USEFULNESS OF SHORT-TERM URINE COLLECTION IN THE NUTRITIONAL MONITORING OF LOW-BIRTH-WEIGHT INFANTS

To establish adequacy of urine collection times shorter than 24h in th e metabolic monitoring of low birthweight infants, we collected urine for 24 h in 39 LBW infants during the third and fourth week of Life. A ll urine voidings over the 24-h period were separately collected, the volume of each sampling and the time of voiding were recorded, and 20% of the volume was removed for pooling. All individual and pooled samp les were analysed for total nitrogen, urea and ammonia, alpha-amino ni trogen, creatinine, sodium, potassium, calcium and phosphorus, and for each compound the ratio to I mol creatinine was established. Individu al sample results were ''pooled'' to obtain 3-, 6- and 12-h period exc retion and than related to the 24-h excretion as measured in the poole d 24-h sample. As the volume of urine obtained in any 6-h collecting p eriod depended on the time of sampling (06:00-12:00 h, 17.5 +/- 3.1% o f total; 12:00-18:00 h, 31.6 +/- 5.1% of total; 18:00-24:00 h, 25.6 +/ - 3.1% of total; and 0:00-06:00 h, 25.3 +/- 2.9% of total), calculatio ns were based on samples obtained from 18:00 to 06:00 h. The correlati on between results of 3- and 24 h-collection periods was weakest, whil e results of the 6-h collection correlated highly with the total daily excretion (r = between 0.82 and 0.93 for the different compounds) and the correlation was only slightly better when the 12-h collection per iod was considered. The correlation between the mean molar substrate/c reatinine ratio of all individual samples of a 24-h collecting period and the and total daily excretion of the respective substrate was weak er (r = between 0.46 and 0.76 for the different compounds) than the co rrelation between the results of a 6-h collecting period and the daily excretion is not as stable than in later life. The data indicate that 6-h urine sampling may be sufficient for metabolic monitoring of LBW infants. By contrast, urinary substrate/creatinine ratios are not good markers of the daily excretions of the respective substrate during th e first weeks of life.