007–2009. Among National Health and Nutrition Examination Survey participants who reported a history of kidney stones, 22.4% had passed three or more stones. Hence, emergency department urolithiasis visit rates have increased significantly, as has the use of computed tomography in the United States. Further research is necessary to determine whether recurrent stone formers receive unnecessary radiation exposure during diagnostic evaluation in the emergency department and allow development of corresponding evidence-based guidelines. Urolithiasis is one of the most common urological diseases in the United States.1Litwin M.S. Saigal C.S. Yano E.M. et al.Urologic diseases in America Project: analytical methods and principal findings.J Urol. 2005; 173: 933-937Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar The lifetime risk of symptomatic kidney stones is ∼13% in men and 7% in women.2Stamatelou K.K. Francis M.E. Jones C.A. et al.Time trends in reported prevalence of kidney stones in the United States: 1976-1994.Kidney Int. 2003; 63: 1817-1823Abstract Full Text Full Text PDF PubMed Scopus (1142) Google Scholar,3Scales C.D. Smith A.C. Hanley J.M. et al.Prevalence of kidney stones in the United States.Eur Urol. 2012; 62: 160-165Abstract Full Text Full Text PDF PubMed Scopus (1540) Google Scholar The prevalence is highest in older white men and lowest in younger black women.2Stamatelou K.K. Francis M.E. Jones C.A. et al.Time trends in reported prevalence of kidney stones in the United States: 1976-1994.Kidney Int. 2003; 63: 1817-1823Abstract Full Text Full Text PDF PubMed Scopus (1142) Google Scholar, 3Scales C.D. Smith A.C. Hanley J.M. et al.Prevalence of kidney stones in the United States.Eur Urol. 2012; 62: 160-165Abstract Full Text Full Text PDF PubMed Scopus (1540) Google Scholar, 4Soucie J.M. Thun M.J. Coates R.J. et al.Demographic and geographic variability of kidney stones in the United States.Kidney Int. 1994; 46: 893-899Abstract Full Text PDF PubMed Scopus (230) Google Scholar Studies suggest that kidney stone presentation also varies by season and geographic region, with greater frequency observed during the warmer months and in the southern United States.4Soucie J.M. Thun M.J. Coates R.J. et al.Demographic and geographic variability of kidney stones in the United States.Kidney Int. 1994; 46: 893-899Abstract Full Text PDF PubMed Scopus (230) Google Scholar, 5Soucie J.M. Coates R.J. McClellan W. et al.Relation between geographic variability in kidney stones prevalence and risk factors for stones.Am J Epidemiol. 1996; 143: 487-495Crossref PubMed Scopus (171) Google Scholar, 6Chauhan V. Eskin B. Allegra J.R. et al.Effect of season, age, and gender on renal colic incidence.Am J Emerg Med. 2004; 22: 560-563Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar, 7Pearle M.S. Calhoun E.A. Curhan G.C. Urologic diseases in America project: urolithiasis.J Urol. 2005; 173: 848-857Abstract Full Text Full Text PDF PubMed Scopus (642) Google Scholar Only limited information is available on urolithiasis recurrence rates. The financial burden of urolithiasis is substantial.8Lotan Y. Economics and cost of care of stone disease.Adv Chronic Kidney Dis. 2009; 16: 5-10Abstract Full Text Full Text PDF PubMed Scopus (117) Google Scholar Total annual medical expenditures for urolithiasis in the United States exceeded $2 billion in 2000, and are expected to increase in the future.7Pearle M.S. Calhoun E.A. Curhan G.C. Urologic diseases in America project: urolithiasis.J Urol. 2005; 173: 848-857Abstract Full Text Full Text PDF PubMed Scopus (642) Google Scholar An important factor in the increased cost is the use of medical imaging technology for the evaluation of urolithiasis patients. However, there is also concern regarding the extent of use of radiation in the evaluation of patients because of potential adverse long-term sequelae, especially in patients with recurrent disease and in younger patients.9Pearce M.S. Salotti J.A. Little M.P. et al.Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study.Lancet. 2012; 380: 499-505Abstract Full Text Full Text PDF PubMed Scopus (2578) Google Scholar Although individuals with kidney stones may be asymptomatic, many may experience severe pain from stone passage.10Teichman J.M. Clinical practice. Acute renal colic from ureteral calculus.N Engl J Med. 2004; 350: 684-693Crossref PubMed Scopus (291) Google Scholar The prevalence and incidence of urolithiasis is increasing globally11Hesse A. Brandle E. Wilbert D. et al.Study on the prevalence and incidence of urolithiasis in Germany comparing the years 1979 vs. 2000.Eur Urol. 2003; 44: 709-713Abstract Full Text Full Text PDF PubMed Scopus (422) Google Scholar, 12Yasui T. Iguchi M. Suzuki S. et al.Prevalence and epidemiological characteristics of urolithiasis in Japan: national trends between 1965 and 2005.Urology. 2008; 71: 209-213Abstract Full Text Full Text PDF PubMed Scopus (178) Google Scholar, 13Romero V. Akpinar H. Assimos D.G. Kidney stones: a global picture of prevalence, incidence, and associated risk factors.Rev Urol. 2010; 12: e86-e96PubMed Google Scholar, 14Turney B.W. Reynard J.M. Noble J.G. et al.Trends in urological stone disease.BJU Int. 2012; 109: 1082-1087Crossref PubMed Scopus (224) Google Scholar and in the United States.2Stamatelou K.K. Francis M.E. Jones C.A. et al.Time trends in reported prevalence of kidney stones in the United States: 1976-1994.Kidney Int. 2003; 63: 1817-1823Abstract Full Text Full Text PDF PubMed Scopus (1142) Google Scholar, 15Penniston K.L. McLaren I.D. Greenlee R.T. et al.Urolithiasis in a rural Wisconsin population from 1992 to 2008: narrowing of the male-to-female ratio.J Urol. 2011; 185: 1731-1736Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 16Sas D.J. Hulsey T.C. Shatat I.F. et al.Increasing incidence of kidney stones in children evaluated in the emergency department.J Pediatr. 2010; 157: 132-137Abstract Full Text Full Text PDF PubMed Scopus (174) Google Scholar, 17Dwyer M.E. Krambeck A.E. Bergstralh E.J. et al.Temporal trends in incidence of kidney stones among children: a 25-year population based study.J Urol. 2012; 188: 247-252Abstract Full Text Full Text PDF PubMed Scopus (206) Google Scholar However, reports of long-term trends in the occurrence of urolithiasis in the United States are lacking. We examined trends in diagnosis, use of imaging and drugs, and regional and seasonal variation for urolithiasis in a nationally representative sample of US emergency department (ED) visits (National Hospital Ambulatory Medical Care Survey, NHAMCS) over nearly a two-decade period beginning in the early 1990s. We determined urolithiasis ED return visits using the NHAMCS database and evaluated the number of episodes of self-reported kidney stones among a nationally representative sample of the population (National Health and Nutrition Examination Survey, NHANES) to identify a subgroup of patients who may receive unnecessary radiation exposure from imaging during evaluation in the ED for suspected urolithiasis. The goals of this study were to assess secular trends in ED urolithiasis visits, including imaging use in the United States, and to estimate urolithiasis recurrence rates and return visits in national and ED populations. A total of 551,577 ED visits were sampled from 1992 to 2009. Upper tract urolithiasis was coded in 3403 visits. Accounting for the sampling weights and complex sample design, over this time, there were an estimated 12.45 million ED visits (95% confidence interval (CI), 11.22–13.67 million) with a diagnosis of upper tract urolithiasis, or about 700,000 visits per year in the United States. These visits accounted for about 0.65% (95% CI, 0.61–0.69%) of all ED visits. To examine temporal trends in urolithiasis, the number of