Breath and Stool Testing
Testing exhaled breath or discharged stool can provide invaluable information in the diagnostic evaluation of disorders of the digestive system. Breath and stool testing can be performed at the point of service or can require more sophisticated biochemical or radiopharmaceutical analysis. Appreciating the spectrum and value of such tests enhances one’s diagnostic acumen.
The number and uses of breath tests continue to be expanded. In ancient times astute diagnosticians reported the ability to detect disorders of digestion and the liver from the patient’s breath. A feculent odor of breath can be associated with achalasia, gastroparesis, small intestinal bacterial overgrowth, and partial small bowel obstruction. Fetor hepaticus is sometimes noted in patients with cirrhosis.
An excess of hydrogen concentration in the breath soon after ingestion of nutrients is used by gastroenterologists with greater precision than can be expected by olfaction alone to identify even mild disorders of small intestinal bacterial overgrowth. The same technology can be used to diagnose common disaccharide malabsorption problems with sucrose or lactose when these sugars are used as a substrate. Collection of radiopharmaceutical-labeled substrates can also be used to diagnose these and other malabsorption disorders.
Breath tests are widely used to diagnose the effectiveness of antibiotic therapy for Helicobacter pylori. These tests capitalize on the fact that these fascinating organisms can influence the pH of their environment within the gastric mucus layer by splitting ammonium ions. Tests of exhaled breath after the ingestion of C13-urea is standard for select patients for whom the presence of this infection is clinically important. This includes patients who have had complications from H. pylori infection, such as complex peptic ulcers and MALT lymphomas.
It is important for clinicians to be familiar with the most common stool tests, their strengths, and their limitations. As in all diagnostic evaluations, an evaluation of abnormal stool output should begin with a description of the stool being evacuated. It must be admitted that observing the consistency, color, and other factors of the appearance of the stool contributes only infrequently to the diagnostic information obtained by a carefully extracted history and the application of appropriate tests. Having stated that, it must be emphasized that the accurate description of stool color continues to have high diagnostic value. An acholic stool will certainly suggest biliary obstruction. A tarry stool indicates gastrointestinal bleeding, usually from the upper levels, and a stool colored red with blood means bleeding in the lower intestinal tract.
Although many descriptors have been considered excessive, attention to the appearance of the stool is rarely useful. Furthermore, patients often complain of increased stool odor, but all stool is malodorous. One widely used scale of stool appearance that has been validated is the Bristol Stool Scale. Using this scale can increase the objective assessment of the severity of disordered defecation and enhance the patient’s ability to communicate concerns.
The stool volume output over 24 hours should be determined in any patient with diarrhea that develops in the hospital or who complains of chronic diarrhea. In both settings, stool output is often normal. Hospitalized patients often complain of diarrhea when the problem is actually fecal incontinence. This may result from inadequate intake of fiber in the diet to produce formed stools, loose stools from medications, and deconditioning or illness that leads to weakness of all muscles, including those contributing to continence. Increased liquid or fecal incontinence may also be due to paradoxic diarrhea, in which there is frequent passage of small volumes. Patients with subacute or chronic diarrhea often suffer from altered bowel habits related to irritable bowel syndrome but do not expel more than 250 mL of stool necessary to make an objective diagnosis of true diarrhea. Quantifying stool output while the patient is fasting is also a very useful way of distinguishing diarrhea due to malabsorption (osmotic diarrhea) from diarrhea due to an inflammatory disorder, a secretagog, or surreptitious diarrhea.
The most common test of the stool is to determine the presence or absence of mucosal breaks leading to bleeding. Fecal occult blood tests and the fecal immune test for human hemoglobin have been reviewed in the section on gastrointestinal bleeding.
Patients suspected of having intestinal infections often undergo stool testing; most patients who develop new onset of diarrhea have an infection. The stool test for infection often begins by looking for fecal leukocytes (stool white blood cells). This test is positive when enteroinvasive, severe enterotoxic, and autoimmune enteritis or colitis is the cause. False-negative tests for fecal leukocytes may occur when stool volumes are very high or when the stool specimen is not collected as a fresh specimen, is not placed in preservative, or is contaminated by urine.
Specific causes for acute diarrhea can be determined by several mechanisms. Enteroinvasive bacterial infections are detected by culturing the stool. When Yersinia colitis is suspected, special culture techniques should be requested using selective media such as MacConkey agar. DNA testing of specific virulence markers is also valuable for bacteria such as Yersinia and Escherichia coli, which are plentiful in the microbiota in non pathogenic strains. Identifying the specific bacterial cause of diarrhea is important to direct antibiotic treatment and identify a specific carrier of transmission in epidemic infections. Helminthic infections and parasitic causes of acute diarrhea can be detected by expert microbiology technicians. False-negative tests are common, however, and fresh specimens should be evaluated three times to be certain that a test for ova and parasites is negative. A specific cause for infectious diarrhea often goes unidentified because most infections are due to viruses, toxins, or a strain of E. coli.
The presence of specific infectious agents can be detected by testing the stool for DNA to common pathogens or the toxins they release. This is most commonly performed to detect C. difficile superinfection in patients whose normal microbiota has been altered by antibiotics. Cultures can be used, but sensitive assays for toxin are the most rapid means of detecting this potentially lethal infection and in detecting if the most lethal form of the disease (NAP-1) is present.
Inflammation in the intestinal tract can be detected by identifying leukocytes or their by-products in the stool including lactoferrin, myeloperoxidase, or calprotectin. Blood tests such as sedimentation rate or C-reactive protein can also reveal gut inflammation, but erroneously normal results are not infrequently encountered.
Evaluation of chronic diarrhea should lead to a consideration of malabsorption. This should begin by collecting several stool specimens in ice and sending them to the laboratory to be evaluated for sodium and potassium and stool osmolality. When malabsorption is present, a correctly performed comparison of specimens will detect excess osmolality not explained by electrolytes alone (hyperosmolar). The specimen should also be tested for phenothalines and magnesium, indicating surreptitious laxative use, and for stool creatinine to rule out contamination by urine. Performing the test is not only unpleasant for the patient and nurse, it is technically challenging, because if it is not iced continuously, it will become uninterruptable because of fermentation or steatorrhea caused by the complexity of mechanisms needed to absorb fatty nutrients into cells that primarily consist of water. Steatorrhea can be detected by using a qualitative Sudan stain slide or quantitatively by accumulating a 24-hour collection of all fecal output while the patient is on a 100-g fat diet. This is a difficult diet to follow and a very challenging collection to make.