AGREEMENT BETWEEN OVERNIGHT AND 24-HOUR URINARY CATION EXCRETIONS IN SOUTHERN CHINESE MEN

Agreement between overnight and 24-hour urinary sodium, potassium, cal cium, and magnesium excretion was studied in a sample of 63 normotensi ve Southwestern Chinese men: 30 Yi farmers and 33 urban residents in A pril 1989. Overnight (8-hour) and 24-hour urine specimens were collect ed on 3 consecutive days. Estimated correlation coefficients between 2 4-hour and overnight mean true values were 0.863 and 0.906 for sodium, 0.736 and 0.816 for potassium, 0.902 and 0.725 for calcium, and 0.733 and 0.703 for magnesium in Yi farmers and urban residents, respective ly. Hourly overnight urinary sodium and potassium excretion rates were significantly lower than the corresponding hourly 24-hour urinary exc retion rates: -0.60 and -1.99 mmol/hour for sodium, -1.24 and -0.48 mm ol/hour for potassium (all p < 0.05) in Yi farmers and urban residents , respectively. In multiple regression analyses, the differences betwe en 24-hour and overnight urinary sodium and potassium excretion rates were significantly and positively related to differences between 24-ho ur and overnight creatinine excretion rates. The ratios of intraindivi dual to interindividual variance were lower for 24-hour collections th an for overnight collections for sodium and calcium, but the differenc es in these ratios for potassium and magnesium were small. For sodium and calcium, twice as many overnight as 24-hour collections were requi red to estimate the correlation between cations and blood pressure wit h the same accuracy; for potassium and magnesium, overnight and 24-hou r collections were equally accurate. These results indicate that in no rmotensive populations such as the one studied, overnight urine collec tions may be used to estimate 24-hour cation excretion. The underestim ate of cation excretion by assessments based on collection of overnigh t specimens may be due to either a lower creatinine clearance or a low er intake of cations at night.