C Y Pak

University of Texas Southwestern Medical Center, Dallas, TX, United States

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Publications (310)1624.38 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Background and objectives: We compared 3 animal protein sources for their effect on urinary stone risk. Design: 15 healthy subjects completed a 3 phase, randomized crossover metabolic study. During each 1-week phase, subjects consumed a standard, metabolic diet containing beef, chicken or fish. Serum chemistries and 24-hour urine samples collected at the end of each phase were compared using a mixed model repeated measures analysis. Results: Serum and urinary uric acid were elevated for each phase. Beef was associated with lower serum uric acid values than either chicken or fish (6.5 mg/dl vs 7.0 mg/dl and 7.3 mg/dl, respectively, p<0.05 for both) and fish (741 mg/day) was associated with higher levels of urinary uric acid than either beef (638 mg/day, p = 0.003) or chicken (641 mg/day, p = 0.04). No significant differences in urinary pH, sulfate, calcium, citrate, oxalate or sodium were noted among phases. Mean saturation index for calcium oxalate was highest for beef (2.48), although the difference reached significance only compared to chicken (1.67, p = 0.02) but not to fish (1.79, p = 0.08). Conclusions: Consumption of animal protein is associated with elevated serum and urine uric acid levels in healthy individuals. The higher purine content of fish compared to beef or chicken is reflected in higher 24-hour urinary uric acid levels. However, stone-forming propensity, as reflected in saturation index, is marginally highest for beef compared to fish or chicken. Stone formers should be advised to limit their intake of all animal proteins, including fish.
    The Journal of urology 07/2014; · 3.75 Impact Factor
  • Khashayar Sakhaee, Charles Pak
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    ABSTRACT: BACKGROUND: Calcium supplementation is commonly recommended for patients after Roux-en-Y gastric bypass to avert bone loss. To test the hypothesis that effervescent (liquid) potassium-calcium-citrate (PCC) might be more bioavailable than a tablet formulation of calcium citrate (Citracal Petite), the present study compared a single dose response of the 2 compounds. The present study was conducted at the University of Texas Southwestern Medical School at Dallas. METHODS: A total of 15 patients who had undergone Roux-en-Y gastric bypass were included in a 2-phase, crossover, randomized study comparing the single-dose bioavailability of PCC versus Citracal Petite. After following a restricted diet for 1 week, the participants ingested either a single dose of 400 mg elemental calcium as PCC or Citracal Petite. Sequential serum and urine samples were collected for a 6-hour period after the dose and analyzed for calcium, parathyroid hormone, and acid-base parameters. RESULTS: Compared with citracal petite, PCC significantly increased the serum calcium concentrations at 2, 3, and 4 hours after the oral load. The peak to baseline variation and increment in serum calcium (area under the curve) were significantly greater after PCC (P = .015 and P = .002, respectively). Concurrently, the baseline to nadir variation and decrement in serum parathyroid hormone (area over the curve) were significantly greater after PCC (P = .004 and P = .005, respectively). Moreover, compared with Citracal Petite, PCC caused a significantly greater increment in urinary citrate (P < .0001) and potassium (P = .0004) and a significantly lower increase in urinary ammonium (P = .045). CONCLUSION: In patients who have undergone Roux-en-Y gastric bypass, PCC was superior to Citracal Petite in conferring bioavailable calcium and suppressing parathyroid hormone secretion. PCC also provided an alkali load.
