Average urinary excretion of sodium in 24 hours can be estimated from a spot-urine specimen.

Abstract

To what extent Na intake can be estimated from the Na content of a spot-urine specimen (spot-UNa) collected within 4 h after the 1st voiding upon awakening but before breakfast was assessed. Subjects were asked to collect spot-urine and 24-h urine specimens other than the spot-urine for a 3-day period either successfully or intermittently. The coefficient of correlation between spot-UNa and urinary excretion of Na in 24 h (24-h UNa) collected on the 1st day was 0.47. When the coefficient of the variation of creatinine excretion in the spot-urine was .apprx. 20%, the sample was discarded as the outlier, and in this case the coefficient of correlation was 0.725. A marked fluctuation of creatinine excretion in spot-urine was considered to represent technical errors at the time of the spot-urine collection. The coefficient of variation of intra-individual 24-h UNa exceeded 20%, suggesting that a single determination of 24-h UNa does not represent the individual average of daily urinary excretion of Na. Apparently, the determination of a substantial number of spot-urine specimens to estimate daily salt ingestion of a given subject may be more reliable than a single determination of 24-h UNa, if the outlier of creatinine excretion in spot-urine specimen is excluded from the determination of spot-UNa. The average urinary excretion of Na in 24 h of Japanese subjects investigated was 218 .+-. 67 meq. [The relation between daily salt intake and the prevalence of hypertension in epidemiologic studies is discussed.].