| Abstract|| |
Low urinary citrate excretion is a risk factor in stone formers (SF). This study aimed to measure the urinary citrate excretion in SF and healthy volunteers at our center from 12 June 2008 to 20 August 2009. There were 28 SF patients (18 males and ten females) and 27 (18 males and nine females) age-matched healthy adult volunteers who participated in this study. Both groups had a similar living environment, extrinsic factors, diet and genetic descent. After collecting 24-h urine, citrate was measured using an enzymatic kit. Routine urinalysis and 24-h creatinine and uric acid were also performed. There was a significant difference in urinary citrate excretion level among SF (mean 310, SD 260 mg/L) and normal volunteer subjects (mean 800, SD 300 mg/L). By applying the previously defined normal values (320 mg/24 h) of urinary citrate in the local population, 43% of the SF in our study group was hypocitric, and none among the controls. We conclude that prevalence of hypocitraturia in stone formers was higher than that in healthy volunteers in our population.
|How to cite this article:|
Goodarzi MT, Forouzanfar F, Moaddab AH, Karimian M, Sabzevar NK. Comparison of 24-hour urinary citrate excretion in stone formers and healthy volunteers. Saudi J Kidney Dis Transpl 2012;23:1227-31
|How to cite this URL:|
Goodarzi MT, Forouzanfar F, Moaddab AH, Karimian M, Sabzevar NK. Comparison of 24-hour urinary citrate excretion in stone formers and healthy volunteers. Saudi J Kidney Dis Transpl [serial online] 2012 [cited 2020 Dec 2];23:1227-31. Available from: https://www.sjkdt.org/text.asp?2012/23/6/1227/103564
| Introduction|| |
Super-saturation of the urine with calcium and other salts, the presence of substances that promote crystallization and a deficiency of inhibitors of crystallization are the main factors that contribute to renal stone disease.  Citrate is normally excreted in the urine as a byproduct of oxidative pathways in the body.  Citrate is known to inhibit precipitation of calcium oxalate and phosphate and growth of their crystals. 
However, there are some reports of low urinary citrate output in stone formers (SF) as compared with healthy subjects, , while other studies found no differences.  Prevalence of hypocitraturia varies widely from 8% to 68.3% in patients with renal stones.  Furthermore, there are some differences in the range of 24-h urinary citrate excretion in normal subjects. ,
Hypocitraturia can be considered as an independent disorder or accompanied with other metabolic disorders for renal stone formation.  Also; it can be secondary to some systemic diseases such as renal tubular acidosis (RTA) or chronic diarrhea. 
Hassani et al, studying urinary citrate in patients with renal stone and healthy subjects in this country, reported a correlation between increase in urinary citrate excretion and decrease in frequency of crystaluria,  while Mithani found no difference in urinary citrate between these two groups. 
This study was aimed to evaluate the 24-h citrate excretion in SF patients and compare it with those of healthy subjects in order to determine the prevalence of this defect in our population.
| Materials and Methods|| |
Twenty eight subjects (18 men and 10 women) participated in this study as a study group. This case-control study was performed at the Research Center for Molecular Medicine of Hamadan University of Medical Sciences (West of Iran) from 12 June 2008 to 20 August 2009. The protocol of this study was approved by the Research Ethics Committee of Hamadan University of Medical Sciences (Hamadan, Iran).
The inclusion criterion for selection of patients was having at least two times history of renal stone removal during the last five years prior to recruitment. Receiving any medication during the collection of the urine specimens and having urinary tract infection were the exclusion criteria in this study. Informed consents were obtained from all studied subjects. Demographic characteristics of all studied subjects were recorded in a questionnaire.
Twenty-seven (18 men and nine women) age-and sex-matched normal adult individuals who had no evidence of urolithiasis and were willing to consent as control subjects participated as the control group. Absence of any renal calculi was confirmed by ultra-sonography and kidney-ureter-bladder (KUB) image in the control group. All the participants in this study had normal kidney function.
The instructions for 24-h urine collection were conducted verbally and in writing to all study subjects. The 24-h urinary excretion of citrate was measured in both groups in the same laboratory. Citrate was measured using enzymatic method (colorimetric kit, LTA, Milano, Italy). Urine creatinine was determined for all the urine specimens using the Jaffe method (Pars Azmoon Co, Tehran Iran). Urine uric acid was determined using the uricase method (Pars Azmoon Co, Tehran, Iran). Also, routine urine analysis was carried out for all study subjects and controls.
| Statistical Analysis|| |
The data were presented as mean ± SD, and statistical analysis was performed by SPSS version 13 and using descriptive methods. Comparison of urinary citrate level between the two groups was carried out using independent t-test, and P <0.05 was set as the significance level.