    Surgery for Obesity and Related Diseases 12/2011; · 4.12 Impact Factor
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    ABSTRACT: The classic definition of hypercalciuria, an upper normal limit of 200  mg/day, is based on a constant diet restricted in calcium, sodium, and animal protein; however, random diet data challenge this. Here our retrospective study determined the validity of the classic definition of hypercalciuria by comparing data from 39 publications analyzing urinary calcium excretion on a constant restricted diet and testing whether hypercalciuria could be defined when extraneous dietary influences were controlled. These papers encompassed 300 non-stone-forming patients, 208 patients with absorptive hypercalciuria type I (presumed due to high intestinal calcium absorption), and 234 stone formers without absorptive hypercalciuria; all evaluated on a constant restricted diet. In non-stone formers, the mean urinary calcium was well below 200  mg/day, and the mean for all patients was 127±46  mg/day with an upper limit of 219  mg/day. In absorptive hypercalciuria type I, the mean urinary calcium significantly exceeded 200  mg/day in all studies with a combined mean of 259±55  mg/day. Receiver operating characteristic curve analysis showed the optimal cutoff point for urinary calcium excretion was 172  mg/day on a restricted diet, a value that approximates the traditional limit of 200  mg/day. Thus, on a restricted diet, a clear demarcation was seen between urinary calcium excretion of kidney stone formers with absorptive hypercalciuria type I and normal individuals. When dietary variables are controlled, the classic definition of hypercalciuria of nephrolithiasis appears valid.
    Kidney International 07/2011; 80(7):777-82. · 8.52 Impact Factor
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    ABSTRACT: Patients undergoing Roux-en-Y gastric bypass (RYGB) surgery are prone to developing bone loss and kidney stones. The goal of the present study was to test the hypothesis that an effervescent formulation of potassium calcium citrate (PCC) would avert metabolic complications by providing bioavailable calcium and alkali. A total of 24 patients with RYGB underwent a 2-phase crossover randomized trial comparing PCC and placebo. During the last 2 days of each 2-week phase, the serum and 24-hour urine samples were analyzed for calcium and bone turnover markers, acid base status, and urinary stone risk factors. Compared with placebo, PCC marginally reduced the serum parathyroid hormone level and significantly decreased urinary deoxypyridinoline by 12% (P <.001) and serum type 1 collagen C-telopeptide by 22% (P <.01). PCC significantly increased the net gastrointestinal alkali absorption, citrate, and pH and significantly lowered the urinary net acid excretion (P <.001). The urinary saturation of uric acid decreased significantly (P <.001). The supersaturation of calcium oxalate and brushite did not change despite an increase in calcium and pH. In untreated urine samples with citrate concentrations altered to mimic those of placebo and PCC, calcium oxalate agglomeration was significantly inhibited by PCC. In RYGB patients, PCC supplementation inhibited bone resorption by providing bioavailable calcium, reduced the urinary saturation of uric acid, and increased the inhibitor activity against calcium oxalate agglomeration by providing alkali that increased urinary pH and citrate.
    Surgery for Obesity and Related Diseases 05/2011; 8(1):67-72. · 4.12 Impact Factor
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    ABSTRACT: We retrospectively analyzed the validity of a simple method of detecting absorptive hypercalciuria type I, a common stone forming condition with hypercalciuria that is believed to be due to high intestinal calcium absorption. The method is based on urinary calcium derived from 24-hour urine collections while on random and restricted diets rather than on a calciuric response to an oral calcium load. A group of 916 well characterized patients with idiopathic calcium oxalate urolithiasis comprised the study group. We also analyzed a subgroup of 695 patients, excluding 221 with dietary abuse, defined as urinary sodium greater than 150 mEq daily and/or sulfate greater than 35 mmol daily, to eliminate potential confounding dietary factors affecting the diagnosis. In each group absorptive hypercalciuria type I was detected by the old criteria, requiring an exaggerated calciuric response to an oral calcium load test, and by the new criteria, based on 24-hour urinary calcium 200 mg or greater daily while on random and restricted diets. Using the old criteria as the gold standard the positive and negative predictive values, sensitivity and specificity of the new criteria were 80.1%, 95.9%, 90.8% and 90.5%, respectively. When excluding patients with dietary abuse the values were 85.9%, 97.2%, 92.4% and 94.5%, respectively. Absorptive hypercalciuria type I may be reliably detected by a simple method based on high 24-hour urinary calcium while on random and restricted diets, especially when excluding patients with evidence of dietary abuse during the restricted diet.