ED visits with a diagnosis code for urolithiasis was calculated as the average annual ED visit rate. Among the entire U.S. noninstitutional population, the rate of ED visits with a diagnostic code for urolithiasis steadily increased from 178 (95% CI, 152–204) per 100,000 individuals in the 1992–1994 survey period to 340 (95% CI, 284–395) per 100,000 individuals in the 2007–2009 survey period (Figure 1), an increase of 91% (Ptrend<0.001). On the basis of a linear regression model by fitting the first five visit rates from the 1992–1994 survey period to the 2004–2006 survey period, we obtained a projected increase of 27 ED visits per 100,000 individuals by each 3-year period (rate=147+27* period). Compared with the predicted rate of 309 (95% CI, 280–337) for the 2007–2009 survey period, the observed rate of 340 (95% CI, 284–395) corresponded to an excess increase of 31 ED visits per 100,000 US population. However, the rate increase in the last period was not significantly greater than the trend seen in the previous surveys (P=0.15). Across all survey periods, ED visit rates for urolithiasis were higher in men than women (Table 1). Although urolithiasis ED visit rates increased in both genders from the survey periods 1992–1994 to 2007–2009, the percentage of increase in women (128%) was nearly twice that of men (70%). In 1992–1994, the highest rate of ED visits for urolithiasis was in individuals aged 45–64 years old. In all subsequent surveys, the highest rate was in individuals aged 25–44 years old. The increase in ED urolithiasis visit rates in each age group over the various surveys was statistically significant, except for individuals >64 years of age. The greatest increase (132%) was found in the 25–44 year age group. The ED visit rate for urolithiasis among whites was about twice that of non-whites for all survey periods except 2001–2003, where the white to non-white rate ratio was nearly 3:1.Table 1Rates of emergency department visits with a diagnosis of upper tract urolithiasis by gender, age, and race: 1992–20091992–19941995–19971998–20002001–20032004–20062007–2009No.RateaRate per 100,000 based on noninstitutional population estimates from National Hospital Ambulatory Medical Care Survey micro-data file documentation: 1992–2009.95% CINo.RateaRate per 100,000 based on noninstitutional population estimates from National Hospital Ambulatory Medical Care Survey micro-data file documentation: 1992–2009.95% CINo.RateaRate per 100,000 based on noninstitutional population estimates from National Hospital Ambulatory Medical Care Survey micro-data file documentation: 1992–2009.95% CINo.RateaRate per 100,000 based on noninstitutional population estimates from National Hospital Ambulatory Medical Care Survey micro-data file documentation: 1992–2009.95% CINo.RateaRate per 100,000 based on noninstitutional population estimates from National Hospital Ambulatory Medical Care Survey micro-data file documentation: 1992–2009.95% CINo.RateaRate per 100,000 based on noninstitutional population estimates from National Hospital Ambulatory Medical Care Survey micro-data file documentation: 1992–2009.95% CIPercent changebPercent change between 1992–1994 and 2007–2009.PtrendcBased on weighted least squares regression analysis.Total435178152, 204326190158, 223400231194, 268731263226, 299708277236, 317803340284, 39591<0.001Sex Male272231189, 274198240193, 288264301250, 351437331275, 387417325276, 374441393322, 464700.002 Female16312799, 154128143109, 176136165127, 203294197164, 231291230187, 273362289235, 343128<0.001Age <25516340, 86397244, 99478255, 110777349, 9610512090, 15010111283, 141780.030 25–44208258205, 310168303237, 370196384305, 462340444368, 521303433345, 521366599472, 7251320.002 45–64139287223, 35083240175, 305125333249, 416258367299, 435234344271, 417275431345, 516500.029 >643713370, 1973616995, 2433211269, 15456176120, 23166224161, 28861216138, 294620.092Race White391191163, 219299217178, 256354252211, 294657300256, 344629309263, 355710384318, 450101<0.001 Non-white4411261, 16327——4613484, 1847410470, 1387914598, 19293165118, 213470.21Abbreviations: CI, confidence interval; No., unweighted visit number.