A Simple Method for Estimating 24 H Urinary Sodium and Potassium Excretion From Second Morning Voiding Urine Specimen in Adults

Abstract

1. An assessment was made of the extent sodium (Na) and potassium (K) intake can be estimated from Na, K and creatinine (Cr) content of a second morning voiding urine (SMU) specimen collected within 4 h after the first voiding upon awakening but before breakfast in 159 clinically healthy, free-living individuals (20-79 years). The SMU and the rest of 24 h urine specimens for a 3-5 day period were collected. 2. The following equations for estimating 24 h urinary Na (24HUNaV) and K (24HUKV) excretions were developed, and the accuracy and the reliability of these equations were evaluated. Estimated value of 24HUNaV (mEq/day) = 16.3 square root of XNa; estimated value of 24HUKV (mEq/day) = 7.2 square root of XK, where XNa (or XK) = SMUNa (or SMUK)/SMUCr x predicted 24 h urinary Cr excretion. 3. Highly statistically significant correlations were detected between the values estimated and measured for both Na (r = 0.728, P < 0.001, n = 159) and K (r = 0.780, P < 0.001, n = 159). 4. These equations were applied to Group 1 subjects, who collected the urine for a single day, and to Group 2, for 3 days. The correlation coefficients between the values estimated and measured for Na and K were 0.531 and 0.443 in Group 1, and 0.821 and 0.590 in Group 2, respectively. No statistically significant differences were observed. 5. The SMU specimens provide a satisfactory alternative to both 24HUNaV and 24HUKV in adults for extensive epidemiological surveys but also for clinical application.

The influence of posture on the estimation of daily salt intake by the second morning urine method

“…Although 24-h urine collection is considered to be the most reliable method for assessing daily salt intake, 4,8,10 complete and accurate collection is not easy to achieve. We excluded 16 patients with incomplete collection and a deviation of creatinine excretion by more than 30% of the predicted value and then compared the SMU and 24-h collection methods in 100 hypertensive patients with a daily salt intake of 7 g during hospital admission.…”

“…Using the second voiding urine specimen, daily sodium excretion was measured by the SMU method, 10 which utilizes the patient's gender, age, body weight, height, and the sodium and creatinine concentrations of the urine sample. As approximately 90% of a person's sodium intake is excreted in the urine, 17 daily sodium excretion was considered to be equal to daily salt intake and was expressed as grams of sodium chloride (NaCl) per day as suggested by the Japanese guideline.…”

“…8 In fact, a survey showed that 88% of general physicians did not estimate the daily salt intake of hypertensive patients attending their outpatient clinics. 9 Kawasaki et al 10 developed the second morning urine (SMU) method to evaluate daily salt intake from the second urine specimen collected in the morning before breakfast. This method is based on the assumption that 24-h creatinine excretion can be predicted from demographic factors (sex, age, height and body weight), and the sodium:creatinine ratio of the 24-h urine specimen is proportional to that of the SMU specimen.…”

“…The usefulness of the SMU method has been confirmed in population studies 5,11,12 and in hypertensive patients. 13 Although the posture that should be adopted for the SMU method was not specified by the original authors, 10 posture is known to affect the urinary sodium:creatinine ratio 14,15 and could possibly influence the estimation of daily salt intake by this method. Therefore, this study was designed to assess the effect of posture on the estimation of daily salt intake by the SMU method compared with the method of 24-h collection in hospitalized patients with hypertension on a saltrestricted diet.…”

Protocol for a cluster randomised controlled trial on information technology-enabled nutrition intervention among urban adults in Chandigarh (India): SMART eating trial

“…Blood samples for haemoglobin, fasting plasma glucose and lipid profile, and urine samples will be analysed using standard laboratory methods. The 24 h urinary sodium excretion (mg/day) will be estimated from a single morning fasting urine sample using the Kawasaki formula [38]. …”