A urinary double-radioisotope technique for the detection of fat malabsorption

The possibility that the unreliability of urinary excretion of radioactivity after the administration of oral 131I-triolein, as a measure of fat absorption, is due to variable renal clearance of iodide prompted investigation of a double-radioisotopic method. Intravenously administered radioiodide should be cleared identically with radioiodide split from absorbed fat and serve as an internal standard. Oral 131I-triolein was given in an 80 gm fat liquid meal; 125I-iodide was given intravenously; urine was collected fractionally for 72 hr. Accepted stool fat excretion data and stool radioactivity excretion data developed in this study were used to define groups of normals (less than 5.4 gm of fat or less than 7.8% 131I) and malabsorbers (greater than 5.4 gm of fat or greater than 7.8% 131I). In 26 normals urinary 131I ranged from 24.6% to 77.1% in 48 hr and 27.7% to 86.8% in 72 hr; in the malabsorbers these values lay between 0.6% and 62.8% in 48 hr and 1.0% and 66.8% in 72 hr, showing much overlap with normal. An index of the cumulative percentage of urinary 131I/125I X 100 at either 48 or 72 hr completely separated the groups, the index in normals ranging from 68.7 to 100 in 48 hr and 72.5 to 100 in 72 hr, whereas the malabsorbers varied between 2.0 and 64 in 48 hr and 2.2 and 67.5 in 72 hr.