    The Journal of urology 03/2011; 185(3):915-9. · 3.75 Impact Factor
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    ABSTRACT: An abnormal urinary pH (UpH) represents an important risk factor for nephrolithiasis. In some stone formers, a fasting urine specimen is obtained instead of a 24-h urine collection for stone risk evaluation. We examined the relationship between 24-h and fasting UpH in non-stone forming individuals and stone formers with various etiologies and a wide range of urine pH to test the validity of fasting UpH. Data from 159 subjects was examined in this retrospective study. We included non-stone forming subjects and stone formers with hypercalciuria, distal renal tubular acidosis, idiopathic uric acid nephrolithiasis, or chronic diarrhea. Participants collected a 24-h urine followed by a 2-h fasting urine. For the entire cohort, a significant correlation was seen between fasting and 24-h UpH (r (2) = 0.49, p < 0.001). Fasting pH was significantly higher than 24-h UpH for the entire cohort (6.02 ± 0.63 vs. 5.89 ± 0.51; p < 0.001), and in the subgroups of non-stone formers and stone formers with hypercalciuria or distal renal tubular acidosis. Fasting UpH was >0.2 pH units different from 24-h UpH in 58% of participants. The difference between fasting and 24-h UpH did not correlate with net gastrointestinal alkali absorption or urine sulfate, suggesting that dietary factors alone cannot explain this difference in UpH. Fasting urine pH correlates moderately with 24-h urine pH in a large cohort of individuals. Significant variability between these two parameters is seen in individual patients, emphasizing the cardinal role of 24-h urine collection for evaluating UpH in nephrolithiasis.
    Urological Research 02/2011; 39(5):367-72. · 1.59 Impact Factor
  • Journal of Urology - J UROL. 01/2011; 185(4).
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    ABSTRACT: The objective of this retrospective data analysis was to test the hypothesis that absorptive hypercalciuria Type II (AH-II) is a less severe variant of absorptive hypercalciuria Type I (AH-I), a common cause of calcareous stones. 24-h urinary calcium obtained on constant metabolic diets was retrieved from several data sources, including those of the authors and another group. On a low calcium diet (10 mmol calcium), 35 patients with AH-II were compared with 70 non-stone formers (NSF) and 76 patients with AH-I. On a high calcium diet (25 mmol calcium/day), 10 patients with AH-II were compared with 35 NSF and 32 with AH-I. On a low calcium diet for all participants, 24-h urinary calcium in AH-II (4.13 ± 0.63 mmol/day) was significantly higher than in NSF (3.06 ± 1.17 mmol/day), but significantly lower than in AH-I (6.11 ± 1.14 mmol/day) (p < 0.001). In a smaller subset, fractional intestinal calcium absorption in AH-II (65.0 ± 11.1%) was intermediate between NSF (50.0 ± 6.4%) and AH-I (71.0 ± 6.7%) (p < 0.001 between AH-II and other groups). On a high calcium diet, the rise in urinary calcium in AH-II was significantly higher than in NSF, but not as marked as in AH-I. Estimated calcium balance in AH-II was similar to NSF, but significantly more positive than AH-I. In conclusion, AH-II shares with AH-I the same metabolic disturbance(s) stimulating intestinal absorption and renal excretion of calcium but to a lesser degree. Bone might be spared in AH-II.