| Results|| |
The means (±SD) of age of patients in the study and control groups were 34.7 ± 14.5 and 39.9 ± 12.6 years, respectively. The ratio of male to female was 18/10 in the study group and 18/9 in the control group. The demographic characteristics of both groups are summarized in [Table 1]. There were no significant differences in these parameters between both groups. There was no documented urinary tract infection in the study group and controls. Calcium phosphate and calcium oxalate were the most prevalent stones (82%) in the study group. Urine characteristics of all participants, including volume, specific gravity, 24-h creatinine, pH and 24-h uric acid are shown in [Table 2]. There were no significant differences in these parameters between the study group and controls. The means of urinary citrate in both groups are shown in [Table 3]. The means of citrate concentration in urine and urinary excretion of citrate were lower in the SF compared with controls (P <0.001). Daily citrate excretion in men and women in the study group and controls is shown in [Table 4]. There was no significant difference in citrate excretion between men and women in both groups. Furthermore, citrate excretion was lower in men and women SF as compared with comparable sex in healthy controls (P <0.001 and P = 0.02, respectively).
|Table 3: Urinary citrate in terms of concentration, daily excretion and excretion rate per gram of creatinine in stone formers and healthy subjects.|
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|Table 4: Comparison of daily citrate excretion between men and women in two studied groups.|
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Also, applying the 320 mg/24 h citrate cutoff value (suggested by kit manufacturer), 43% of the urolithiasis patients were hypocitraturic, while there were no hypocitric subjects among the controls.
| Discussion|| |
Despite intensive research, the knowledge of stone pathogenesis remains limited. Recurrence of stone formation continues to be a significant clinical problem. Recurrent nephrolithiasis is a burden to the individual patients as well as to the healthcare system.  In the absence or low concentration of urinary inhibitors, urine can supersaturate with crystal of salts.  Because citrate is one of these inhibitors, hypocitraturia is an important factor associated with idiopathic renal stone disease.  In most studies in this field, hypocitraturia is known as the major metabolic abnormality in urolithiasis. , Increased renal tubular reabsorption that usually occurs in RTA leads to decrease in urinary citrate.  In addition to RTA, chronic diarrhea syndrome, hypokalemia and urinary tract infection are known as the cause of hypocitraturia. Nevertheless, most cases of hypocitraturia have unknown etiology, which is known as idiopathic hypocitraturia.  Metabolic acidosis may also be associated with hypertension, and decreases urinary citrate levels. 
In the present study, we found lower citrate excretion in SF as compared with healthy subjects. Comparing the urinary citrate excretion in forms of mg/24 h and mg/g creatinine, and also citrate urine concentration (mg/mL), produced similar results. These findings exclude some intervening factors such as 24-h urine volume or renal function. There is no consistency in lower acceptable level of urine citrate. We used 320 mg/24 h citrate as cutoff that was recommended by the kit manufacturer, and this is similar to what has been suggested in other studies. 
Close prevalence of hypocitraturia in patients with renal stones has been reported; for example, 38.4% in Japan  and 37.3% in Brazil,  even in a study on obese patients with stone disease it is 54%.  In a report that was carried out in India, the prevalence of hypocitrateuria was found to be 43%,  which is comparable with the percent (43%) found in our study. In our study, most of the cases had history of calcium stones in the form of oxalate or phosphate. Hypocitraturia has been found as the most common risk factor in Thai recurrent idiopathic calcium SF. 
In a study that was published in 2005,  studying urinary citrate in 40 SF patients and 40 control subjects with similar nutritional and environmental status, no significant difference was observed in urinary citrate between the two groups. The incidence of hypocitraturia was found to be similar in both groups.  The investigators concluded that the role of genetics and its correlation with hypocitraturia can be considered as an important factor. In a recent Iranian study that was carried out to identify metabolic and anatomical abnormalities associated with urinary calculi in children, hypocitraturia was found in 2.1% of the studied subjects.  At the same time, they did not find any reason for calculus formation in 46.2% of the studied subjects. 
The reason for these differences in results can be the different method of urinary citrate assay. However, nutritional, environmental and genetics factors must be considered. The most common method for urine citrate assay is the enzymatic method using citrate lyase, which used in this study. The results of this method are comparable to liquid chromatography tandem mass spectrometry (r = 0.964). 
Our small study population made it difficult to extrapolate the results to the whole population of this area. The other limitation in this study was the high number of young subjects.
High prevalence of hypocitraturia found in this study supports the idea that an assessment of urinary citrate excretion should be an integral element of metabolic evaluation in urolithiasis. We believe that metabolic study and follow-up in patients with recurrent lithiasis, and using appropriate treatment such as potassium-sodium citrate supplementation, can decrease the recurrence rate of urinary calculi.
| Acknowledgment|| |
The authors wish to thank Dr. S. H. Mousavi Bahar and Vice-Presidency for Research of Hamadan University of Medical Sciences (Iran).
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Mohammad Taghi Goodarzi
Research Center for Molecular Medicine, Hamadan University of Medical Sciences, 65178 Hamadan
[Table 1], [Table 2], [Table 3], [Table 4]