When ordering stool cultures, tell the lab which organism you suspect

In some cases only a brief evaluation is necessary

THOMAS HELTON, DO
Department of General Internal Medicine
Division of Medicine
The Cleveland Clinic Foundation

DAVID D.K. ROLSTON, MD
Clinical Director, A91
Department of General Internal Medicine
The Cleveland Clinic Foundation

A: Evidence to answer this question is scarce. The general consensus is, however, that a patient with any of the following should be evaluated:

• Fever (temperature > 38.5°C; 101.3°F)
• Dysentery (passage of blood and mucus in the stool)
• Symptoms of dehydration, particularly postural lightheadedness, decreased urine output, and excessive thirst
• Worsening diarrhea after 48 hours
• Six or more stools in 24 hours
• Advanced age (> 70 years)
• Compromised immune system
• Age greater than 50 with severe abdominal pain.^1-3

If none of these is present, no further evaluation other than a brief history is generally necessary.⁴ Oral hydration and over-the-counter loperamide usually suffice.

The rest of this article focuses on a cost-effective approach to the evaluation of acute diarrhea.

DEFINITION AND CLASSIFICATION

Diarrhea is defined as stool weight greater than 200 g/day, which is best determined by a 72-hour stool collection.⁵

Given the difficulty and impracticality of obtaining such information, a more useful definition is an increase in the frequency of stools with a decrease in consistency compared with the patient's baseline pattern. Another frequently
used and practical definition is three or more stools in 24 hours.

Diarrhea that has lasted less than 14 days is referred to as acute, while chronic diarrhea has a duration greater than 1 month. Diarrhea that lasts 14 days and resolves within 1 month is generally referred to as persistent acute diarrhea.^1,2

SOCIOECONOMIC IMPACT

Diarrhea is common: the estimated incidence of acute diarrhea in adults in the United States is approximately 99 million cases per year. It is also costly: approximately 50% of patients restrict their daily activities by at least 1 full day, resulting in an estimated $20 billion in lost productivity for those who do not seek physician care. Approximately
250,000 patients are hospitalized, at an estimated medical cost of $560 million. Combining patients who are hospitalized and those who seek medical care but are not hospitalized, the total cost to society is greater than $23 billion annually.^2,6

EVALUATION

Primary care physicians and gastroenterologists often encounter this problem in the office and indeed over the telephone and need to decide whether the patient needs further evaluation. In the initial encounter, the physician
should ask about several important symptoms to determine if further evaluation is necessary (FIGURE 1).

If these symptoms are absent, hospitalization is probably not needed. Patients without these symptoms can usually be treated with oral hydration and over-the-counter drugs such as loperamide, kaolin, or bismuth subsalicylate for symptomatic relief. Loperamide is generally recommended for most cases of acute diarrhea because it is safe and effective.^2,7,8

Thus far, there are few data to support withholding antidiarrheals in the absence of fever greater than 101.3°F or bloody stools.^9,10

History and physical examination
After deciding to proceed with a medical evaluation, the physician faces the dilemma of deciding what is an appropriate evaluation. The history can give clues to the specific pathogen responsible for the illness (TABLE 1).^3,6

Ask about:

• Travel
• Sexual practices
• Antibiotic use within 2 months
• Other medications (eg, laxatives, antacids, digoxin, immunosuppressive drugs)
• Ill contacts
• Group gatherings after which other attendees also developed similar illness
• Recent surgeries or procedures
• Recent meals
• Water source
• Pets (particularly turtles)
• The onset and duration of the illness

The physical examination is helpful in determining the severity of dehydration, but rarely in determining the etiology of diarrhea. It should include:

• General appearance and mental status
• Vital signs, including orthostatic changes in blood pressure and heart rate, as well as temperature
• Abdominal examination
• Stool for occult blood
• Skin turgor and mucus membrane moisture (considered to be of value in children, but less reliable in adults, particularly the elderly).^1,3,6

Characteristic skin lesions can in rare cases point to a specific cause of diarrhea due to underlying systemic diseases,^10,11 eg:

• Migratory necrolytic erythema — glucagonomas
• "Pinch" purpura — amyloidosis
• Generalized hyperpigmentation — Addison disease
• Dermatitis herpetiformis — celiac sprue
• Urticaria pigmentosa, telangiectasia macularis eruptiva persistens, and diffuse cutaneous mastocytosis — mastocytosis
• Carcinoid flush and venous telangiectasia — carcinoid syndrome
• Rose spots — typhoid fever.
  

