Situation: A 34-year-old woman has had chronic diarrhea with pale, bulky, foul-smelling stools that float, a 6-kg weight loss over 6 months, and iron-deficiency anemia. Celiac disease is suspected. Before the diagnosis was confirmed, a 72-hour fecal fat collection was done while she ate about 100 g of fat per day. The total fat in the 72-hour collection was 27 g. The laboratory reports more than 7 g of fecal fat per day as abnormal. Which interpretation is correct?

The collection covers 3 days, so 27 g ÷ 3 = 9 g of fat per day, which exceeds the 7-g/day limit and confirms fat malabsorption. Put another way, she absorbed 273 of 300 g (91%), less than the about 93% expected when no more than 7 g/day is lost on a 100-g diet.

The question presents a 72-hour fecal fat collection result and asks for correct interpretation. The total fat measured over three days was 27 g, while the patient consumed approximately 100 g of fat per day. The laboratory defines abnormal as more than 7 g of fecal fat per day.

The first step is to convert the three-day total into a daily average. Dividing 27 g by 3 days yields 9 g/day. This value exceeds the 7 g/day upper limit of normal, so the result indicates fat malabsorption. The correct answer is therefore option 4.

A daily fecal fat loss greater than 7 g while consuming 100 g of fat per day is the standard threshold for diagnosing steatorrhea. This cutoff is referenced in the literature on exocrine pancreatic insufficiency, where quantitative fecal fat estimation remains the gold standard for steatorrhea, defined as fecal fat exceeding 7 g/day ^[4]. The patient’s clinical picture—chronic diarrhea with pale, bulky, foul-smelling stools that float, weight loss, and iron-deficiency anemia—is consistent with malabsorption, and the fecal fat result provides objective confirmation.

Calculating the coefficient of fat absorption provides another way to understand the result. Over three days, the patient ingested 300 g of fat (100 g/day × 3 days). She excreted 27 g, meaning she absorbed 273 g. The absorption percentage is therefore 91% (273 ÷ 300 × 100). When fecal fat loss is at the upper limit of 7 g/day on a 100-g fat diet, the expected absorption is approximately 93%. A value of 91% falls below that threshold, confirming impaired absorption. Option 3 states that 91% absorption is within normal limits, which is incorrect because the expected value is closer to 93% or higher when fat handling is intact.

Option 1 incorrectly treats the 27 g total as a daily value rather than a three-day total. Option 2 correctly calculates 9 g/day but incorrectly labels it as normal. The distinction between the total collection and the daily average is a common source of error in test interpretation.

Watch out! A 72-hour collection must always be divided by three to obtain a daily excretion rate. Interpreting the raw total as a single day’s output leads to a falsely severe assessment of malabsorption.

The rationale for using a 72-hour collection rather than a single random stool sample is that fecal fat excretion varies from day to day. Collecting over three days smooths out this variability and provides a more reliable estimate of average daily fat loss. Although the test is considered the gold standard for quantifying fat malabsorption, its use has declined in some settings because it is cumbersome and inconvenient for patients ^[4]. Alternative markers such as fecal elastase-1 have been proposed, but their diagnostic utility for steatorrhea is variable, and quantitative fecal fat remains the reference method ^[4].

In this patient, the finding of 9 g/day of fecal fat supports the suspected diagnosis of celiac disease. In celiac disease, villous atrophy in the small intestine reduces the surface area available for nutrient and fat absorption, leading to steatorrhea. The presence of iron-deficiency anemia further supports malabsorption of iron in the proximal small intestine, which is the same region affected by gluten-induced mucosal damage.

Key point! The diagnostic threshold for steatorrhea is 7 g/day of fecal fat on a 100 g/day fat diet. A three-day total of 27 g corresponds to 9 g/day, which is abnormal and indicates fat malabsorption.