    Urological Research 11/2010; 39(2):147-52. · 1.59 Impact Factor
  • Journal of bone and mineral research: the official journal of the American Society for Bone and Mineral Research 12/2009; 11(5):561 - 564. · 6.04 Impact Factor
  • Joseph E. Zerwekh, Peter Antich, Charles Y. C. Pak
    Journal of bone and mineral research: the official journal of the American Society for Bone and Mineral Research 12/2009; 11(9):1370 - 1371. · 6.04 Impact Factor
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    ABSTRACT: Estimating calcium oxalate saturation in human urine is critical for nephrolithiasis clinical research and practice. The Joint Expert Speciation System (Mayhem Unit Trust and Council for Scientific and Industrial Research, Pretoria, South Africa) computer program has questioned the validity of the widely used EQUIL 2 program in estimating calcium oxalate urinary saturation. To attempt resolution the computer model based supersaturation index (by the Joint Expert Speciation System) and the relative saturation ratio (by EQUIL 2) were compared with the experimentally derived activity product ratio, that is the ratio of activity products before and after incubating urine with synthetic calcium oxalate. To determine the experimental conditions required to attain calcium oxalate steady state solubility the filtrate concentration product of calcium and oxalate was determined after incubating 8 urine samples with 2 to 15 mg/ml calcium oxalate for various intervals. In 20 urine samples the activity product ratio of calcium oxalate was compared with the relative saturation ratio and the supersaturation index. Steady state solubility occurred after incubating 15 mg calcium oxalate per ml urine for 72 hours. The mean +/- SD supersaturation index of 4.92 +/- 2.57 in the original urine samples closely approximated the activity product ratio of 5.08 +/- 3.10 but the relative saturation ratio of 7.47 +/- 3.89 was significantly higher than the activity product ratio. The supersaturation index was recalculated after omitting soluble complexes recognized by the Joint Expert Speciation System but not by EQUIL 2, including Ca(2)H(2)(PO(4))(2) and (CaCitPO(4))(4-). The corrected supersaturation index was compared with the relative saturation ratio. After correction the supersaturation index increased to 7.28 +/- 3.81, which was not significantly different from the relative saturation ratio. The Joint Expert Speciation System is more accurate than EQUIL 2 to estimate calcium oxalate saturation, probably by accounting for Ca(2)H(2)(PO(4))(2), (CaCitPO(4))(4-), and other minor complexes of calcium and oxalate.
    The Journal of urology 12/2009; 182(6):2951-6. · 3.75 Impact Factor
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    ABSTRACT: The Westernized diet is acidogenic due to the high content of sulfur-containing amino acids and relative deficiency of potassium organic anions. Chronic acid loads result in hypercalciuria and negative calcium balance often associated with loss of bone mineral. Alkali therapy tends to reverse the hypercalciuria but little is known regarding its effect on bone as assessed by bone histomorphometry. The present study utilized dynamic bone histomorphometry to evaluate the effects of alkali therapy on acid-induced changes in bone turnover. Serum and urine analyses and bone histomorphometry were assessed in adult rats after 2 months of either a low casein (LC) or high casein (HC) diet supplemented with either potassium chloride (KCl) or potassium citrate (KCit). Compared to animals on LC-KCl diet, HC-KCl diet delivered a substantial acid load as shown by significant increases in urinary sulfate, ammonium, and net acid excretion, and a lower urinary pH and citrate excretion without detectable changes in serum parameters. The acid load also resulted in hypercalciuria. Dynamic and static bone histomorphometry disclosed a significant reduction in cancellous bone volume and trabecular number associated with a 2.5-fold increase in eroded and a 3.5-fold increase in osteoclastic surfaces. There was also a near 2-fold increase in bone formation rate in rats on the HC-KCl diet. When animals on the HC diet were given KCit instead of KCl, all of the aforementioned changes in urine biochemistry and bone turnover were significantly attenuated or entirely prevented. These findings underscore the deleterious effects of high animal protein intake in promoting hypercalciuria and increasing bone turnover. Co-administration of potassium alkali attenuates or prevents these changes. In this animal model of high dietary animal protein intake, the major skeletal effect of alkali therapy is to reduce bone resorption, with little or no effect on bone formation.
