Limitations of ultracentrifugation and in vivo dialysis as methods of stool analysis.

Extracellular stool fluid has been sampled by in vivo dialysis, in vitro dialysis, and high speed centrifugation. Detailed comparisons of amino acids and electrolytes have been made between each fluid; normal values and significant differences are summarized. Evidence is provided which suggests that there is release of intracellular contents during normal handling of stool in the laboratory. The amino acids which display the greatest differences between centrifugate and dialysate from given stool specimens are those which are liberated by ultrasonication or aerobic incubation of stool. Factors affecting the chemical stability of stool are discussed. In most circumstances in vivo dialysis would seem to provide the closest approximation to extracellular fluid, providing the faeces remain in the descending colon or rectum for at least two hours.