—: Data not shown have fewer than 30 records in the corresponding category. Estimates are considered unreliable.a Rate per 100,000 based on noninstitutional population estimates from National Hospital Ambulatory Medical Care Survey micro-data file documentation: 1992–2009.b Percent change between 1992–1994 and 2007–2009.c Based on weighted least squares regression analysis. Open table in a new tab Abbreviations: CI, confidence interval; No., unweighted visit number. —: Data not shown have fewer than 30 records in the corresponding category. Estimates are considered unreliable. Although ED visit rates varied by season and by region of the country, none of these differences were statistically significant (Figure 2). The proportion of ED urolithiasis visits with any imaging used increased from 56% (95% CI, 48–64%) in 1995–1997 to 79% (75–83%) in 2007–2009 (Ptrend=0.015) (Table 2). The percent of ED urolithiasis visits at which X-rays were ordered decreased from 48% (95% CI, 40–55%) in 1995–1997 to 17% (12–22%) in 2007–2009 (Ptrend=0.005). In contrast, the use of computed tomography (CT) increased from 21% (95% CI, 15–26%) in 1998–2000 to 71% of visits (65–77%) in 2007–2009 (Ptrend=0.029). The frequency of the use of ultrasound was low: 5–6% in the survey periods 2001–2003 and 2004–2006.Table 2Percentage of use of imaging and drugs at emergency department visits with a diagnosis of upper tract urolithiasis: 1992–20091992–19941995–19971998–20002001–20032004–20062007–2009%95% CI%95% CI%95% CI%95% CI%95% CI%95% CIPercent changeaPercent change between 1995–1997 and 2007–2009 in any imaging and X-ray. Percent change between 2001–2003 and 2004–2006 in ultrasound. Percent change between 1998–2000 and 2007–2009 in CT. Percent change between 1992–1994 and 2007–2009 in medication use.PtrendbBased on weighted least squares regression analysis.Imaging use Any imagingcX-ray, ultrasound, or CT was ordered/provided at visit.NANA5648, 645549, 606661, 717874, 817975, 83410.015 X-rayNANA4840, 553428, 402824, 322117, 261712, 22-650.005 UltrasoundNANA————64, 853, 7——-17NA CTNANA——2115, 264438, 496560, 707165, 772380.029Medication use Visits with medication provided or prescribeddVisits at which one or more medications were provided or prescribed.8884, 929086, 958985, 939593, 979593, 979593, 9780.020 Visits with analgesics provided or prescribedeBased on National Drug Code directory (17XX) in 1992–2005 and Multum Lexicon second-level therapeutic drug category (058) in 2006–2009.7773, 818479, 908278, 869189, 949087, 939189, 94180.022 Visits with MET provided or prescribedNANANANANANA————1410, 17NANA Number of medications provided or prescribed (mean, 95% CI)2.11.9, 2.22.42.2, 2.62.12.0, 2.32.82.7, 3.03.13.0, 3.33.43.1, 3.6620.008Abbreviations: CI, confidence interval; CT, computed tomography; MET, medical expulsive therapy; NA, not available.—: Data not shown have fewer than 30 records or have a relative s.e. >30% in the corresponding category.Estimates are considered unreliable.a Percent change between 1995–1997 and 2007–2009 in any imaging and X-ray. Percent change between 2001–2003 and 2004–2006 in ultrasound. Percent change between 1998–2000 and 2007–2009 in CT. Percent change between 1992–1994 and 2007–2009 in medication use.b Based on weighted least squares regression analysis.c X-ray, ultrasound, or CT was ordered/provided at visit.d Visits at which one or more medications were provided or prescribed.e Based on National Drug Code directory (17XX) in 1992–2005 and Multum Lexicon second-level therapeutic drug category (058) in 2006–2009. Open table in a new tab Abbreviations: CI, confidence