Estimation of the 3-day faecal fat excretion and fat concentration as a differential test of malabsorption and maldigestion.

The diagnostic efficiency of estimating the faecal fat excretion and faecal fat concentration in uncontrolled 3-day collections was estimated from a prospective series of 87 consecutive patients. The diagnoses were established on the basis of morphologic criteria, and the assimilation was established from the faecal 14C-triolein/3H-oleic acid test. These investigations showed the diagnostic sensitivity of the 3-day faecal fat excretion to be 90%. The 3-day mean faecal fat concentration was less than or equal to 5.1% w/w in patients with normal lipid assimilation. Of the patients with malassimilation 90% had a faecal fat concentration greater than or equal to 5.1% w/w. The faecal fat concentration was especially high in patients with maldigestion. When 10% w/w was used as the discrimination level between malabsorption and maldigestion, the faecal fat concentration correctly distinguished between these in 66% of the patients with malassimilation. The day-to-day variation of the faecal fat concentration was modest. Diagnosis of the lipid assimilation as estimated by the 1-day faecal fat concentration agreed with the diagnosis of the mean 3-day faecal fat concentration in 115 of the 122 1-day samples. The 1-day faecal fat concentration seems useful as a first screening test when malassimilation is suspected.