Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1351 page)

   Limitations

   Significant infections caused by a number of enteric pathogens, including
Vibrio
spp., enterohemorrhagic
E
.
coli
, and viral agents, do not show an increase in fecal leukocytes. An increase in fecal leukocytes is not specific for infection and may be caused by other conditions, such as inflammatory bowel disease.

FRANCISELLA TULARENSIS
CULTURE (RULE OUT)

   Definition

   
Francisella tularensis
is a slow-growing, fastidious, gram-negative bacillus capable of producing severe infection, including localized and systemic disease. Infections have typically been acquired by zoonotic tick-borne transmission or direct contact. The common reservoir for organisms includes rabbits, rodents, deer, squirrels, and other wild mammals. Domestic animals may harbor the organism. This organism is easily transmissible, so it is critical that the laboratory be informed whenever tularemia is suspected. Typical disease syndromes include glandular, oculoglandular, and ulceroglandular tularemia; oropharyngeal tularemia; typhoidal tularemia; and pneumonic tularemia. There is great concern regarding the use of this organism for a bioterror-related attack.

   Use

   This culture is used to isolate
F
.
tularensis
from clinical specimens.

   
Method:

   Specimens for
F
.
tularensis
isolation should be inoculated onto cysteineenriched agar media. Blood–cysteine–glucose agar is recommended; most clinical isolates will grow on chocolate, Thayer-Martin, and nonselective buffered charcoal–yeast extract (BCYE) agar. Enriched broth media, such as thioglycollate broth, should also be inoculated. Blood agar and MacConkey agar are typically inoculated for clinical specimens for isolation of other possible pathogens.

   Because of the risk of laboratory-acquired infection and because isolation of
F
.
tularensis
may represent a sentinel event in a bioterror attack, most clinical microbiology laboratories limit the workup of suspected isolates to simple tests to rule out suspicious colonies, referring isolates that fail to “rule out” to their local public health laboratory for identification and further characterization. Final results for testing, therefore, may be delayed compared to common bacterial isolates.

   
Turnaround time:
Isolation and a preliminary identification are usually available within 3–6 days. Additional time is required for transfer to the local public health laboratory, confirmation of identification, and further testing.

   Special Collection and Transport Instructions

   Lymph node aspirate, ulcerative lesions, sputum, BAL, or other localized specimens, depending on the clinical presentation, are usually submitted for diagnosis.