Measuring urea kinetics: oral versus intravenous method - abstract

ABSTRACT

Urea kinetics have generally been measured using labelled urea. We became interested in using a non-invasive technique, more appropriate for young children and less threatening to adults, and therefore measured urea kinetics using labelled urea only. The results showed that this method is reproducible with low variability when a single individual was measured repeatedly over four years. In adopting the method, it was assumed that urea is metabolised in the same way if the isotope is oral or intravenous and this assumption was not adequately tested. Therefore we decided to compare this oral method with the more commonly used intravenous method, as a way of validating the oral method. Urea kinetics were measured in 5 normal healthy adults, using an oral or intravenous primed/intermittent infusion of 15N 15N-urea (urea-30) over 18 hours. A standard intake of food (I) was given to supply 1.2g protein and 44 kcal/kg/d. The isotopic enrichment in urinary urea and daily urea excretion (Eu) were used to calculate urea production rate (P) and rate of urea hydrolysis (T) in the bowel. In three of the five subjects, the values obtained for P, Eu an T were not altered by the route by which the isotope was given. In the remaining two subjects, P* and T* were elevated when the isotope was oral but not when it was intravenous. Urea excretion was not affected by the way in which the kinetics were measured. These data demonstrate that in most cases the oral method to measure urea kinetics will give results in agreement with those obtained using the intravenous method. In some cases, however, higher values are obtained with the oral method, suggesting that in some individuals there are sites other than the colon where effective urea hydrolysis may occur (AU)