Chapter 15 - Fecal Analysis

# Physiology

* Many species of bacteria make up the normal flora of the intestines, and they produce the strong odor associated with feces and intestinal gas (flatus).
* Carbohydrates resistant to digestion and lactose (for those with intolerance) passes through the upper intestine and are metabolized by bacteria in the lower intestine, producing large amounts of flatus.
* Although digestion of ingested proteins, carbohydrates, and fats takes place throughout the alimentary tract, the small intestine is the primary site for the final breakdown and reabsorption of these compounds.
* Digestive enzymes secreted into the small intestine by the include trypsin, chymotrypsin, amino peptidase and lipase by the pancreas, and bile salts by the liver.
* If undigested or unreabsorbed material appear in the feces, the patient may be exhibiting symptoms of maldigestion and malabsorption.
* Water and electrolytes are readily absorbed in both the small and large intestines, resulting in a fecal electrolyte content that is similar to that of plasma.
* The large intestine is capable of absorbing approximately 3000 mL of water.
* When the amount of water reaching the large intestine exceeds this amount, it is excreted with the solid fecal material, causing diarrhea.
* When the amount is below this amount, water is reabsorbed from the feces and produces small hard stools, causing constipation.

## Diarrhea

* Diarrhea is defined as an increase in daily stool weight above 200 g with increased liquidity and frequency of more than three times per day.
* Diarrhea classification can be based on four factors: duration of the illness, mechanism, severity, and stool characteristics.
* Diarrhea lasting less than 4 weeks is defined as acute, and diarrhea persisting for more than 4 weeks is termed chronic diarrhea.
* The major mechanisms of diarrhea are secretory and osmotic.
* The laboratory tests used to differentiate these mechanisms are fecal electrolytes (fecal sodium, fecal potassium), fecal osmolality, and stool pH.
* The total fecal osmolarity is close to the serum osmolality (290 mOsm/kg).
* The fecal sodium and fecal potassium results are used to calculate the fecal osmotic gap, and it is calculated as follows:

Osmotic gap = 290 – \[2 (fecal sodium + fecal potassium)\]

| | Secretory Diarrhea | Osmotic Diarrhea |
|----|----|----|
| Causative Agent | bacterial, viral and protozoan infections, drugs, hormones, inflammatory bowel disease, endocrine disorders, neoplasms and collagen vascular disease | disaccharidase deficiency, malabsorption, laxatives, magnesium-containing antacids, amebiasis and antibiotic administration |
| Osmotic Gap |