Stool Electrolytes and pH
The composition of normal stool must be known to understand the consequences of diarrhea:
- Normal stool has an alkaline pH.
- Sodium and potassium salts are the primary stool solutes. The sodium plus potassium concentration in stool usually ranges between 130 and 150 meq/L. Other cations, such as calcium and magnesium, are present at much lower concentrations.
- The main inorganic stool anions are bicarbonate (about 30 meq/L), chloride (about 10 to 20 meq/L), and a small amount of phosphate and sulfate.
- Various organic acid anions (eg, propionate and butyrate, about 80 to 90 meq/L) account for most of the anions in normal stool.
- Stool water osmolality is similar to the osmolality of serum (approximately 300 mOsm/kg). Although the stomach is capable of maintaining an osmotic gradient, more distal segments of the bowel are permeable to water, and therefore, the luminal contents have osmotically equilibrated with the serum by the time the fluid has passed the ligament of Treit
It should be emphasized that, normally, only about 100 mL of water is excreted in the stool each day. Thus, despite the high stool electrolyte concentrations, only about 4 meq of sodium and 9 meq of potassium are lost in normal stool each day, and the total daily loss of bicarbonate plus organic acid anions is only about 11 meq/day
For watery stools, fecal electrolyte concentrations and pH are measured in stool water after homogenization of a fresh specimen (by manual stirring or in a mechanical blender) and centrifugation of an aliquot to obtain supernatant for analysis. Fecal pH is measured with nitrazine paper on a fresh stool sample.
Normal stool has an osmolality close to plasma (280 - 303 mOsm/kg), a sodium concentration of about 30 mmol/L, a potassium concentration of about 75 mmol/L, and a magnesium concentration between 12 and 240 mg/dL (depending on diet) although it is usually less than 110 mg/dL.
The osmotic gap of fecal fluid can be used to estimate the relative contributions of electrolytes and nonelectrolyte osmoles to retention of water in the intestinal lumen. In osmotic diarrhea, nonelectrolytes cause water retention. The osmotic gap is calculated from electrolyte concentrations in stool water by the following formula: 290 - 2([Na+] + [K+]) This formula is preferred over those that use measured stool osmolalities, because the latter may be falsely elevated due to post-collection changes or to contamination of the sample with concentrated urine.
| Laboratory test differentiation between secretory and osmotic diarrhea | ||
| Laboratory tests | Secretory | Osmotic |
| Osmotic gap | <50 mOsm/kg | >75 mOsm/kg |
| pH | Often >6.0 | Often <6.0 |
Normal osmolar gap using this formula is <125 mOsm.
Secretory diarrhea is due to the entrance of more fluid into the colon than can be absorbed. Secretory diarrhea can be caused by many laxatives, other drugs, tea, coffee, cola, ethanol, bacterial toxins, chemical toxins, and VIPomas. Secretory diarrhea is characterized by an osmolar gap <50 mOsm, Na >90 nM/L, and pH >5.6. However, this pattern can also be seen in osmotic diarrhea due to sodium-containing laxatives.
Osmotic diarrhea is characterized by an osmolar gap of >100 mOsm, stool sodium of <60 meq/L and pH <5.3. Common causes include an increased osmolar load due to polyethylene glycol (PEG), Mg salts, Na phosphate, sorbitol, mannitol, lactulose, xylitol. It can also be associated with malabsorption due to infections, sprue, disaccharidase deficiency, Olestra, bacterial overgrowth, pancreatic insufficiency, short bowel, and inflammatory bowel disease.
A fecal sodium concentration of greater than 150 mmol/L and an osmolality of greater than 400 mOsm suggest contamination of stool with concentrated urine.
A fecal osmolality of less than 250 mOsm suggests contamination of stool with hypoosmotic urine or water. Fecal osmolality is helpful in the diagnosis of factitious diarrhea. In this case, fecal osmolality is lower than serum osmolality.
Storage of stool at room temperature can artifactually increase stool osmolality due to bacterial metabolism and degradation of carbohydrates. Stool osmolality may exceed 350 mOsm.
