Fecal specimens for parasitic examination should be collected before initiation of antidiarrheal therapy or antiparasitic therapy. The highest yield on hospitalized patients occurs when diarrhea is present on admission or within 72 hours of admission. The onset of diarrhea more than 72 hours after admission is usually caused by Clostridium difficile toxin rather than parasites or the usual stool pathogens. The following recommendations are made for efficient and cost-effective diagnosis of diarrheal disease in patients admitted with gastroenteritis. * Submit one or two specimens per diarrheal illness immediately. Consider requesting EIA for Giardia (see tests Giardia lamblia, Direct Detection EIA  and Giardia lamblia, EIA and Ova and Parasite Examination ) if that is the primary suspected organism. * If those are negative, submit an additional specimen after 5 days. * Patients who are immunocompromised by AIDS, malignancy, or immunosuppressive therapy may require additional testing for other unusual stool pathogens (eg, Cyclospora ).