Second morning urine method is superior to the casual urine method for estimating daily salt intake in patients with hypertension

Abstract

The accuracy of the casual urine (CU) method for estimating daily salt intake was compared with the second morning urine (SMU) method and with 24-h urine collection (24UC) method as the gold standard. Data were obtained from three previously reported studies, in which we evaluated the daily salt intake by the SMU method. Using SMU samples from 1315 outpatients, the estimated salt intake was lower with the CU method than the SMU method. In inpatients with a daily salt intake of 7, 8 or 18 g, the CU method was applied to SMU specimens. It underestimated salt intake compared with the 24-h collection method whereas the SMU method and 24UC method gave similar results. In the present study, 24UC was done and then urine was collected at 0800, 1100, 1400, 1700 and 1900 hours, with a daily salt intake of 6 g in 8 inpatients, 10 g in 11 inpatients or 15 g in 5 inpatients. In comparison with the 24UC method, the CU method underestimated a high salt intake (15 g) when morning specimens were used and overestimated a low salt intake (6 g) when afternoon specimens were used. The correlation between the CU method and 24UC method was weaker (R¼0.57) than that between the SMU method and 24UC method (R¼0.85). In conclusion, the CU method is heavily influenced by the timing of urine collection and by the actual daily salt intake, so the SMU method provides a better estimate of individual salt intake.

Sodium intake in Germany estimated from sodium excretion measured in spot urine samples

“…Two studies found results with Toft's equation to correlate better with 24-h urine samples compared with Tanaka's equation [31,52]. The precision of estimates seems to be influenced by the level of sodium intake [51,53], as well as the time of spot urine sampling [24,49,53]. The timing of spot urine sampling could indeed partly explain deviating results.…”

“…Cogswell et al [49] concluded that results obtained with Brown's equation are least biased. Using Tanaka's equation resulted in over-and underestimation of measured daily sodium excretion in various evaluation studies [31,32,[49][50][51][52][53]. Particularly, sodium excretion of women seems to be overestimated with Tanaka's equation [31,49,52], and sodium excretion of men is probably underestimated [31].…”

“…Several studies have evaluated the accuracy of some equations already by comparing extrapolated sodium excretion with measured 24-h urine collections [31,32,[49][50][51][52][53]. The assessment revealed contradictory conclusions.…”

“…The acrophase is between 15:00 and 16:00 [56], and the amplitude of the circadian rhythm is approximately 25 % [57] or 50 % to 60 % [56], respectively. This diurnal rhythm is expressed in lower estimated daily sodium excretion based on morning spot urines and higher estimated daily sodium excretion based on afternoon spot urines [53]. Thus, Kawasaki et al (1993) developed their equation based on generally lower sodium concentrations in morning urine samples.…”

Dietary Salt Intake: History, Assessment, and Benefit in Hypertensive Treatment

“…Japanese investigators including Kawasaki and Tanaka have demonstrated that it is possible to estimate 24-hrs urinary sodium from single spot urine. Using second-morning void urine, Kawasaki successfully achieve correlation of R = 0.85 between second-morning void to 24-hrs urine collection [23]. Tanaka reported a correlation of 0.65 between both methods and concluded that while spot urine is not suitable to estimate individual sodium excretion, it can be used to measure population intake of sodium [24].…”