Differentiation of osmotic and secretory diarrhoea by stool carbohydrate and osmolar gap measurements
Departments of Paediatrics
and Medicine, Universidad Peruana Cayetano Heredia and Instituto de
Investigación Nutricional, Lima, Perú
Correspondence to: and reprint requests to: Dr José Antonio Castro-Rodríguez, Instituto Médico Infantil, Barros Errazurriz 1919, Providencia, Santiago, Chile.
Accepted 3 May
1997
Clinical features and laboratory tests that determine
carbohydrate in faeces were evaluated to determine which was best able to distinguish between osmotic and secretory diarrhoea in infants and
children. For this purpose 80 boys aged 3 to 24 months, with acute
watery diarrhoea, were studied prospectively. The faecal osmolar gap
(FOG) was calculated as: serum osmolarity 
[2 × (faecal sodium + potassium concentration)]. Fifty eight patients were classified as having predominantly osmotic diarrhoea (FOG >100 mosmol/l), and 22 as having predominantly secretory diarrhoea (FOG
100 mosmol/l). The two groups were comparable in their clinical features on admission, in the results of blood and urine tests, and in
the evolution of their diarrhoeal illness. Evidence of steatorrhoea (by
positive Sudan III test) and of acid faecal pH on admission were
significantly more frequent in patients with osmotic diarrhoea. Mean
(SD) faecal osmolarity was not significantly different between the two
groups (319 (80) mosmol/l in secretory diarrhoea v 361 (123) mosmol/l in osmotic diarrhoea). Tests for reducing substances in
faeces such as Benedict's test
with and without hydrolysisand
glucose strip, all showed a positive and significant association with
osmotic diarrhoea (p <0.05, <0.025, <0.05, respectively). The
presence of excess reducing substances (Benedict's test with
hydrolysis >++) on admission was the most sensitive and specific test
with the best predictive value for differentiating between the two
types of watery diarrhoea.
© 1997 by Archives of Disease in Childhood
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