Correlation of 24-Hour Urine Sodium , Potassium and Calcium Measurements with Spot Urine

In this study, we analyzed the correlation of sodium(Na), potassium(K) and calcium(Ca) amounts with spot urine(SU) and 24-hour urine(24-HU) collected as 8-hour portions. 21 hypertensive patients and 21 normotensive individuals were enrolled in the study. Na, K and Ca were studied in three sequential 8-hour urine samples and with the sum of three, 24-HU samples of the cases. Sodium/creatinine, potassium/creatinine, and calcium/creatinine were studied in fasting SU and serum Na, K and creatinine were studied. A significant correlation was found between the SU sodium/creatinine ratio and the Na amount in each 8-hour urine collections, and the Na amount in the 24-hour urine in hypertensive patients, the control group, and when all patients were evaluated together. When all group were evaluated together for Ca, a significant correlation was determined between the SU calcium/ creatinine ratio and the Ca amount in all three 8-hour urine collections. The Na amount in the 24-HU may be estimated both with the SU Na/creatinine ratio and the 8-hour urine collection; SU does not have clinical significance for potassium. However, it may be estimated with 8-hour urine collections; SU may help roughly for calcium, and the collected urine gives higher correlation


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Sodium, potassium and calcium were studied in three sequential 8-h urine collections obtained following the first urination into the toilet at 08.00.Furthermore, the sodium/creatinine, potassium/creatinine, and calcium/ creatinine ratios were studied in the first spot urine obtained during the fasting period following the 24-h urine.Na, K, and Ca excretions were calculated in the 8-h urines of the cases included in the study.The 24-h values were measured with the sum of these three.
The correlation of Na, K, and Ca excretion in the 24-h urine with three portions of urine collected for 8 hours and with the spot urine was studied.The first part (Na-K-Ca)1 was collected between 08-16 hours, the second part (Na-K-Ca)2 was colected between 16-24 hours, and the third part (Na-K-Ca)3 was collected between 24-08 hours.The time intervals were written on the bottles and given to the patients in order to avoid confusion.The 24-hour and the spot urine samples were analyzed with the enzymatic colorimetric method using the Integra 800 device.

Statistical analysis:
The obtained data were statistically analyzed using the SPSS 15.0 program.The Student's t test was used to determine the difference between patient and the control groups in terms of the studied parameters; the Pearson correlation analysis was used to determine the relationship between the parameters of the patient and the control groups.The results were recorded as mean±standard deviation.A p level of <0.05 was accepted as statistically significant.

Sodium:
When the hypertensive patients and the control group were compared, no significant difference was found between the Na amount in the 8-hour urine collection (Na1, Na2,Na3), the spot urine Na/Cr ratio and the and 24-hour excretion (Table 1).Sodium chloride intake was found as 10.8 ± 3.98 gr/day in the hypertension group and 12.5 ± 6.27 gr/day in the control group.

INTRODUCTION
High sodium intake is a well-known risk factor for hypertension and cardiovascular disease (1,2).Sodium excretion in the 24-hour urine is an important parameter as it indicates the daily salt consumption (3,4).Potassium is an important mineral for the nervous system and cardiac rhythm.Potassium excretion in the 24-hour urine indicates the dietary potassium intake (5,6).Calcium excretion in the 24-hour urine reflects the dietary calcium intake, calcium absorption, serum calcium and calcium loss from the skeletal system, and is important for studying hypercalciuria (7).Generally, 24-h urine collection is considered to be the most reliable method to evaluate these intakes.However, this method poses a considerable burden on the subjects and it is difficult to collect complete and accurate 24-h urine samples (8)(9)(10).This method may not be practical in clinical practice or in large-scale epidemiological surveys.The protein/creatinine ratio in spot urine enables the prediction of the protein amount excreted in the 24-h urine by making protein excretion independent from dilution of the urine (11).Similarly, in this study, we studied whether the 24-h urine excretion of these electrolytes could be predicted or not by using the ratio of sodium, potassium and calcium to creatinine in the spot urine, which was more practical.

