Simultaneous assessment of fat maldigestion and fat malabsorption by a double-isotope method using fecal radioactivity.

[14C]Triolein and [3H]oleic acid are useful tracers of dietary triglycerides and free fatty acids. The combustion/liquid scintillation counting technique was found a practicable method of estimating the fecal activity of these tracers. When ingested with the nonabsorbable marker 51CrCl3, the fecal excretion of 14C and 3H could be estimated accurately from samples of two stools. This technique was investigated as a test of lipid assimilation in a prospective, blind study of 84 consecutive patients suspected of malassimilation. The fecal excretion of 14C was a useful alternative to fecal fat: the patients with "normal lipid assimilation" excreted less than or equal to 10.4% of the dose ingested, whereas 25 of the 26 patients with steatorrhea ("unequivocal malassimilation") excreted greater than 10.4%. Fecal fat was normal in 18 patients who exhibited other signs of malassimilation ("equivocal malassimilation"); in 5 of these patients fecal excretion of 14C was greater than 10.4%. Lastly, there was a significant correlation between fecal 14C and fecal fat (r = 0.82, p less than 0.001). The fecal 14C/3H ratio was found useful as an index of pancreatic digestive function, being less than 1.3 in patients with "normal" lipid digestion and greater than or equal to 1.3 in 18 of the 19 patients with pancreatic steatorrhea and in 19 of the 27 patients with severely reduced exocrine pancreatic function. The test result was found widely independent either of the amount of carrier fat or the quality of the fecal collections and was therefore useful as an outpatient test. As it provided information about the pathogenesis of malassimilation and was at least as sensitive and specific as a fat "balance study," the test may be a useful alternative to fecal fat measurement.