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.
STOOL TESTING
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.