interval; CT, computed tomography; MET, medical expulsive therapy; NA, not available. —: Data not shown have fewer than 30 records or have a relative s.e. >30% in the corresponding category. Estimates are considered unreliable. Among urolithiasis ED visits with any imaging, the proportion of CT or X-ray/ultrasound use in the survey periods 1998–2003 and 2004–2009 is shown in Figure 3. In 1998–2003, approximately half the visits used CT, increasing to 88% in 2004–2009. The use of CT increased from 19% (95% CI, 11–27%) in 1998–2000 to 73% (65–80%) in 2007–2009 in patients aged 25–44 years old (Ptrend=0.018) and from 42% (28–56%) in 2001–2003 to 64% (51–77%) in patients <25 years in 2007–2009 (Ptrend<0.001) (Table 3). The use of CT did not differ proportionally between men and women with urolithiasis visits during any of the time periods assessed, but increased in men (Ptrend=0.028). The use of CT did not differ proportionally between white and non-white patients with urolithiasis visits during any of the time periods assessed, but the proportion of use increased significantly in white patients (Ptrend=0.036).Table 3Percentage of computed tomography use at emergency department visits with a diagnosis of upper tract urolithiasis, by selected characteristics: 1998–20091998–20002001–20032004–20062007–2009%95% CI%95% CI%95% CI%95% CIPtrendaBased on weighted least squares regression analysis.Total2115, 264438, 496560, 707165, 770.029Age <25——4228, 565341, 656451, 77<0.001 25–441911, 273932, 476457, 727365, 800.018 45–64——4739, 557162, 797468, 800.24 >64————7257, 866045, 74NA P valuebBased on Wald χ2-tests.NA0.230.110.21Sex Female——4437, 516155, 686860, 760.20 Male2416, 314436, 516862, 757468, 790.028 P valuebBased on Wald χ2-tests.NA0.990.150.14Race White2115, 274438, 506560, 707074, 760.036 Non-white——4228, 576652, 808068, 920.10 P valuebBased on Wald χ2-tests.NA0.880.880.11Abbreviations: CI, confidence interval; NA, not available.—: Data not shown have fewer than 30 records or have a relative s.e. >30% in the corresponding category.Estimates are considered unreliable.a Based on weighted least squares regression analysis.b Based on Wald χ2-tests. Open table in a new tab Abbreviations: CI, confidence interval; NA, not available. —: Data not shown have fewer than 30 records or have a relative s.e. >30% in the corresponding category. Estimates are considered unreliable. The proportion of ED visits for codes indicating other disorders of the urethra and urinary tract, symptoms involving the urinary system, and other symptoms involving the abdomen and pelvis with any imaging used increased from 38% (95% CI, 35–40%) in 1995–1997 to 55% (53–57%) in 2007–2009 (Ptrend<0.001). CT use in this group increased from 2% (95% CI, 2–3%) in 1995–1997 to 30% (28–32%) in 2007–2009 (Ptrend<0.001) (data not shown). Prescribed drugs were commonly used over the survey periods (range 88–95%). Analgesic prescriptions increased from 77% (95% CI, 73–81%) to 91% (89–94%) (Ptrend=0.022) of visits. Medical expulsive therapy (MET) use was infrequent before 2007. In 2007–2009, MET was prescribed in 14% (95% CI, 10–17%) of ED urolithiasis visits. The mean number of prescribed drugs per ED urolithiasis visit increased from 2.1 (95% CI, 1.9–2.2) in 1992–1994 to 3.4 (3.1–3.6) in 2007–2009 (Ptrend=0.008). In the survey period 2001–2003, 5% (95% CI, 3–7%) of urolithiasis ED visits were return visits, increasing to 10% (7–13%) in the survey period 2007–2009. A total of 528 NHANES 2007–2008 participants reported a history of kidney stones. Among them, 22.4% (95% CI, 17.3–28.6%) reported that they had passed three or more stones and 13.7% (9.7–19.0%) reported having passed four or more stones. These findings suggest that ED visits resulting in a diagnosis of urolithiasis may include a substantial number of individuals with recurrent episodes of kidney stones. Over nearly 20 years beginning