Urinary Citrate Excretion in Patients With Renal Calculi

0022-534 7/83/1296-1158$02.00/0 Vol.129, June

THE JOURNAL OF UROLOGY

Printed in U.S.A.

Copyright © 1983 by The Williams & Wilkins Co.

URINARY CITRATE EXCRETION IN PATIENTS WITH RENAL CALCULI M. MENO.N*

C. J. MAHLE

AND

From the John Cochran Veterans Administration Hospital and Division of Urology, Washington University Medical Center, St. Louis, Missouri

ABSTRACT

Urinary citrate excretion was measured with a specific enzymatic technique in normal subjects and in an unselected group of patients with recurrent calcium oxalate stones. Hypocitraturia (citrate levels less than those present in 95 per cent of the normal population) was detected in 7 of 46 patients with stones (15 per cent). Hypocitraturia was the only metabolic abnormality in 6 patients. A role for decreased urinary citrate excretion in the pathogenesis of calcareous calculi has been postulated since the 1930s, after Sabbatini's demonstration of the ability of citric acid to form a soluble complex with calcium. 1 In addition, citrate has been reported to be a direct inhibitor of calcium phosphate precipitation2 and of calcium oxalate crystal growth. 3 For these reasons urinary citrate concentrations may have important implications in renal stone formation. We herein report on the excretion of citrate by an unselected group of patients with calcium oxalate stones and appropriate control subjects. METHODS

Study groups. Group 1 consisted of 13 men and 12 women, between 20 and 40 years old, who served as healthy controls. Group 2 consisted of 6 men and 11 women considered to form a control group with no defect in citrate metabolism but with evidence of other disease. All patients were insulin-dependent diabetics attending the Diabetes Research Center outpatient clinic at Barnes Hospital. Group 3 consisted of 46 consecutive patients (41 men and 5 women, between 22 and 56 years old) referred to the urolithiasis clinic at Barnes Hospital. Of these patients 11 were evaluated after the first episode of stone disease, while 35 had a history of recurrent calcium oxalate nephrolithiasis. No patient had hypercalcemia, renal tubular acidosis, malabsorption, azotemia or urinary tract infection. All patients were evaluated according to an ambulatory protocol similar to that of Pak and associates. 4 Group 4 was composed of 16 patients who had undergone intestinal surgery, 8 of whom had recurrent calcium oxalate stone disease. Group 5 included 12 male patients with varying degrees of renal insufficiency attending the renal clinic at the Veterans Hospital. All subjects were studied in an ambulatory setting on an umestricted diet. All patients had normal serum potassium and bicarbonate levels. Urine cultures were negative in all patients. Normal controls had no history of urinary tract infection and no evidence of bacteriuria on microscopic examination of the urinary sediment. All stones were analyzed by a commercial laboratory and were established to be predominantly calcium oxalate in composition. Urine samples were analyzed immediately after collection or after acidification to pH 1 and freezing. Frozen samples were thawed and heated to 50C for 30 minutes before analysis. Routine chemical analyses were performed by

the clinical chemistry laboratories at Barnes Hospital. Citrate was measured by enzymatic analysis,5 oxalate by ion chromatography6 and calcium by atomic absorption spectrophotometry7 in the urolithiasis research laboratory. Statistical analysis was performed by the Mann-Whitney U test. Allowance for multiple testing was made by Bonferroni's inequality method and a p value <0.01 was considered significant.8 RESULTS

Total urinary citrate. Mean urinary citrate was 2.29 mmol. per 24 hours in the normal subjects (table 1). No significant difference was detected between levels in men (2.13 mmol.) and women (2.48 mmol.) (p = 0.22). Urinary citrate excretion was significantly reduced in patients with renal calculi, intestinal resection or renal insufficiency but was unaltered in those with diabetes. Citrate-creatinine ratio. The citrate-creatinine ratio was significantly lower in normal men than in women, and in men with stones than in male controls. The latter observation suggests that the decrease in urinary citrate excretion seen in patients with stones is real and is not because a larger number of women were included in the normal population than in the group with stones (table 2). Relationship of urinary calcium excretion to urinary citrate excretion. The relationship between urinary calcium and citrate excretion was examined in some detail. In 24 normal subjects and 34 patients with stones in whom calcium and citrate excretions were determined on a random diet the correlation between absolute levels of urinary calcium and urinary citrate was not significant (r = 0.33, p >0.05 for normals and r = 0.19, p >0.05 for patients with stones). The 24-hour citrate levels also were measured in 13 patients on a random and restricted (300 mg.) calcium diet. 9 Urinary calcium excretion decreased in 12 of 13 patients after dietary calcium restriction but urinary citrate levels increased in 8. Because this increase in urinary citrate may be attributed to substitution of calcium rich foods (dairy products) with citrate rich foods, urinary citrate excretion also was measured in 2 normal subjects and 22 patients with recurrent stones during the 4 hours after a 1.0 gm. oral calcium load. Urinary citrate excretion increased in 1 normal subject and in 8 patients with stones after the load, whereas it decreased in the second normal control and in 14 of the 22 patients. Relative incidence of metabolic abnormalities in patients with stones. The excretion of more calcium or oxalate and less citrate than that excreted by 95 per cent of normal subjects (mean ± 1.645 standard deviation) was considered abnormal. Thus, hypercalciuria was defined as urinary calcium excretion >7.13 mmol. in men and 3.48 mmol. in women, hyperoxaluria was defined as urinary oxalate excretion >0.66 mmol. in men and 0.41 mmol. in women, and hypocitraturia was defined as

