Stool Studies

Stool Studies


The elimination of digestive waste products from the body is essential to health. These excreted waste products are known as stool or feces. Stool examination is often done for evaluation of gastrointestinal (GI) disorders. These studies are helpful in detecting GI bleeding, GI obstruction, obstructive jaundice, parasitic disease, dysentery, ulcerative colitis, and increased fat excretion.

An adult excretes 100 to 200 g (wet weight, <66 g dry weight) of fecal matter a day, of which as much as 75% may be water. The feces are what remain of the 8 to 10 L of digested fluid-like material that enters the intestinal tract each day, and oral food and fluids, saliva, gastric secretions, pancreatic juice, and bile add to the formation of feces.

Feces are composed of the following materials:

  • Waste residue of indigestible material (e.g., cellulose) from food eaten during the previous 4 days

  • Bile (pigments and salts): stool color is normally due to bile pigments that have been altered by bacterial action.

  • Intestinal secretions

  • Water and electrolytes

  • Epithelial cells that have been shed

  • Large numbers of bacteria

  • Inorganic material (10%-20%), chiefly calcium and phosphates

  • Undigested or unabsorbed food (normally present in very small quantities)

The output of feces depends on a complex series of absorptive, secretory, and fermentative processes. Normal function of the colon involves three physiologic processes: (1) absorption of fluid and electrolytes; (2) contractions that churn and expose the contents to the GI tract mucosa and transport the contents to the rectum; and (3) defecation.

The small intestine is approximately 23 feet (7 m) long, and the large intestine is 4 to 5 feet (1.2-1.5 m) long. The small intestine degrades ingested fats, proteins, and carbohydrates to absorbable units and then absorbs them. Pancreatic, gastric, and biliary secretions exert their effects on the GI contents to prepare this material for active mucosal transport. Other active substances absorbed in the small intestine include fat-soluble vitamins, iron, and calcium. Vitamin B12, after combining with intrinsic factors, is absorbed in the ileum. The small intestine also absorbs as much as 9.5 L of water and electrolytes for return to the bloodstream. Small intestine contents (i.e., chyme, which contains fragments of proteins, droplets of fat, salt, and water) begin to enter the rectum as soon as 2 to 3 hours after a meal, but the process is not complete until 6 to 9 hours after eating.

The large intestine performs less complex functions than the small intestine. The proximal or right colon absorbs most of the water remaining after the GI contents have passed through the small intestine. Colonic absorption of water, sodium, and chloride is a passive process. Fecal water excretion is only about 100 mL/day. The colon mainly moves the luminal contents to and fro by seemingly random contractions of circular smooth muscle. Increased propulsive activity (i.e., peristalsis) occurs after eating. Peristaltic waves are caused by the gastrocolic and duodenocolic reflexes, which are initiated after meals and stimulated by the emptying of the stomach into the duodenum. The muscles of the colon are innervated by the autonomic nervous system. Additionally, the parasympathetic nervous system stimulates movement, and the sympathetic system inhibits movement. Massive peristalsis (progressive waves of muscular contractions pushing the food ahead) usually occurs several times a day. Resultant distention of the rectum initiates the urge to defecate. In persons with normal motility and a mixed dietary intake, normal colon transit time is 24 to 48 hours.


Stool analysis determines the various properties of the stool for diagnostic purposes. Some of the more frequently ordered tests on feces include tests for leukocytes, blood, fat, ova, parasites, and pathogens (Table 4.1). (Stool culture is explained in Chapter 7.) Stool is also examined by chromatographic analysis for the presence of gallstones. The recovery of a gallstone (precipitated by cholesterol or bile pigments) from feces provides the only proof that a common bile duct stone has been dislodged and excreted. Stool testing also screens for colon cancer and asymptomatic ulcerations or other masses of the GI tract and evaluates GI diseases in the presence of diarrhea or constipation. Stool testing is done in immunocompromised persons for parasitic diseases. Fat analysis is used as the gold standard to diagnose malabsorption syndrome. The Centers for Disease Control and Prevention recommend that stool samples submitted for enteric pathogen testing should also be tested for Shiga toxin (produced by Escherichia coli O157:H7 and other strains of E. coli.).

