Feces is solid waste discharged from the large intestine through the anus. It is made of cellulose and other indigestible food matter, water, and bacteria. Other contents include epithelial cells shed from the GI tract, small amounts of fat, bile pigments as urobilinogen, GI and pancreatic secretions, electrolytes, and trypsin (a proteolytic enzyme from the pancreas).
An average adult excretes about 100 to 300 gram of fecal material daily, the residue of roughly 10 liters of liquid that enters the GI tract each day. Lab analysis covers macroscopic exam (volume, odor, shape, color, consistency, mucus), microscopic exam (leukocytes, epithelial cells, meat fibers), and chemical tests for specific substances (occult blood, trypsin, carbohydrate estimation).
Your role centers on the specimen: proper collection and handling, accurate identification, the right supplies, and prompt transport to the lab.
Indications
- Diagnose disorders of GI bleeding or medication-induced bleeding
- Assist in diagnosing pseudomembranous enterocolitis after broad-spectrum antibiotic therapy
- Help diagnose suspected inflammatory bowel syndrome (IBS)
- Identify the cause of diarrhea of unknown origin
- Investigate disorders of protein digestion
- Screen for colorectal cancer
- Screen for cystic fibrosis
- Determine intestinal parasitic infestation
- Evaluate therapy for intestinal malabsorption or pancreatic insufficiency
Interfering Factors
- Medications that irritate the gastric mucosa (NSAIDs, anticoagulants, colchicine, corticosteroids, phenylbutazone, iron preparations) can cause false-positive occult blood
- High-dose vitamin C (more than 250 mg per day) can cause false-negative occult blood
- A diet high in red meat, certain vegetables (radish, turnips, cauliflower, broccoli), and fruits (bananas, apples, cantaloupe) can cause false-positive occult blood
- Constipated stool may show no trypsin activity from prolonged exposure to intestinal bacteria
Procedure
- Collect the sample. Wearing clean gloves, collect stool directly into a leak-proof container with a tight-fitting lid. For a bedridden patient, use a clean, dry bedpan, then transfer with a tongue blade into a labeled container. For a rectal swab, insert the swab 2-3 cm through the rectal sphincter, rotate gently, remove, and transfer into the container.
- Complete the request form. Note the patient's demographics, stool consistency, and the date and time of collection. Document current or previous antibiotic therapy and any pertinent history.
- Transport promptly. Send the specimen to the lab immediately. A liquid or soft stool that cannot be processed within 30 minutes of passage goes into preservative. A formed stool that cannot be studied immediately goes into preservative or the refrigerator.
Nursing Responsibilities
Before the procedure
- Assess comfort. Stool collection can embarrass the patient.
- Have the patient void first to avoid contaminating the stool with urine.
- Avoid laxatives. No laxatives, enemas, or suppositories for 3 days before collection.
- Set the diet for occult blood testing. A red-meat-free, high-residue diet with generous chicken, turkey, and tuna, plus raw vegetables and fruits such as spinach, celery, prunes, and bran cereal, for 2 days before the test.
After the procedure
- Have the patient wash up. Clean the hands and perianal area thoroughly.
- Resume activities. Normal diet and medications resume unless otherwise specified.
- Recommend screening. The American Cancer Society recommends regular colorectal cancer screening starting at age 45 for people at average risk, which can be done with a yearly high-sensitivity stool-based test such as a guaiac FOBT (gFOBT) or fecal immunochemical test (FIT) (American Cancer Society). Any positive stool test must be followed by a timely colonoscopy. For home gFOBT screening, samples are usually collected from several separate bowel movements, because bleeding can be intermittent and blood is not evenly distributed in stool (StatPearls).
Normal Results
Stool is examined two ways:
- Macroscopic exam for appearance and color
- Microscopic exam for cell count and meat fibers; leukocyte esterase for leukocytes; Benedict's solution (copper sulfate) for reducing substances; guaiac for occult blood; x-ray paper for trypsin
Abnormal Results
- Unusual appearance:
- Mucous: intestinal wall inflammation
- Bloody: excessive intestinal wall irritation or malignancy
- Frothy or bulky: malabsorption
- Ribbonlike or slender: obstruction
- Unusual color:
- Red: beets and food coloring, lower GI bleed, phenazopyridine hydrochloride compounds, rifampicin
- Black: bismuth (antacid) or charcoal ingestion, iron therapy, upper GI bleeding
- Yellow: rhubarb
- Green: antibiotics, biliverdin, green vegetables
- Grayish white: barium ingestion, bile duct obstruction
- Increased:
- Blood: GI bleeding
- Occult blood: diverticular disease, esophagitis, gastritis, esophageal varices, anal fissure, hemorrhoids, infectious diarrhea, inflammatory bowel disease, polyps, tumors, ulcers, Mallory-Weiss tears
- Leukocytes: bacterial infection of the intestinal wall, ulcerative colitis, shigellosis, salmonellosis
- Epithelial cells: inflammatory bowel disorders
- Carbohydrates/reducing substances: inability to digest some sugars, malabsorption syndromes
- Fats: sprue (celiac disease), cystic fibrosis, pancreatitis
- pH: inflammation from colitis, cancer, or antibiotic use
- Meat fibers: pancreatitis, impaired protein digestion
- Decreased:
- Leukocytes: toxins, cholera, viral diarrhea, amebic colitis, parasites
- Carbohydrates/reducing substances: cystic fibrosis, sprue, malnutrition, medications such as colchicine or birth control pills
- pH: poor absorption of carbohydrate or fat
- Trypsin: malabsorption syndrome, pancreatic deficiency, cystic fibrosis
Frequently Asked Questions
What is a fecal occult blood test looking for?
The FOBT detects hidden (occult) blood in stool that is not visible to the eye. It is used mainly to screen for colorectal cancer and to investigate GI bleeding, since tumors, polyps, ulcers, and inflammatory bowel disease can bleed in small amounts before any symptoms appear.
At what age should colorectal cancer screening start?
The American Cancer Society recommends average-risk adults begin regular screening at age 45 and continue through 75 for those in good health. Screening can use a yearly stool-based test (gFOBT or FIT) or a visual exam such as colonoscopy (American Cancer Society).
What foods and medications can cause a false positive or false negative?
Red meat, certain raw vegetables (radish, turnips, cauliflower, broccoli), and drugs that irritate the gastric mucosa (NSAIDs, anticoagulants, corticosteroids, iron) can cause false-positive guaiac results. High-dose vitamin C over 250 mg per day can cause a false negative. Diet and medication restrictions before a guaiac test reduce these errors.
Does the patient need to follow a special diet before the test?
For guaiac occult blood testing, a red-meat-free, high-residue diet is typically followed for about 2 days, and laxatives, enemas, and suppositories are avoided for 3 days before collection. Newer FIT tests react specifically to human hemoglobin and usually require no dietary restriction.
Why must the specimen be transported promptly?
A liquid or soft stool that cannot be processed within 30 minutes of passage goes into preservative, and a formed stool that cannot be examined immediately is preserved or refrigerated. Delay allows degradation that can alter results, especially for parasite, leukocyte, and trypsin studies.
Does a positive stool test mean the patient has cancer?
No. A positive occult blood test only means blood was detected, which can come from hemorrhoids, ulcers, diverticular disease, or inflammation as well as polyps or cancer. A positive result is followed by a colonoscopy to find the source.