(gms fat ingested - gms fat excreted) x 100 / gms fat ingested

Should not vary w/ dietary intake.

Need strict record of dietary fat intake & output so best reserved for patients whose fecal fat excretion is borderline abnormal.

Normal :

premature infants - 60-75%

newborn infants - 80-85%

10 months - 3 years - 85-95%

> 3 years - 95%