Investigation and diagnosis of diarrhea caused by sodium phosphate

Because there are no published reference values for fecal phosphate concent ration or output, diagnosing surreptitious use of phosphate laxatives has b een difficult, The purposes of this study were to determine normal fecal ph osphate levels and to quantitate and chemically analyze diarrhea produced b y sodium phosphate. Timed stool collections were obtained from 20 normal su bjects during 25 study periods (normal controls), from 27 normal subjects w ith diarrhea induced by a variety of laxatives not containing phosphate dur ing 234 study periods (diarrhea controls), and from 10 normal subjects duri ng 14 periods after ingestion of 45 or 22.5 mi of a commercially available 66% sodium phosphate solution (Fleet Phospho-Soda). All stools were analyze d for soluble phosphate concentration, and daily output was calculated. The upper limits of normal for soluble fecal phosphate concentration and outpu t, derived from the normal controls and diarrhea controls, respectively, we re 33 mmol/liter and 15 mmol/day. Diarrhea produced by 45 mi of sodium phos phate was watery and voluminous, with fecal weights averaging 1078 g/day (r ange 601-1713 g/day). Measured fecal phosphate concentrations and outputs a veraged 85 mmol/liter and 92 mmol/day, respectively, and all values were si gnificantly elevated. Soft, less voluminous stools were produced with 22.5 mi of sodium phosphate but all had an abnormally high soluble phosphate con centration 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.