Accepted for publication November 19, 1982. Supported in part by Biomedical Research Grant 3375 51054B from Washington University School of Medicine, United States Public Health Service Award IR23AM 29831-01 from National Institute of Arthritis, Diabetes, Digestive and Kidney Diseases, Veterans Administration Research Grant and Public Health Service Research Grant No. RR-36 from General Clinical Research Center Branch, Division of Research Facilities and Resources, Bethesda, Maryland. * Requests for reprints: Division of Urology, Washington University School of Medicine, St. Louis, Missouri 63110. 1158

'Ol-iiNARY CiT:?,ATE EXCRETIO}J 11\J PATIEf'7TS VlITH FtEI"-Jfo.,.L CALCULI 1. 24-hour urinary citrate

TABLE

No. Pts. Normal Pts. with stones Intestinal resection Diabetes Renal insufficiency

Ivl:ean ± Standard Error (mmol.)* 2.29 1.55 0.54 2.89 0.85

25 46 16 17 12

± ± ± ± ±

0.17 0.13 0.28 0.48 0.31

Range (mmol.)

P Valuet

0.91-3.81 0.32-3.38 0.00-2.03 0.34-5.48 0.00-3.28

0.0005 0.00002 0.7 0.0003

' Multiply value by 192 to convert mmol. to mg. t Compared to normals.

TABLE

2. Citrate- creatinine ratio Male Pts.

No. Pts. Normal Pts. with stones Intestinal resection Diabetes Renal insufficiency

12 34 6 6

12

Female Pts.

Mean± Standard Error (mrnol./ mrnol.)* 0.17 0.11 0.04 0.18 0.08

± ± ± ± ±

0.02 0.01 0.02 0.03 0.02

No. Pts. 12 5 10

11

Mean± Standard Error (mmol./ mmol.)* 0.32 0.18 0.07 0.25

± ± ± ±

0.03 0.04 0.08 0.04

p = 0.003 (normal men versus normal women) and 0.005 (men with stones versus normal men). * Divide value by 0.59 to convert mmol. citrate/mmol. creatinine into mg./mg.

citrate excretion <0.60 mmol. in men and 1.03 mmol. in women. With these definitions 12 of 36 patients (33 per cent) had hypercalciuria, 18 of 45 (40 per cent) had hyperoxaluria and 7 of 46 (15 per cent) had hypocitraturia. Hypocitraturia occurred as an isolated abnormality in 6 of these patients. Of the 32 patients with all 3 parameters determined 25 (78 per cent) had at least 1 biochemical abnormality. DISCUSSION

In 1934 Boothby and Adams showed that urinary citrate excretion was decreased in 2 patients with renal calculi. 10 These findings were corroborated by several other investigators. 11 , 12 In contrast, Conway and associates stated that urinary citrate excretion was decreased only in those patients with renal calculi who had concomitant urinary tract infection, and suggested that hypocitraturia was the result of bacterial infection. 13 Similarly, Hodgkinson concluded that slthough urinary citrate excretion was decreased in patients with stones overt hypocitraturia (defined as urinary citrate excretion <400 mg. per day in men and 200 mg. per day in women) was present only in patients with urinary tract infection or renal failure. 14 Perhaps because of the conflicting data reported urinary citrate excretion is not measured routinely during the metabolic diagnostic studies of patients with renal calculi. However, several recent investigators have emphasized the importance of low urinary citrate levels in at least some patients with renal stones. Dedmon and Wrong, 15 and McSherry and Pokroy 16 have demonstrated that urinary citrate excretion is low in patients with type I renal tubular acidosis, a disease characterized a high incidence of nephrocalcinosis and nephrolithiasis. Alkali therapy has increased citrate excretion and prevented nephrocalcinosis in some of these patients. Rudman and associates found low urinary citrate levels in patients with intestinal malabsorption. 17 These investigators suggested that hypocitraturia, in addition to hyperoxaluria, may be implicated in the higher incidence of calcium oxalate renal calculi seen in malabsorptive disorders. Finally, Shorr18 and Jenkins 19 and their associates have found that calculi are seen in hyperparathyroid patients with low urinary citrate but not in those with normal citrate excretion. In contrast to the data available in patients with specific metabolic disorders, such as renal tubular acidosis, malabsorption or hyperparathyroidism, there is little contemporary infor-