Diagnostic studies
Stool cultures. Routine stool cultures grow a pathogenic organism in 1.5% to 5.6% of cases, at a cost of $950 to $1,200 per positive result.^2,12

Stool cultures should probably be ordered only if the patient has fever and bloody diarrhea or is immunocompromised, or as part of a research project when investigating an outbreak of diarrhea.

Suspected traveler's diarrhea would also be an indication for stool culture, but many experts believe empiric antibiotics are appropriate in this situation, since these drugs can decrease the duration of illness by 2 to 3 days. Furthermore,
stool cultures may be of little value if the patient responds to empiric therapy.¹³

In hospitalized patients, the yield of stool cultures drops to zero by the third day of hospitalization. If stool cultures are ordered, it is important to inform the laboratory which organism is suspected.

"Routine cultures" will detect Shigella, Salmonella, and Campylobacter species. Special culture media and conditions are required for Aeromonas, Yersinia, and Vibrio species and Escherichia coli O157:H7.⁴ The pretest probability
is based almost entirely on clues from the history and the physician's knowledge of the epidemiology of acute diarrhea.

Testing the stool for ova and parasites is not routinely indicated and should be done only in the following situations:

• Patients with a history of recent travel to Africa, South and Southeast Asia, Russia, the Far East, and the mountainous areas of North America where Giardia, Cryptosporidium, and Cyclospora infections are endemic
• Homosexuals who are immunocompromised (who are at risk for Giardia and Entamoeba histolytica infections)
• Epidemiologic studies for community outbreaks of the aforementioned parasites
• Dysentery with few leukocytes.^11,12

If the patient has recently been hospitalized or has received antibiotics, stool cultures or stool testing for ova and parasites is not warranted, although sending stools for Clostridium difficile toxin testing is justified.

In addition to the aforementioned reasons for the low yield of stool cultures, it is also important to realize that viruses are a common cause of diarrhea and are not detected by routine stool cultures.

Fecal leukocyte testing can further aid in the decision to order stool cultures. A positive fecal leukocyte test suggests an inflammatory cause of acute diarrhea (although the sensitivity of this test is relatively low) and in the
appropriate clinical setting can support obtaining stool cultures.^3,14

Endoscopy is generally not of value in the evaluation of acute diarrhea. However, it may help to determine the cause of the diarrhea and can potentially change the treatment plan if ischemic colitis or inflammatory bowel disease
is in the differential diagnosis. Another situation in which endoscopy may be of value is if the patient is immunocompromised and possibly has cytomegalovirus colitis.^6,10

Other tests. Depending on the clinical severity of the illness, other tests to be considered are:

• Plain film radiography of the abdomen — to evaluate the possibility of colitis, toxic megacolon, or ileus
• A basic metabolic panel — to evaluate renal function and electrolyte status if the patient appears moderately to severely dehydrated
• A complete blood cell count — although rarely helpful in determining the cause of diarrhea, it can suggest an invasive organism if there is leukocytosis with bandemia. It can also be useful to evaluate for neutropenia if the history and physical indicate the patient may be at risk.¹⁵

REFERENCES

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2. DuPont HL. Guidelines on acute infectious diarrhea in adults. The Practice Parameters Committee of the American College of Gastroenterology. Am J Gastroenterol 1997; 92:1962-1975.
3. Park S, Giannella R. Approach to the adult patient with acute diarrhea. Gastroenterol Clin North Am 1993; 22:483-497.
4. Rolston DDK. Acute diarrhea in adults. In: Edmundowcz ED, editor. Twenty Common Problems in Gastroenterology. New York, McGraw-Hill, 2002:159-176.
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9. Cimolai N, Carter J, Morrison B, et al. Risk factors for the progression of E. coli O157:H7 enteritis to hemolytic-uremic syndrome. J Pediatr 1990; 116:589-592.
10. Schiller L. Advances in gastroenterology: diarrhea. Med Clin North Am 2000; 84:1259-1271.
11. Fitzpatrick TB, Johnson RA, Wolf K, et al. In Cooke D, Englis M, Morriss J, editors. Color Atlas & Synopsis of
    Clinical Dermatology, 4th ed. New York, McGraw-Hill, 2001:334,424,490,514.
12. Thielman N, Guerrant R. Acute infectious diarrhea. N Engl J Med 2004; 350:38-47.
13. Guerrant R, Gilder T. Practice guidelines for the management of infectious diarrhea. Clin Infect Dis 2001; 32:331-350.
14. Gore J, Surawicz C. Severe acute diarrhea. Gastroenterol Clin North Am 2003; 32:1249-1267.
15. Bennett RG, Greenough WB. Approach to acute diarrhea in the elderly. Gastroenterol Clin North Am 1993; 22:517-533.
    

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