TABLE 4.1 Stool Testing for Infections

Source of Stool Infection

Clinical Signs or Symptoms

Laboratory Test (Sequence or Follow-up)

Community Acquired from intermediate hosts, such as:

Diarrhea, bloody, purulent


Cramping, bloating, belching

Small bowel obstruction, weight loss

Generalized skin rash

Huge swelling of legs, arms, or scrotum

Huge lymphatic swelling

Fever, chills, night sweats

Screen stool for ova and parasites

Microscopic exam of stool for ova and parasites

Stool culture—Clostridium difficile assay (some patients may require more than one assay)

Eosinophilia in blood sample

Exam for worms in stool or around anus

Fecal smear for leukocytes and yeast

Home—pets, dogs, contaminated water Occupational

Fishing—snails and worms

Meat cutters—from contaminated animals

Health care workers—from patients

Farm workers—animals (cows, pigs), garden, flies, mosquitoes, insects, fleas, bugs

Recreational—backpacking, poor sanitation

Travel—Third World—contaminated water supply

Contact with fair animals

Hookworm infection—not fecal or oral; it is direct penetration of skin by larva in contaminated soil or in animal droppings

Nosocomial Acquired from institutions such as hospitals or nursing homes


Medication history of antibiotic use

Eosinophilia in blood sample

Exam for worms in stool or around anus

Microscopic exam of stool for ova and parasites

Stool culture for Clostridium difficile

Fecal smear for leukocytes and yeast

Personal Contact with an infected host when patient is compromised (weakened immune system, i.e., HIV) or debilitated as in frail children or elderly

Screen stool for ova and parasites

Microscopic exam of stool for ova and parasites

Stool culture for Clostridium difficile

Toxin stool assay

Acid-fast bacilli (AFB) in stool for tuberculosis


Patients and health care personnel may dislike collecting and examining fecal material; however, this natural aversion must be overcome in light of the value of a stool examination for diagnosing disturbances and diseases of the GI tract, the liver, and the pancreas.

Random Collection and Transport of Stool Specimens

  • Observe standard/universal precautions (see Appendix A) when procuring and handling specimens to avoid infectious pathogens (e.g., hepatitis A, Salmonella, and Shigella).

  • Collect feces in a dry, clean, urine-free container that has a properly fitting cover.

  • The fresh specimen should be uncontaminated with urine or other bodily secretions such as menstrual blood. Stool can be collected from the diaper of an infant or incontinent adult. Samples can be collected from temporary ostomy bags.

  • While wearing gloves, collect the entire stool specimen and transfer it to a clean container using a clean tongue blade or similar object. A sample 2.5-cm (1-inch) long or 64.7 mg (1 oz) of liquid stool may be sufficient for some tests.

  • For best results, cover specimens and deliver to the laboratory immediately after collection. Depending on the examination to be performed, the specimen should be either refrigerated or kept warm. If you are unsure of how to handle the specimen, contact the laboratory for detailed instructions concerning the disposition of the fecal specimen before collection is begun.

  • Post signs in bathrooms that say “DO NOT DISCARD STOOL” or “SAVE STOOL” to serve as reminders that fecal specimen collection is in progress.

Collection and Transport of Specimens for Ova and Parasites

  • Wear gloves. Observe standard precautions (see Appendix A). Collect feces in a dry, clean, urine-free container. If unsure of how to collect specimen, contact the laboratory before collection is begun.

  • Warm stools are best for detection of ova and parasites. Do not refrigerate specimens for ova and parasites.

  • Special vials that contain 10% formalin and polyvinyl alcohol (PVA) fixative may be used for collecting stool samples to test for ova and parasites. In this case, specimen storage temperature is not critical.

  • Because of the cyclic life cycle of parasites, three separate random stool specimens for analysis are recommended.

  • Place the specimen in a biohazard bag.

Collection and Transport of Specimens for Enteric Pathogens

  • While wearing gloves, collect feces in a dry, clean, urine-free container. If unsure of how to collect the specimen, contact the laboratory before collection is begun. Observe standard precautions.

  • Some coliform bacilli produce antibiotic substances that destroy enteric pathogens. Refrigerate the specimen immediately to prevent this from happening in the sample.

  • A diarrheal stool will usually give accurate results.

  • A freshly passed stool is the specimen of choice.

  • Collect stool specimens before antibiotic therapy is initiated and as early in the course of the disease as possible.

