Stool Studies
OVERVIEW OF 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
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 | |||||||||||||||||||||||||||||||||||
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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.
Interventions
Pretest Patient Care
Explain the collection purpose, procedure, and interfering factors in language the patient understands. Because the specimen cannot be obtained on demand, it is important to provide detailed instructions before the test so that the specimen is collected when the opportunity presents itself. Provide written instructions if necessary.
Provide proper containers and other collection supplies. Instruct the patient to defecate in a large-mouthed plastic container, bag, or clean bedpan. Provide for and respect the patient’s privacy.
Instruct the patient not to urinate into the collecting container or bedpan.
Do not place toilet paper in the container or bedpan because it interferes with testing.
If the patient has diarrhea, a large plastic bag attached by adhesive tape to the toilet seat may be helpful in the collection process. After defecation, the bag can be placed into a gallon container.
Specimens for most tests can be produced by a warm saline enema or Fleet Phospho-Soda enema.
Tests for both ova and parasites and cultures for enteric pathogens may be ordered together. In this case, the specimen should be divided into two samples, with one portion refrigerated for culture testing and one portion kept at room temperature for ova and parasite testing. There are commercial collection kits that require the stool to be divided and placed into separate vials for better recovery of ova and parasites and enteric pathogens. (See Chapter 7, Microbiologic Studies.)
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Provide patient privacy and the opportunity to cleanse perineal area and hands. Assist as necessary.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.
CLINICAL ALERT
Any stool collected may harbor highly infective pathogens. Use extreme caution and proper handling techniques at all times.
Instruct patients in proper handwashing techniques after each use of the bathroom.
STOOL STUDIES
Stool Consistency, Shape, Form, Amount, and OdorInspection 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
Normal
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).
Procedure
Collect a random, fresh stool specimen in a clean, dry, leakproof container.
Clinical Implications
Fecal consistency alterations
Diarrhea due to the following:
Infection—Salmonella, Shigella, Yersinia, human immunodeficiency virus (HIV) enteropathy, Campylobacter
Inflammatory disorder—Crohn’s disease, ulcerative colitis
Steatorrhea—sprue, celiac disease
Carbohydrate malabsorption—lactose or sucrose deficiency
Endocrine abnormalities—diabetes mellitus, hyperthyroidism or hypothyroidism, adrenal insufficiency
TABLE 4.2 Normal Values in Stool Analysis
Macroscopic Examination
Normal Value
Amount
100-200 g/24 h (100-200 g/d)
Color
Brown
Odor
Varies with pH of stool and depends on bacterial fermentation and putrefaction
Consistency
Plastic; not unusual to see fiber, vegetable skins, and seeds; soft and bulky in high-vegetable diet; small and dry in high-meat diet
Size and shape
Formed
Gross blood
None
Mucus
None
Pus
None
Parasites
None
Microscopic Examination
Normal Values
Fat
Colorless, neutral fat (18%) and fatty acid crystals and soaps
Undigested food, meat fibers, starch, trypsin
None to small amount
Eggs and segments of parasites
None
Bacteria and viruses
None
Yeasts
None
Leukocytes
None
Chemical Examination
Normal Values
Water
Up to 75% (0.75)
pH
Neutral to weakly alkaline (pH 7.0-7.5)
Occult blood
Negative
Urobilinogen
50-300 mg/24 h (50-300 mg/d)
Porphyrins
Coproporphyrins: 400-1200 µg/24 h (611-1832 nmol/d)
Uroporphyrins: 10-40 µg/24 h (12-48 nmol/d)
Nitrogen
<2.5 g/24 h (<178 mmol/d)
Apt test for swallowed blood
Negative in adults; positive in newborns
Trypsin
20-95 U/g
Osmolality, used with stool
200-250 mOsm
Na + K to calculate osmotic gap
Sodium
5.8-9.8 mEq/24 h (5.8-9.8 mmol/d)
Chloride
2.5-3.9 mEq/24 h (2.5-3.9 mmol/d)
Potassium
15.7-20.7 mEq/24 h (15.7-20.7 mmol/d)
Lipids (fatty acids)
0-6 g/24 h (0-21 mmol/d)
Carbohydrates (as reducing substances)
<0.25 g/dL (<2.5 g/L)
Note: Reference values for electrolytes differ greatly from laboratory to laboratory.