115'9

mation about urinary citrate excretion in with the of calcium oxalate renal Perhaps the most authoritative report is that of Welshman and McGeown, who measured citrate excretion in 108 healthy subjects, 158 controls and 164 patients with stones. 12 These investigators demonstrated that urinary citrate excretion was decreased in male and female stone patients, with the decrease being more marked in female patients" Normal women excreted more citrate than normal men irrespective of whether citrate excretion was expressed as absolute amounts or as a factor of creatinine excretion. Our results corroborate their findings that urinary citrate excretion is decreased in patients with renal calculi. In contrast, we did not find a significant difference in total urinary citrate excretion between normal men and women. The most likely reason for this discrepancy is our small number of control subjects. Nevertheless, other reasons may be implicated. Urinary citrate excretion is dependent on estrogen secretion and varies with the menstrual cycle. 18 Detailed menstrual histories were not obtained either by Welshman and McGeown or by us. It is possible that a relatively lower proportion of our patients were in the estrogenic phase of the menstrual cycle than in their study. Also, we performed urinalyses but not urine cultures in our control subjects. Although unlikely, it is possible that some of our normal controls might have had asymptomatic bacteriuria, causing a decrease in urinary citrate. The relationship between urinary citrate excretion and urinary calcium excretion requires further discussion. In 1942 Shorr and associates stated that changes in urinary calcium excretion were associated normally with parallel changes in urinary citrate excretion. 18 They presented data from a patient with hyperparathyroidism to substantiate their conclusions, and noted that 2 patients with hypercalciuria and low fecal calcium had an unusually low content of citric acid. (Although details were not given these patients presumably had absorptive hypercalciuriao) These investigators suggested that an impairment of the mechanism by which the kidney adjusted citric acid output to calcium output may be important in the production of renal stones. In contrast to these observations, Hodgkinson found no relationship between calcium and citrate excretion in a group of normal subjects. 14 Welshman and McGeown found that citrate excretion increased with calcium excretion in patients with stones as well as in normal subjects. 12 We found no significant correlation between calcium and citrate excretion in either controls or stone patients. Dietary calcium restriction did not cause a uniform decrease in urinary citrate excretion in patients with stones. Finally, urinary citrate excretion increased in only 9 of 23 subjects following an oral calcium load. These observations indicate that there is no strong correlation between urinary calcium excretion and urinary citrate excretion, either in normal subjects or in patients with stones. In summary, we have studied urinary citrate excretion in 46 unselected patients with calcium oxalate stones without hyperparathyroidism, urinary infection, renal tubular acidosis or malabsorption. We found that urinary citrate levels were low in 7 of the 46 patients (15 per cent). In 6 of these 7 patients no other metabolic abnormality was detected. We used a rather stringent definition of hypocitraturia, that is urinary citrate excretion <95 per cent of a healthy control population, and compared male patients to male controls and female patients to female controls. Because of the small numbers of subjects in the control groups this definition may underestimate the prevalence of hypocitraturia. In the ultimate analysis it may not be the exact prevalence of hypocitraturia that is of clinical relevance but whether increasing citrate excretion decreases the rate of stone formation. We have initiated oral citrate therapy in a small subset of stone patients and will report on the results at a later date. Josephine M. Dickson, Rica Cuenca and Dr. Mohammed S. Khalifa provided urinary citrate measurements.

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REFERENCES

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1, chapt. 22, p. 878, 1978. 10. Boothby, W. M. and Adams, M.: The occurrence of citric acid in urine and body fluids. Amer. J. Physiol., 107: 471, 1934. 11. Kissin, B. and Locks, M. 0.: Urinary citrates in calcium urolithiasis. Proc. Soc. Exp. Biol. Med., 46: 216, 1941. 12. Welshman, S. G. and McGeown, M. G.: Urinary citrate excretion in stone-formers and normal controls. Brit. J. Urol., 48: 7, 1976. 13. Conway, N. S., Maitland, A. I. L. and Rennie, J.B.: Urinary citrate excretion in patients with renal calculi. Brit. J. Urol., 21: 30, 1949. 14. Hodgkinson, A.: Citric acid excretion in normal adults and in patients with renal calculus. Clin. Sci., 23: 203, 1962. 15. Dedmon, R. E. and Wrong, 0.: The excretion of organic anion in renal tubular acidosis with particular reference to citrate. Clin. Sci., 22: 19, 1962. 16. McSherry, E. and Pokroy, M.: The absence of nephrocalcinosis in children with type 1 RTA on high-dose alkali therapy since infancy. Abstract. Clin. Res., 26: 470a, 1978. 17. Rudman, D., Dedonis, J. L., Fountain, M. T., Chandler, J. B., Gerron, G. G., Fleming, G. A. and Kutner, M. H.: Hypocitraturia in patients with gastrointestinal malabsorption. New Engl. J. Med., 303: 657, 1980. 18. Shorr, E., Almy, T. P., Sloan, M. H., Taussky, H. and Toscani, V.: The relation between the urinary excretion of citric acid and calcium; its implications for urinary calcium stone formation. Science, 96: 587, 1942. 19. Jenkins, A. D., Purnell, D. C., Scholz, D. A., Werness, P. G. and Smith, L. H.: Urolithiasis in primary hyperparathyroidism. Read at annual meeting of American Urological Association, abstract 362, Kansas City, Missouri, May 16-20, 1982.