The fecal osmotic gap: technical aspects regarding its calculation.

The fecal osmotic gap is considered to be useful in the investigation of chronic diarrhea. However, two important questions regarding the calculation of the osmotic gap remain unanswered: the effect of the method of measuring osmolality and the most appropriate means for its calculation. In 41 patients with varying causes of diarrhea, stool osmolalities and electrolyte concentrations were measured. Measurement of stool osmolality by a dew point technique underestimated results by a variable factor, which makes it an inappropriate method. Fecal osmolality measured by freezing point depression gave reliable results and so was considered to be preferable. The osmotic gap was calculated by two methods: subtraction of the calculated osmolality from (1) stool osmolality and (2) plasma osmolality. The former approach was found to be inappropriate on the basis of large variations in results when consecutive stool samples were analyzed, a poor correlation with the severity of osmotic diarrhea, poor resolution between results from patients with secretory diarrhea and those with osmotic diarrhea, and practical difficulties in ensuring adequate collection of stool samples.