Stool Examination

Macroscopic evaluation and color significance

The first stage of diagnostics involves a visual assessment of the physical properties of stool: its color, consistency, form, and odor. The laboratory technician also pays attention to the presence of pathological impurities, which include pus, mucus, visible blood, parasites, and undigested food residues.

Stool color is of utmost diagnostic importance:

• Normal stool is brown, which is due to the presence of stercobilin (a bilirubin derivative).
• Acholuric stool appears grayish-white. This is a direct sign of impaired bile excretion, as stercobilin is absent.
• Steatorrhea may visually resemble "yeast dough", indicating fat malabsorption syndrome.
• Melena is black, tarry stool. It is characteristic of bleeding from the upper gastrointestinal tract (stomach, duodenum), where blood has time to oxidize under the action of hydrochloric acid.
• Hematochezia is the appearance of bright red blood. It indicates a bleeding source in the lower gastrointestinal tract (e.g., colon tumors, ulcerative colitis, hemorrhoids, or acute intestinal infections).

Differential diagnosis: upon detecting black stool, pseudo-melena must be excluded. Such discoloration can be triggered by taking a number of medications, particularly iron and bismuth preparations.

Microscopic examination (coprogram)

For microscopy, a stool sample is mixed with an isotonic sodium chloride solution. This method aims to detect cellular elements, parasites, and evaluate the digestive capacity of the digestive tract.

Detection of pathological elements:

1. Cellular elements: the detection of a large number of erythrocytes and macrophages is very characteristic of ulcerative lesions of the colon.
2. Parasitology: searching for protozoan cysts and helminth eggs. To obtain a reliable result, this study requires repeated testing.

Digestive disorders syndromes:

• Creatorrhea is the detection of undigested muscle fibers retaining transverse striation in the visual field. It indicates defects in gastric digestion or a decrease in the proteolytic function of the pancreas.
• Steatorrhea is an excessive content of neutral fat and fatty acids. It occurs when bile flow into the intestine ceases (acholia), with a lack of pancreatic lipase, or in malabsorption syndrome.
• Amylorrhea is an excess of starch. It indicates a lack of the enzyme amylase (pancreatic insufficiency) or too rapid evacuation of the food bolus (chyme) through the small intestine.
• Presence of plant fiber is a sign of accelerated transit of contents through the colon, due to which intestinal bacteria simply do not have time to break down cellulose.

Chemical examination: occult blood test

The occult blood test plays a colossal clinical role. It is used to diagnose unexplained causes of anemia and serves as a crucial screening tool for colorectal cancer. Often, a positive result of this test turns out to be the sole and earliest symptom of a developing tumor.

Rules for biomaterial collection: Since bleeding from tumors or ulcers is often episodic, a single test may be falsely negative. It is necessary to perform at least a three-time examination on different days. If the patient has concomitant hemorrhoids, stool collection is performed using a rectoscope from the upper sections of the intestine. This is necessary to exclude a false-positive result due to trauma to hemorrhoidal nodes.

Weber test: A classical method for chemical detection of occult blood. Guaiac resin is used as a reagent. The reaction principle is based on the fact that hemoglobin possesses pronounced peroxidase activity. It oxidizes guaiac resin, resulting in a characteristic blue coloration.

Biochemical and bacteriological examination

In addition to the coprogram and blood tests, in complex diagnostic cases, additional laboratory tools are used:

• Stool biochemistry: includes determining total volume, electrolyte levels, and osmolarity. These parameters are critically important for the differential diagnosis of the nature of diarrhea (allowing secretatory diarrhea to be distinguished from osmotic).
• Bacteriology: seeding biomaterial onto nutrient media. Used for precise identification of pathogens when acute intestinal infections are suspected.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of the peroxidase reaction in the Weber test
• Differential diagnosis of secretory and osmotic diarrhea by electrolyte levels
• Features of detecting protozoan cysts and helminth eggs (concentration methods)
• Pathophysiology of stercobilin formation from bilirubin
• The role of pancreatic lipase and amylase in the formation of steatorrhea and amylorrhea

Other topics in the "Digestive System" section

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