MATERIALS AND METHODS
Twenty-one hypertensive patients (16 females, 5 males, mean age: 55.8 ±8.4 years), and 21 normotensive healthy individuals (15 females, 6 males; mean age: 49.3 ± 5.4 years ) were enrolled in the study.This study was conducted according to the ethical principles of the Declaration of Helsinki.Approval for the study was obtained from the Ethics Committee of Yuzuncu Yıl University Medical Faculty.All patients provided written informed consent upon enrollment.The patients were informed about the purpose and the content of the study and were included in the study after consents were obtained.Those who had findings suggestive of secondary hypertension or another systemic disease according to the medical history and physical examination and in whom a pathology was detected in the routine biochemical and hematological analyses, were excluded from the study.A blood pressure of above 140/90 mmHg on admission was accepted as hypertension according to the JNC 7 criteria.creatinine ratio and Na1, Na3 and sodium excretion in the 24-hour urine (r =0.882 (p<0.01),r=0.541 (p<0.05),r=0.814 (p<0.01),respectively).There was also a significant correlation between the 24-hour sodium excretion and Na1, Na2, Na3 (r=0.911(p<0.01),r=0.592 (p<0.01),r=0.793 (p<0.01),respectively).In the evaluation of all urine samples, there was a significant correlation between the spot urine sodium/creatinine and Na1, Na3 and 24-hour urine sodium (r =0.651 (p<0.01),r=0.617 (p<0.01),r = 0.713 (p<0.01),respectively).Furthermore, a significant correlation was found between the 24-hour sodium excretion and Na1, Na2, Na3 (r=0.854(p<0.01),r=0.602 (p<0.01),r=0.741 (p<0.01),respectively)(Table 2).In this way, we found a linear regression equality as the 24-hour-Na excretion (mmol)=103.42+56.39xNa/Cr (mmol/L-mg/dl) to calculate 24-h Na excretion from spot urine Na/Cr (R2 = 0.51) (Figure 1).

