Investigation and diagnosis of diarrhea caused by sodium phosphate

Because there are no published reference values for fecal phosphate concentration or output, diagnosing surreptitious use of phosphate laxatives has been difficult. The purposes of this study were to determine normal fecal phosphate levels and to quantitate and chemically analyze diarrhea produced by sodium phosphate. Timed stool collections were obtained from 20 normal subjects during 25 study periods (normal controls), from 27 normal subjects with diarrhea induced by a variety of laxatives not containing phosphate during 234 study periods (diarrhea controls), and from 10 normal subjects during 14 periods after ingestion of 45 or 22.5 ml of a commercially available 66% sodium phosphate solution (Fleet Phospho-Soda). All stools were analyzed for soluble phosphate concentration, and daily output was calculated. The upper limits of normal for soluble fecal phosphate concentration and output, derived from the normal controls and diarrhea controls, respectively, were 33 mmol/liter and 15 mmol/day. Diarrhea produced by 45 ml of sodium phosphate was watery and voluminous, with fecal weights averaging 1078 g/day (range 601-1713 g/day). Measured fecal phosphate concentrations and outputs averaged 85 mmol/liter and 92 mmol/day, respectively, and all values were significantly elevated. Soft, less voluminous stools were produced with 22.5 ml of sodium phosphate but all had an abnormally high soluble phosphate concentration and 24-hr output. In conclusion, the upper limits of normal for soluble fecal phosphate concentration and output established in this study should be useful in the chemical diagnosis of phosphate-induced diarrhea.