    Bone 08/2009; 45(5):1004-9. · 4.46 Impact Factor
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    ABSTRACT: The Equil 2 computer program has been questioned by the new Joint Expert Speciation System program (Mayhem Unit Trust and Council for Scientific and Industrial Research, Pretoria, South Africa) for estimating the urinary saturation of stone forming salts to gauge the propensity for stone formation. To attempt resolution the supersaturation index according to the Joint Expert Speciation System and the relative saturation ratio according to Equil 2 were compared with the semi-empirically derived concentration-to-product ratio. Data were obtained from a recent article in The Journal of Urology(R), in which pH, calcium and citrate were varied over a wide range in 72 urine samples. We calculated the relative saturation ratio and the supersaturation index of brushite, and compared them with the available concentration-to-product ratio derived from the growth or dissolution of synthetic brushite. The mean concentration-to-product ratio did not differ from the supersaturation index but the concentration-to-product ratio and the supersaturation index were significantly lower than the relative saturation ratio (p <0.004). On the saturation value and urinary variable plot the relative saturation ratio could be readily distinguished from the concentration-to-product ratio because it was consistently and significantly higher. While the supersaturation index pattern was similar to the concentration-to-product ratio, the supersaturation index was slightly lower at high urinary pH and calcium, and slightly higher at lower urinary pH and calcium (p <0.001). When the Ca(2)H(2)(PO(4))(2) complex was deleted from the Joint Expert Speciation System, the corrected supersaturation index was not significantly different from the relative saturation ratio determined by Equil 2. The relative saturation ratio overestimates brushite saturation by about 80%. The supersaturation index yields a good approximation of brushite saturation at modest degrees of saturation but it overestimates saturation at low pH or calcium (low saturation) and underestimates it at high pH or calcium (high saturation).
    The Journal of urology 01/2009; 181(3):1423-8. · 3.75 Impact Factor
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    ABSTRACT: Hypercalciuria of intestinal origin has been linked with bone loss in calcium nephrolithiasis and idiopathic osteoporosis. This retrospective data analysis was performed to explore potential pathogenetic link between intestinal hyperabsorption of calcium and postmenopausal osteoporosis. Data were retrieved from postmenopausal women who were evaluated for osteoporosis or osteopenia at the Mineral Metabolism Clinic of UT Southwestern Medical Center. A total of 319 patients underwent the test of calciuric response to oral calcium load to obtain an indirect measure of intestinal calcium absorption. Serum and urinary biochemistry and L2-L4 bone mineral density (BMD) were compared between five quintiles of calciuric response. There was a statistically significant trend toward a rise in 24-h urinary calcium and a decrease in urinary deoxypyridinoline (DPD) and BMD, with increasing order of quintiles. The presentation of those in the 1st quintile was consistent with vitamin D insufficiency or deficiency, with impaired calcium absorption, secondary hyperparathyroidism, and stimulated bone turnover (high normal urinary DPD). In contrast, patients in the 5th quintile displayed a picture of absorptive hypercalciuria of stone disease, with intestinal hyperabsorption of calcium, high or high normal urinary calcium and suppressed bone turnover (low or low normal urinary DPD). Thus, the assessment of intestinal calcium absorption in a seemingly homogeneous group of postmenopausal women with osteoporosis or osteopenia revealed a spectrum of calciuric response whose extremes may represent two physiologically distinct subtypes that have important diagnostic and therapeutic implications.