  • If mucus or blood is present, it definitely should be included with the specimen because pathogens are more likely to be found in these substances. If only a small amount of stool is available, a walnut-sized specimen is usually adequate.

  • Accurately label all stool specimens with the patient’s name, date, and tests ordered on the specimen. Keep the outside of the container free from contamination and immediately send the sealed container to the laboratory.

  • For best preservation and transport of pathogens, a Cary-Blair solution (contains sodium thiogly-collate, disodium phosphate, sodium chloride, agar, and distilled water) vial with indicator should be used.

Interfering Factors for All Types of Stool Collection

  • Stool specimens from patients receiving tetracyclines, antidiarrheal medications, barium, bismuth, oil, iron, or magnesium may not yield accurate results.

  • Bismuth found in paper towels and toilet tissue interferes with accurate results.

  • Do not collect or retrieve stool from the toilet bowl or use a specimen that has been contaminated with urine, water, or toilet bowl cleaner. A clean, dry bedpan may be the best receptacle for defecation.

  • Inaccurate test results may result if the sample is not representative of the entire stool evacuation.

  • Lifestyle, personal habits, travel, home and work environments, and bathroom accessibility are some of the factors that may interfere with proper sample procurement.

  • Specimen not transported promptly. Trophozoites (protozoans [e.g., Sporozoea], which includes malaria) in liquid stool disintegrate rapidly after defecation; therefore, the specimen needs to be examined 30 minutes from start of collection of specimen, not 30 minutes from end of collection. Semi-formed stool should be examined within 60 minutes after defecation. No trophozoites are seen in formed stool.


Stool Consistency, Shape, Form, Amount, and Odor

Inspection of the feces is an important diagnostic tool. The quantity, form, consistency, and color of the stool should be noted. When diarrhea is present, the stool is watery. Large amounts of mushy, frothy, foul-smelling stool are characteristic of steatorrhea (excess fat in the feces). Constipation is associated with firm, spherical masses of stool. Feces have a characteristic odor that varies with diet and the pH of the stool.

Normally, evacuated feces reflect the shape and caliber of the colonic lumen as well as the colonic motility. The normal consistency is somewhat plastic and neither fluid, mushy, nor hard. Consistency can also be described as formed, soft, mushy, frothy, or watery. The odor of normal stool is caused by indole (produced in intestine by breakdown of tryptophan) and skatole (formed from decomposition of protein), formed by bacterial fermentation and putrefaction.

Reference Values


  • 100-200 g/24 hours or 100-200 g/day

  • Characteristic odor present; plastic, soft, formed; soft and bulky on a high-fiber diet; small and dry on a high-protein diet; seeds and small amounts of vegetable fiber present (as opposed to muscle fiber) (Table 4.2).

Assessment of Diarrhea and Constipation

  • When performing a workup for the differential diagnosis of diarrhea or constipation, a patient history is most important. The following factors should be charted:

    • An estimate of volume and frequency of fecal output

    • Stool consistency and presence of blood, pus, mucus, oiliness, or bad odor in specimen; evaluate through direct observation

    • Decrease or increase in frequency of defecation

    • Sensations of rectal fullness with incomplete stool evacuation

    • Painful defecation

  • Assess dietary habits and food allergies.

  • Assess emotional state of patient—psychological stress may be major cause of altered bowel habits.

  • Assess for signs of laxative abuse.

Posttest Patient Care

  • Evaluate outcome; interpret, report, and record findings. If abnormalities are detected, counsel patient appropriately. If patient has watery diarrhea, note history of contact with affected family members, travel to a developing country, involvement in vacation or resort backpacking, community and municipal water supply, or contact with farm animals. Explain that additional testing (e.g., colonoscopy) may be necessary.

  • Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Stool Color

The brown color of normal feces is probably due to stercobilin (end-product of heme catabolism), a bile pigment derivative, which results from the action of reducing bacteria in bilirubin and other undetermined factors.

The first indication of GI disturbances is often a change in the normal brown color of the feces. A change in color can provide information about pathologic conditions, organic dysfunction, or intake of drugs. Color abnormalities may aid the clinician in selection of appropriate diagnostic chemical and microbiologic stool tests.

Reference Values



Interfering Factors

Jun 11, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Stool Studies

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