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HISTORY The definition of diarrhea varies within the medical literature, so it is not surprising that patients' definitions vary too. Many patients come to the emergency department complaining of "diarrhea" when what they really have is soft stools or two stools per day compared with their usual one. Strictly speaking, diarrhea is present when the daily stool weight exceeds 200 g.10 Practically speaking, however, diarrhea is present when the patient is making more stools of lesser consistency, more frequently.

Once a true diarrheal illness is confirmed, the physician's focus should change toward attempting to ascertain the cause of the diarrhea. As already mentioned, there are many causes of diarrhea (IabJ.e...,7.,9.-.1). Fortunately, the history usually leads the physician to the etiology. The first step is to determine whether the diarrhea is acute (<3 weeks) or chronic (>3 weeks). The acute diarrheas are of greatest concern to the emergency physician, for they are more apt to be a manifestation of an immediately life-threatening illness (infection, ischemia, intoxication, or inflammation). 10 The next step is to define the diarrhea. Is it bloody or melanotic? Is it associated with the ingestion of certain foods, such as milk or sorbitol? What symptoms accompany the diarrhea? Is there fever or abdominal pain, which may suggest diverticulitis or infectious gastroenteritis? Seizures accompanying diarrhea often point toward shigellosis but could also indicate theophylline toxicity. Does the patient have heat intolerance and anxiety, suggesting thyrotoxicosis, or paresthesias and reverse temperature sensation, suggesting ciguatera?

Finally, define the host. A patient's medical and surgical history often assists in narrowing the differential diagnosis. For example, diarrhea resulting from malabsorption secondary to pancreatic exocrine insufficiency need not be considered in an otherwise healthy host. Conversely, the differential diagnosis for diarrhea is broadened for a patient with acquired immunodeficiency syndrome (AIDS). Medications commonly have diarrhea as a side effect or sequel. Is the patient taking medication that may have contributed to the diarrhea (e.g., antibiotics, lithium, chemotherapy, colchicine, and laxatives)? Has the patient traveled outside the United States or to the countryside recently? Rural hiking places the patient at risk for Giardia, particularly if water purification procedures were not strictly followed, and travel to Third World countries increases the chances of parasitic infection. Sexual and occupational histories are also important. A patient's sexual preference or occupation may be the physician's only clue to a diagnosis of gay bowel disease or organophosphate poisoning.

PHYSICAL EXAMINATION As with a vomiting patient, the examination begins with the ABCs. Thus, assessment of hydration status occurs shortly after the physician arrives at the bedside. Like the history, a careful physical examination can help narrow the differential diagnosis. Only by doing a thyroid examination can the physician discover a thyroid mass that may be contributing to diarrhea. Abdominal and rectal examinations are critical. Especially in the elderly, fecal impaction may result in diarrhea as liquid stool passes around the impaction. Special attention should be given to the presence or absence of surgical scars, tenderness, masses, or peritoneal signs. Checking the stool for the presence or absence of blood is also important, since bloody diarrhea can be caused by inflammation, infection, or ischemia. An elderly patient with bloody diarrhea and abdominal pain out of proportion to the physical examination may have mesenteric ischemia—a true emergency.

DIAGNOSTIC STOOL EVALUATION Tests specific to the emergency department evaluation of a patient with diarrhea include Wright's stain for fecal leukocytes; stool culture for bacteria, ova, and parasites; and stool analysis for Clostridium difficile toxin. Diagnostic testing, while rarely helpful to the emergency physician, occasionally is helpful to the primary care physician. Who should be tested? Stool cultures for bacteria should be obtained in children, toxic patients, patients with a protracted diarrheal illness lasting longer than 3 days, and immunocompromised patients in whom infectious diarrhea is suspected. A request for an ova and parasite evaluation can be made for patients at risk for parasitic disease. In adddition, patients at risk for C. difficile colitis should have a stool sample sent for C. difficile toxin assay.

Wright's Stain When applied to a stool sample, Wright's stain allows detection of fecal leukocytes. A positive Wright's stain has a sensitivity of 82 percent and a specificity of 83 percent for the presence of bacterial pathogen.11 Historically, Wright's stain for fecal leukocytes has been used to differentiate invasive and noninvasive infectious diarrheas. In the past, this was an important distinction because physicians were reluctant to prescribe antibiotics for patients with infectious diarrhea because of the fear of prolonging the Salmonella carrier state. They therefore reserved antimicrobial treatment for the toxic patients who they felt truly had invasive diarrhea. Recently, this dictum has been questioned, and many physicians now treat patients with diarrheal illness with antibiotics regardless of whether or not the diarrhea is invasive or bacterial in origin.15 Therefore, ascertaining the presence or absence of fecal leukocytes is superfluous if it does not change management.

Bacterial Stool Culture Bacterial stool culture is an expensive and labor-intensive diagnostic test that plays a minor role in the emergency department evaluation of a patient with diarrhea. Most laboratories culture for only three common bacterial pathogens: Salmonella, Shigella, and Campylobacter. Because of a low sensitivity for detecting pathogens, each positive routine stool culture costs the laboratory approximately $950 to $1200, depending on the number of samples tested and the number of tests run on each sample.9 To limit cost and increase yield, diagnostic testing should be limited to patients with high pretest probabilities of bacterial disease: severely dehydrated or toxic patients, children, immunocompromised patients, and patients with diarrhea lasting longer than 3 days. 9 In addition, if other enteric pathogens are suspected, the laboratory should be notified so that appropriate testing may be performed.

Ova and Parasitic Evaluation Patients in whom a parasitic cause of diarrhea is suspected should have stool sent for evaluation for ova and parasites. These tests lack sensitivity, since many parasites are fastidious. Recently, direct immunoflouresence staining has been shown to improve the sensitivity for detecting Giardia and Cryptosporidium.12

Clostridium Difficile Toxin Assay Diarrhea in a patient with an antecedent history of antibiotic use may be caused by pseudomembranous colitis from C. difficile infection. This is diagnosed with the C. difficile toxin assay. Unfortunately, this assay has a 10 percent false-negative rate and is rarely available to the emergency physician, since the turnaround time on the test approaches 24 h.13

Other Diagnostic Tests If diarrhea is not felt to be infectious in origin, data acquisition should be dictated by the differential diagnosis. Rarely are serum chemistry results or complete blood count results diagnostic or necessary. Electrolyte measurements are warranted in severely dehydrated patients, regardless of the cause of dehydration. They can also be helpful in patients experiencing an addisonian crisis. Serum drug levels can assist the physician in making the diagnosis of theophylline, lithium, or heavy metal intoxication. In patients with a history of abdominal surgery, abdominal films may help rule out partial obstruction as a cause of diarrhea. Finally, a chest radiograph may help diagnose an occult pneumonia (i.e., Legionella) in a patient with diarrhea and a cough. For patients in whom mesenteric ischemia is suspected, mesenteric angiography is the test of choice.

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