    Urological Research 10/2008; 36(5):233-9. · 1.59 Impact Factor
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    ABSTRACT: Brushite crystallization might be important in stone formation and prevention. To explore this question new methods for the saturation and crystal growth of brushite were devised that are applicable to whole urine without any computer program. The saturation value (concentration-to-product ratio) was determined by dividing the molar concentration product of Ca ([Ca]) and phosphate ([P]), that is [Ca] x [P], of original urine by the steady state solubility obtained after incubating with an excess of brushite (10 mg/ml) for 5 hours. Crystal growth was measured from the depletion of filtrate ([Ca] x [P]) 3 hours after seeding with brushite (0.25 mg/ml). To test the effect of pH, Ca and citrate the saturation value and crystal growth were determined in 24-hour urine samples from 4 normal volunteers and 2 stone formers, and modified artificially to produce 4 ranges of pH, Ca and citrate by adding acid, base, Ca or citrate. The saturation value and crystal growth of brushite increased with an increase in pH or the Ca concentration but they decreased when the citrate concentration increased. The saturation value correlated strongly with crystal growth. The new methods of brushite saturation value and crystal growth should help discern how abnormalities in urinary pH, Ca and citrate interact to influence the formation of Ca stones in cases of distal renal tubular acidosis and alkali therapy.
    The Journal of urology 09/2008; 180(4):1532-7. · 3.75 Impact Factor
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    Charles Y C Pak
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    ABSTRACT: The urological community has had a vital role in the author's 35 years of research on the medical management of urolithiasis. The goal of this article is to review the progress made from the perspective of collaborating urologists and urological journals in which the findings were reported. The author's work appeared in 94 articles in urological journals, including 63 in The Journal of Urology, and in 28 other journals with collaborating urologists. Progress on various aspects of medical management of stone disease was reviewed based on these articles. Pathophysiological exploration was performed by elucidating metabolic-dietary etiologies of hypocitraturia, separating hypercalciuria into 3 types, and linking gouty diathesis (uric acid stones) with obesity and insulin resistance. Physicochemical consequences of hypocitraturia were delineated and semi-empirical methods were developed to assess calcium salt saturation. Potassium-rich fruit juices differed from potassium-poor fruit juices and excessive salt intake increased the stone forming risk. Vital to diagnostic separation was a comprehensive analysis of urine for stone risk factors. As an example of selective treatment, potassium citrate was shown to be useful for controlling uric acid stones by urinary alkalinization as well as calcareous stones by hypercitraturia. During the last 35 years much progress has been made on the pathophysiology of stone formation, crystallization of stone forming salts, diagnostic separation and prevention of stone recurrence. The author's contribution in this effort would not have been possible without the active participation and support of the urological community.
    The Journal of urology 09/2008; 180(3):813-9. · 3.75 Impact Factor
  • Charles Y C Pak
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    ABSTRACT: Kidney stones are associated with various biochemical disturbances in urine. Various drugs and dietary changes have been recommended to halt stone recurrence. To determine whether a correction of urinary abnormalities by appropriate pharmacological agents and dietary modification may ameliorate stone disease. A review of the literature was conducted on the role of diet, metabolic disorders and drugs for stone prevention. Metabolic risk factors are hypercalciuria, hypocitraturia, hyperoxaluria, hyperuricosuria, and abnormally low urinary pH. Dietary-environmental risk factors include high urinary sodium and low urine volume. Rare or less commonly encountered risk factors are high urinary cystine, and alkaline urine from an infection with urea-splitting organisms. To ameliorate or correct the above disturbances, all patients should be offered dietary modification, including restricted intake of sodium, oxalate and animal proteins. Useful drugs include thiazide or indapamide to control hypercalciuria, potassium citrate to correct hypocitraturia and undue urinary acidity, and allopurinol for co-existing hyperuricemia or marked hyperuricosuria.
