Usefulness of short-term urine collection in the nutritional monitoring of low birthweight infants.

To establish adequacy of urine collection times shorter than 24h in the metabolic monitoring of low birthweight infants, we collected urine for 24 h in 39 LBW infants during the third and fourth week of life. All 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 samples were analysed for total nitrogen, urea and ammonia, alpha-amino nitrogen, creatinine, sodium, potassium, calcium and phosphorus, and for each compound the ratio to 1 mol creatinine was established. Individual sample results were "pooled" to obtain 3-, 6- and 12-h period excretion and than related to the 24-h excretion as measured in the pooled 24-h sample. As the volume of urine obtained in any 6-h collecting period depended on the time of sampling (06:00-12:00 h, 17.5+/-3.1% of total; 12:00-18:00 h, 31.6+/-5.1% of total; 18:00-24:00 h, 25.6+/-3.1% of total; and 0:00-06:00h, 25.3+/-2.9% of total), calculations were based on samples obtained from 18:00 to 06:00 h. The correlation between results of 3- and 24 h-collection periods was weakest, while 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 period was considered. The correlation between the mean molar substrate/creatinine ratio of all individual samples of a 24-h collecting period and the and total daily excretion of the respective substrate was weaker (r = between 0.46 and 0.76 for the different compounds) than the correlation 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 the first weeks of life.