in the early 1990s, the rate of ED visits with a diagnosis of upper tract urolithiasis in the United States has steadily increased, nearly doubling from 178 to 340/100,000 population. In contrast, total ED visit rates increased by only 16% over the same time period.18Burt C.W. McCaig L.F. Trends in hospital emergency department utilization: United States, 1992–99.Vital Health Stat. 2001; 13: 1-34Google Scholar,19NHAMCS National Hospital Ambulatory Medical Care Survey: 2008 emergency department summary tables.http://www.cdc.gov/nchs/data/ahcd/nhamcs_emergency/nhamcsed2008.pdfGoogle Scholar Thus, the increase in ED urolithiasis visits was more than five times greater than the proportional increase for all ED visits. The greatest increases in ED urolithiasis visit rates occurred among women, whites, and individuals aged 25–44 years old. The use of all prescription drugs, including analgesics, increased during the study period. The frequency of the use of CT to aid in the diagnosis and management of urolithiasis in the ED also increased significantly. Because the use of MET began in 2007–2009, we are unable to report time trends. Several recent US studies have also documented an increase in the use of health-care resources7Pearle M.S. Calhoun E.A. Curhan G.C. Urologic diseases in America project: urolithiasis.J Urol. 2005; 173: 848-857Abstract Full Text Full Text PDF PubMed Scopus (642) Google Scholar,20Hyams E.S. Korley F.K. Pham J.C. et al.Trends in imaging use during the emergency department evaluation of flank pain.J Urol. 2011; 186: 2270-2274Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar and in the incidence and prevalence15Penniston K.L. McLaren I.D. Greenlee R.T. et al.Urolithiasis in a rural Wisconsin population from 1992 to 2008: narrowing of the male-to-female ratio.J Urol. 2011; 185: 1731-1736Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 16Sas D.J. Hulsey T.C. Shatat I.F. et al.Increasing incidence of kidney stones in children evaluated in the emergency department.J Pediatr. 2010; 157: 132-137Abstract Full Text Full Text PDF PubMed Scopus (174) Google Scholar, 17Dwyer M.E. Krambeck A.E. Bergstralh E.J. et al.Temporal trends in incidence of kidney stones among children: a 25-year population based study.J Urol. 2012; 188: 247-252Abstract Full Text Full Text PDF PubMed Scopus (206) Google Scholar of urolithiasis. In a study using the NHAMCS, hospital outpatient urolithiasis visits increased nationally by 40% between 1994 and 2000.7Pearle M.S. Calhoun E.A. Curhan G.C. Urologic diseases in America project: urolithiasis.J Urol. 2005; 173: 848-857Abstract Full Text Full Text PDF PubMed Scopus (642) Google Scholar Although we confirmed prior studies that show urolithiasis to be more common in men, we found the percentage of increase in ED visits to be nearly twice as great in women. This finding is consistent with prior reports of an increase in incident symptomatic stone disease15Penniston K.L. McLaren I.D. Greenlee R.T. et al.Urolithiasis in a rural Wisconsin population from 1992 to 2008: narrowing of the male-to-female ratio.J Urol. 2011; 185: 1731-1736Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar,21Lieske J.C. Pena de la Vega L.S. Slezak J.M. et al.Renal stone epidemiology in Rochester, Minnesota: an update.Kidney Int. 2006; 69: 760-764Abstract Full Text Full Text PDF PubMed Scopus (201) Google Scholar and hospital discharges for ureteral and renal calculi.22Strope S.A. Wolf J.S. Hollenbeck B.K. Changes in gender distribution of urinary stone disease.Urology. 