    Expert Opinion on Pharmacotherapy 06/2008; 9(9):1509-18. · 2.86 Impact Factor
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    ABSTRACT: Urinary magnesium and pH are known to modulate urinary calcium excretion, but the mechanisms underlying these relationships are unknown. In this study, the data from 17 clinical trials in which urinary magnesium and pH were pharmacologically manipulated were analyzed, and it was found that the change in urinary calcium excretion is directly proportional to the change in magnesium excretion and inversely proportional to the change in urine pH; a regression equation was generated to relate these variables (R(2) = 0.58). For further exploration of these relationships, intravenous calcium chloride, magnesium chloride, or vehicle was administered to rats. Magnesium infusion significantly increased urinary calcium excretion (normalized to urinary creatinine), but calcium infusion did not affect magnesium excretion. Parathyroidectomy did not prevent this magnesium-induced hypercalciuria. The effect of magnesium loading on calciuria was still observed after treatment with furosemide, which disrupts calcium and magnesium absorption in the thick ascending limb, suggesting that the effect may be mediated by the distal nephron. The calcium channel TRPV5, normally present in the distal tubule, was expressed in Xenopus oocytes. Calcium uptake by TRPV5 was directly inhibited by magnesium and low pH. In summary, these data are compatible with the hypothesis that urinary magnesium directly inhibits renal calcium absorption, which can be negated by high luminal pH, and that this regulation likely takes place in the distal tubule.
    Journal of the American Society of Nephrology 05/2008; 19(8):1530-7. · 8.99 Impact Factor
  • Journal of Pediatric Urology 04/2008; 179(4):382-382. · 1.37 Impact Factor
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    ABSTRACT: Reference values for stone risk factors in 24-hour urine samples for nonstone forming children are limited. We measured urinary stone risk factors in healthy children 3 to 18 years old, and sought to determine whether the risk factors are affected by age. A total of 48 healthy subjects with no history of stone disease, endocrine abnormalities or urological surgery were recruited from the Naval Medical Center in San Diego. Subjects were then further divided into 4 age groups, each separated by 5 years. A single outpatient 24-hour urine sample was obtained and analyzed. Urine chemistries were adjusted for urinary creatinine and body weight. After excluding under collected samples 46 urine samples were analyzed. Urinary pH and volume decreased with increasing age, although the difference in pH did not reach statistical significance. Unadjusted urinary parameters failed to show statistical difference among the age groups. When adjusted for urinary creatinine and body weight all urinary parameters (calcium, oxalate, uric acid, citrate, magnesium, sodium, phosphorus and potassium) decreased with increasing age (statistically significant except for calcium). Stone risk factors in 24-hour urine samples decrease with increasing age in healthy, nonstone forming children. Normative data, derived by adjustment with urinary creatinine or body weight and stratified according to quintiles of age, should be useful in defining abnormal stone risk factors in children with stones.
    The Journal of urology 02/2008; 179(1):290-4; discussion 294. · 3.75 Impact Factor

Publication Stats

9k Citations
1,624.38 Total Impact Points

Institutions

  • 1975–2011
    • University of Texas Southwestern Medical Center
      • • Department of Internal Medicine
      • • Department of Physical Medicine and Rehabilitation
      • • Department of Urology
      • • Pak Center for Mineral Metabolism and Clinical Research
      • • Medical School
      Dallas, TX, United States
  • 2007
    • National Heart, Lung, and Blood Institute
      Maryland, United States
  • 2005
    • Mayo Clinic - Rochester
      • Department of Urology
      Rochester, Minnesota, United States
  • 2002
    • University of Chicago
      • Section of General Internal Medicine
      Chicago, IL, United States
  • 1978–2000
    • University of Texas at Dallas
      Richardson, Texas, United States
  • 1995
    • Vanderbilt University
      • Department of Internal Medicine
      Nashville, MI, United States
  • 1994
    • Mount Sinai School of Medicine
      • Department of Medicine
      Manhattan, NY, United States
  • 1989
    • Cornell University
      • Department of Medicine
      Ithaca, NY, United States
  • 1987–1988
    • Texas Tech University Health Sciences Center
      • Department of Internal Medicine
      Lubbock, TX, United States
  • 1974–1983
    • University of Texas Health Science Center at Tyler
      Tyler, Texas, United States
  • 1979
    • University of Texas Health Science Center at San Antonio
      • Department of Neurology
      San Antonio, Texas, United States