Hormone-producing tumors—Zollinger-Ellison syndrome (pancreatic gastrin-secreting tumor leading to increased acid activity in the stomach), gastrinoma, medullary thyroid carcinoma, villous adenoma
Colon carcinoma
Infiltration of lesions due to lymphoma, scleroderma of bowel
Drugs, antibiotics, cardiac medications, chemotherapy
Osmotically active dietary items—sorbitol, psyllium fiber, caffeine, ethanol
GI surgery—gastrectomy, stomach stapling, intestinal resection
Factitious—self-induced laxative abuse associated with psychiatric disorders
“Pasty” stool associated with high-fat content can be caused by the following:
Common bile duct obstruction
Celiac disease (sprue and steatorrhea); stool resembles aluminum paint
Cystic fibrosis—greasy “butter” stool appearance due to pancreatic involvement
Bulky or frothy stool is usually due to steatorrhea and celiac disease.
Alterations in stool size or shape indicate altered motility or colon wall abnormalities.
A narrow, ribbon-like stool suggests the possibility of spastic bowel, rectal narrowing or stricture, decreased elasticity, or a partial obstruction.
Excessively hard stools are usually due to increased fluid absorption because of prolonged contact of luminal contents with colon mucosa during delayed transmit time through the colon.
A large-circumference stool indicates dilation of the viscus.
Small, round, hard stools (i.e., scybala) accompany habitual, moderate constipation.
Severe fecal retention can produce huge, firm, impacted stool masses with a small amount of liquid stool as overflow. These must be removed manually, occasionally under light anesthesia.
Fecal odor should be assessed whenever a stool specimen is collected.
A foul odor is caused by dehydration of undigested protein and is produced by excessive carbohydrate ingestion.
A sickly sweet odor is produced by volatile fatty acids and undigested lactose.
Mucus in stool occurs in constipation, malignancy, and colitis.
Interventions
Pretest Patient Care
Ensure that the patient avoids barium procedures and laxative preparations for 1 week before stool specimen collection.
Advise the patient of the purpose of test and instruct him or her in collection techniques and refrigeration of specimen. Provide clean, dry, leakproof collection container.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
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 ColorThe 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
Normal
Brown
Procedure
Collect a random stool specimen. Observe standard precautions.
Clinical Implications
The color of feces changes in some disease states as follows:
Yellow, yellow-green, or green: severe diarrhea
Black, with a tarry consistency: usually the result of bleeding in the upper GI tract (>100 mL blood)
Maroon, red, or pink: possibly the result of bleeding of the lower GI tract from tumors, hemorrhoids, fissures, or an inflammatory process
Clay colored (tan-gray-white): biliary obstruction
Pale, with a greasy consistency: pancreatic deficiency causing malabsorption of fat
CLINICAL ALERTGrossly visible blood always indicates an abnormal state.
Blood streaked on the outer surface of stool usually indicates hemorrhoids or anal abnormalities.
Blood present in stool can also be caused by abnormalities higher in the colon. If transit time is sufficiently rapid, blood from the stomach or duodenum can appear as bright red, dark red, or maroon in stool.
Interfering Factors
Stool darkens on standing.
The color of stool is influenced by diet (certain foods), food dyes, and drugs (see Appendix E).
Yellow-rhubarb, yellow to yellow-green color occurs in the stool of breast-fed infants who lack normal intestinal flora.
Pale yellow, white, or gray stools are due to barium intake.
Green color occurs with diets high in chlorophyll-rich green vegetables such as spinach or with some drugs (see Appendix E). An increase in biliverdin, green pigment formed during hemoglobin breakdown, may also contribute to green color.
Black color may be due to foods such as cherries, an unusually high proportion of dietary meat, artificially colored foods such as black jelly beans, or drugs and supplements such as charcoal, bismuth, or iron.
Light-colored stool with little odor may be due to diets high in milk and low in meat.
Clay-like color may be due to a diet with excessive fat intake or to barium intake.
Red color may be due to a diet high in beets or tomatoes, red food coloring, or peridium compounds.
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