2010; 75: 543-546Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar,23Scales C.D. Curtis L.H. Norris R.D. et al.Changing gender prevalence of stone disease.J Urol. 2007; 177: 979-982Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar In contrast, we did not observe regional or seasonal differences in ED visit rates for urolithiasis, using a large, comprehensive national survey. However, the rates for both 9-year survey periods were numerically highest in summer and in the South, in agreement with several other studies.2Stamatelou K.K. Francis M.E. Jones C.A. et al.Time trends in reported prevalence of kidney stones in the United States: 1976-1994.Kidney Int. 2003; 63: 1817-1823Abstract Full Text Full Text PDF PubMed Scopus (1142) Google Scholar, 5Soucie J.M. Coates R.J. McClellan W. et al.Relation between geographic variability in kidney stones prevalence and risk factors for stones.Am J Epidemiol. 1996; 143: 487-495Crossref PubMed Scopus (171) Google Scholar, 6Chauhan V. Eskin B. Allegra J.R. et al.Effect of season, age, and gender on renal colic incidence.Am J Emerg Med. 2004; 22: 560-563Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar The increase in ED visits with urolithiasis diagnoses might result from a combination of factors including an actual change in disease incidence, improvement in diagnostic methods, and/or an increase in the number of people who use the ED for primary medical care. However, we cannot identify the reasons for this increase from our data. We believe the contribution of enhanced detection as a result of improved imaging technology on the increase in urolithiasis was likely modest, as several recent studies, using the NHAMCS, have found that the increased use of CT was not associated with a change in the proportion of diagnosis of kidney stones among ED patients presenting with acute flank or groin pain.20Hyams E.S. Korley F.K. Pham J.C. et al.Trends in imaging use during the emergency department evaluation of flank pain.J Urol. 2011; 186: 2270-2274Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar,24Westphalen A.C. Hsia R.Y. Maselli J.H. et al.Radiological imaging of patients with suspected urinary tract stones: national trends, diagnoses, and predictors.Acad Emerg Med. 2011; 18: 699-707Crossref PubMed Scopus (131) Google Scholar Moreover, although we could not exclude the possibility that more people with kidney stone episodes use the ED for primary medical care, we would not expect most of the increases observed in our study to be because of this reason. Recent work suggests adults with Medicaid accounted for most of the increase in ED visits from 1997 to 2007 because of their increasing difficulties in accessing primary care.25Tang N. Stein J. Hsia R.Y. et al.Trends and characteristics of US emergency department visits, 1997-2007.JAMA. 2010; 304: 664-670Crossref PubMed Scopus (581) Google Scholar As we could not envision that patients with kidney stones are more likely to be Medicaid recipients, and the increase in ED visits for urolithiasis was much greater than the increase for all ED visits, our findings are consistent with other reports of increased hospital outpatient visits for urolithiasis.7Pearle M.S. Calhoun E.A. Curhan G.C. Urologic diseases in America project: urolithiasis.J Urol. 2005; 173: 848-857Abstract Full Text Full Text PDF PubMed Scopus (642) Google Scholar Because medical, lifestyle, or dietary information on individual patients was not collected in NHAMCS, we can only speculate on the reasons for the increased urolithiasis rate. Over the period of the surveys, three important risk factors for urolithiasis, obesity,26Taylor E.N. Stampfer M.J. Curhan G.C. Obesity, weight gain, and the risk of kidney stones.JAMA. 2005; 293: 455-462Crossref PubMed Scopus (845) Google Scholar diabetes,27Taylor E.N. Stampfer M.J. Curhan G.C. Diabetes mellitus and the risk of nephrolithiasis.Kidney Int. 2005; 68: 1230-1235Abstract Full Text Full Text PDF PubMed Scopus (383) Google Scholar and the use of dietary supplemental calcium28Wallace R.B. Wactawski-Wende J. O'Sullivan M.J. et al.Urinary tract stone occurrence in the Women's Health Initiative (WHI) randomized clinical trial of calcium and vitamin D supplements.Am J Clin Nutr. 2011