DISCUSSION
Although the recommended daily salt amount for healthy life is 6 gr, in the SALTURK study carried out on 1767 subjects regarding the salt consumption habit and hypertension in Turkey, the mean salt consumption amount was found as 18 gr (4).In our study, the sodium chloride intake was 10.8 ± 3.98 gr/day in the hypertension group, and 12.5 ±6.27 gr/day in the control group.
According to the formula we have found, the sodium/ creatinine ratio should be 0.8 in the spot urine for a daily excretion of 150 mmol sodium (R2 = 0.51).This is equal to 8.8 gr/day sodium chloride intake.In the study of Mann et al. (12) in the correlation analysis of 24-hour Na excretion and Na/creatinine ratio in the spot urine  samples obtained when the patients were fasting in the morning at the beginning (AM), middle (PM) and just after urine collection, it was found to be correlated only with PM sample (late afternoon/early evening) (r=0.86,p<0,01).In another study of Mann et al. (13) the spot urine, the Na/creatinine ratio in the PM sample and the 24-hour Na excretion were correlated.In the study of Tsai et al. (14) the Na excretion in the 9-hour night urine collection was correlated with the 24-hour urine (r=0,78,p<0,01,) and the Na excretion in the spot urine was not correlated.
In this study, we found a significant correlation between the three portions of the 8-hour urine and the spot urine and the 24-hour Na amount in the hypertension and the control groups.When we evaluated all the cases together, we detected that the correlation coefficient between the 24-h Na excretion and Na1 (r=0.854) was higher than the correlation coefficient between Na2 (r=0.602) and Na3 (r=0.741).Hence, Na1 (between 08-16 hours) better reflects the 24-hour Na excretion, but the Na3 urine that is collected at night is more practical.Spot urine also reflects the 24-hour Na excretion, but as understood from the distribution figure, this correlation may lead to errors in routine use, it may be used as a screening test and for detection of excessive salt consumption by knowing this limitation.In our study, in the evaluation of all urine samples, the correlation coefficient between the spot urine potassium/creatinine and K3 (r=0.315,p<0.05) and the 24-hour potassium excretion (r=0.315,p<0.05) was found to be significant, but the r is very low.The correlation coefficient between potassium excretion in the urine collected between 08-16 hours (K1) and in the 24-hour urine was found to be higher (r=0,828, p=0.01).However, as patients collect urine easier at night, we may conclude that K3 urine is more practical, despite lower correlation.
In our study, in the hypertension and the control groups and when all the urine samples were analyzed, a sig-nificant correlation was detected between the calcium excretion in all three 8-hour urine collections and 24-h urine collections.It was seen that collecting three portions of 8-hour urine collection reflected the 24-hour calcium excretion, but the Ca excretion in the urine collected at night (Ca3) provided a better idea about the calcium excretion in the 24-hour urine.According to our formula, the spot urine calcium/creatinine ratio (mg/ dl-mg/dl) should be 0.19 for 300 mg daily calcium excretion (R2 = 0.27).In order to calculate the 24-hour calcium excretion using Ca3, we found a linear regression equality as 24-hour Ca excretion (mg)=32.01+2.5xCa3(mg) (R2 = 0.78).In the study of Gokce et al.( 15) a significant corelation was found between the 24-hour calcium excretion and the spot urine calcium/creatinine (r=0.946,p<0.01).In our study, a significant correlation was detected between the spot urine calcium/creatinine ratio and the 24-hour calcium excretion, although the correlation coefficient was low (r=0.521).Hong et al. (16) could not find a correlation between the 24-hour urinary Na excretion and early morning (AM) spot urine Na excretion.They found a significant but low correlation with K (r=0.39).A significant correlation was found in terms of calcium excretion (r=0.669,p=0.01).In the study of Illich et al. (17) carried out on 143 healthy individuals, although there was a positive correlation between Ca, Na, and K excretion in the spot urine given just after the 24-hour urine collection and the 24-hour excretions (r=0.603,0.452, 0.396 respectively), the significance and the need for 24-hour urine collection for evaluation of the electrolyte excretion was emphasized.Tanaka et al. (10) stated that although the formulas found for evalauting the total mean values of 24hour sodium and potassium excretion using spot urine samples were not sufficient for estimating the individual sodium and potassium excretion, they were beneficial for estimating the mean values of the population.Kawasaki et al. (9) stated that the formulas they found for estimating the spot urine and the 24-hour sodium and potassium excretion, were sufficient for estimating the individual sodium and potassium excretion.In our study, we consider that the formula found for sodium is more practical for outpatient clinic use.According to our formula, the spot urine sodium/creatinine ratio (mmol/L-mg/dl) should be 0.8 for 150 mmol daily sodium excretion (R2=0.51).The small number of cases is an important limitation of our study.In conclusion, the 24-hour urine Na amount may be predicted, either with the spot urine Na/creatinine ratio, or with the 8-hour urine collection.Spot urine is not of clinical importance for potassium; however, it may be estimated using the collected 8-hour urine samples.For calcium, spot urine may only give a rough idea, and collected urine samples give higher correlation coefficients.

Table 1 .
Sodium excretion in the 8-hour urine collection, in the 24-hour urine and the spot urine in the hypertension and the control groupsbetween the calcium values in the all the cases have been showed in the table 4.
Figure 1.The 24 hours Na and spot Na/creatinin ratio in all cases Figure 2. 24-hour Ca and Ca3 for all urine samples

Table 2 .
Correlation analysis between the sodium parameters in all cases *

Table 3 .
Potassium excretion in the 8-hour urine collection, in the 24-hour urine and the spot urine in the hypertension and the control groups.

Table 6 .
Correlation analysis between the Ca parameters in all cases.