What is the most likely cause of chronic diarrhea with high stool output and normal osmolality?

Factitious Diarrhea (Dilutional Type)

The most likely cause of this patient's diarrhea is factitious diarrhea due to spurious addition of water or urine to the stool specimen, as evidenced by the abnormally low measured stool osmolality of 200 mosmol/L (normal plasma osmolality is 290 mosmol/kg). 1

Key Diagnostic Features

The critical finding here is the measured stool osmolality of 200 mosmol/L, which is significantly below the normal plasma osmolality of 290 mosmol/kg. 1 This definitively indicates that water or a hypotonic solution has been added to the stool specimen. 1

Understanding the Stool Osmolality Gap

• The calculated stool osmolality (210 mosmol/L) is similar to the measured osmolality (200 mosmol/L), yielding a minimal osmotic gap of approximately 10 mosmol/L 2
• A small osmotic gap (<50 mosmol/kg) would typically suggest secretory diarrhea, but this interpretation is invalid when the measured stool osmolality itself is abnormally low 2
• When faecal osmolality is less than 290 mosmol/kg, the specimen has been diluted with water or hypotonic fluid, indicating dilutional factitious diarrhea 1

Clinical Context Supporting Factitious Diarrhea

Epidemiology

• Factitious diarrhea represents 20% of cases at tertiary referral centers, making it the most common cause of diarrhea of previously undetermined origin 1
• In patients who have undergone extensive evaluation, 33% were found to be taking laxatives or diuretics 1

Clinical Presentation Consistent with This Diagnosis

• Normal vital signs and physical examination despite reported severe symptoms (8-10 bowel movements daily, 8 lb weight loss) 1
• Normal serum laboratory studies despite purported significant volume losses 1
• High stool output (500 g/24 hours) with otherwise unremarkable findings 1

Recommended Next Steps

Perform supervised inpatient stool collection with concurrent laxative screening to confirm the diagnosis. 1

Specific Testing Protocol

• Collect stool specimens under direct observation to prevent specimen tampering 1
• Screen urine for anthraquinones, bisacodyl, and phenolphthalein 1
• Screen stool for magnesium (>45 mmol/L suggests magnesium-induced diarrhea) and phosphate 1
• Use spectrophotometric or chromatographic analysis rather than alkalinization assays, which lack sufficient sensitivity 1
• Perform repeated testing, as patients may ingest laxatives intermittently 1

Important Clinical Pitfalls

• Do not rely on osmotic gap calculations when measured stool osmolality is abnormally low - the primary abnormality is specimen dilution, not the gap itself 1
• Patients with factitious diarrhea often have few physical cues despite potentially severe reported symptoms 1
• These patients may have psychiatric history, particularly eating disorders, or connections with health professions 1
• Hospital admission may be required to document true stool volumes while preventing specimen tampering 1