Fat digestion tests

Estimation of the fat assimilation is one of the keystones when examining a patient with diarrhoea. Three fat assimilation tests, using 14C- and/or 3H-labelled long chain fatty acids as test substance, have been investigated: the 14C-triglyceride breath test is not sufficiently effective in diagnosing malassimilation, and two tests on alternative days are needed to establish lipid digestion. The serum 14C-triolein/3H-oleic acid test estimates lipid assimilation from blood samples and may discriminate between lipid malabsorption and lipid maldigestion from the ratio between 3H (from oleic acid) and 14C (from triolein). The test is, however, not sufficiently effective to be useful in clinical practice. The faecal 14C-triolein/3H-oleic acid test estimates lipid assimilation correctly in approximately 90% of the patients with diarrhoea from samples of faeces and discriminates between malabsorption and maldigestion with an efficiency of approximately 100% from the ratio between 14C and 3H in faeces. The traditionally used estimation of the 3-day faecal fat excretion classifies lipid assimilation correctly in approximately 85% of the patients, even when performed as simple out-patient collections. Furthermore, the faecal fat concentration is often useful in discriminating between normal lipid assimilation, malabsorption and maldigestion, even when only a single-day collection is done. The so-called fat balance studies do not seem to improve the usefulness of the faecal fat measurement. When quantitative data about the magnitude of malassimilation are sought, none of the fat assimilation tests are optimal, all of the tests having coefficients of variation of approximately 30%.