4. Stool assays for Clostridium difficile lack a high degree of sensitivity.
North America is experiencing an explosion in illness related to Clostridium difficile (primarily C. difficile associated diarrhea, or CDAD) (3). C. difficile is most often a nosocomial infection, and CDAD has become a very common disease in patients hospitalized for any length of time who are given broad-spectrum antibiotics. Hospitalization is the “perfect storm” for CDAD. C. difficile spores exist in the hospital environment and are ingested by patients on broad-spectrum antibiotics that inhibit the normal flora, creating the perfect environment for C. difficile to flourish. The assays to detect C. difficile toxins in the stool are often not highly sensitive; under the best of circumstances the techniques used by most hospital labs will produce a false-negative result 10–20% of the time (4). Not all labs detect all toxin types, and not all kits are highly efficient for detecting toxins. Patients in whom there is a strong suspicion for CDAD should be treated even in the face of negative toxin assays unless there is another likely source of diarrhea.
5. Hospitalized patients who develop diarrhea after admission almost never have enteric infections other than CDAD.
Patients who develop diarrhea after being in the hospital 1 or 2 days almost never have infection with Salmonella, Campylobacter, Entamoeba histolytica, or Giardia sp. It is not uncommon to see hospitalized patients who develop diarrhea after several days in the hospital with shotgun orders for “SSYC and O&P.” Unless there is a history of immunosupression or risk factors for enteric infection such as international travel, these tests are almost always unnecessary (5). 6. The most common cause of leukocytosis of unknown etiology in hospitalized patients is CDAD. There is something about the pathogenesis of CDAD that produces a leukemoid reaction much more often than other infections do. It is not unusual to see white blood cell counts of 30,000 with CDAD, and counts of 50,000 and higher in patients with CDAD are not rare. Patients who develop leukocytosis in the hospital while on antibiotics, or who present from long-term care facilities with marked leukocytosis and recent antibiotic exposure, have a high pretest probability of having CDAD (6). In this setting, the higher the white count, the more likely the patient has CDAD.
6. Blood cultures should always be obtained before parenteral antibiotics are given for a febrile illness.
Patients who are given broad-spectrum antibiotics have 1 opportunity to have interpretable blood cultures obtained: before antibiotics are administered. Once patients are given broad-spectrum antibiotics, blood cultures have a very limited value in diagnosing infections that might not be initially suspected on admission. A common example in our hospital is a patient presenting with pneumonia. About a third of the patients who come through the emergency room with a diagnosis of community-acquired pneumonia end up having another diagnosis. Often the alternative diagnosis is suspected based on blood cultures obtained prior to the patient receiving broad-spectrum antibiotics in the emergency room. In the last 3 months we have seen patients with liver abscesses, endocarditis, and osteomyelitis initially felt to have community-acquired pneumonia whose blood cultures initiated prior to antibiotic therapy revealed a pathogen that caused a search for an alternative source of infection. The vast majority of patients only need 2 blood cultures from 2 sites 20 minutes apart before initiation of antibiotic therapy. Patients in whom common skin contaminants may easily be interpreted as pathogens (such as patients with prosthetic heart valves) should have 3 sets of blood cultures to aid in the interpretation of cultures that are positive for skin contaminants such as coagulase negative staph.