Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1423 page)

   
Shigella
species may not survive broth enrichment techniques.

   
Common pitfalls:

   Rectal swabs can collect only a small amount of feces; their use should be restricted to infants.

   
Shigella
species are fastidious and may not survive changes in stool pH that occur after passage. Rapid transport and/or the use of transport media is important for reliable isolation by culture.

   Other Considerations

   Direct detection of
Campylobacter
antigen in stool is an alternative to culture for diagnosis of enteric campylobacteriosis. Sensitivity of antigen testing is 80–96% with specificity approximately 98%.

   Absence of normal fecal flora or the presence of predominant growth of yeast,
S
.
aureus
, or
Pseudomonas aeruginosa
can be recognized by inspection of the SBA plate, providing clinically relevant information about alternative diagnoses.

   Oxidase testing may be performed on SBA isolated with heavy growth to screen for unexpected enteric infection caused by
Vibrio
,
Aeromonas
, or
Plesiomonas
species.

STREPTOCOCCUS, GROUP A, DIRECT DETECTION (ANTIGEN, NUCLEIC ACID)

   Definition

   The results of direct tests for group A streptococci may guide early therapy. In antigen tests, the group A cell wall polysaccharide is extracted from a throat swab.

   Use

   Direct detection tests for group A beta-hemolytic streptococci (
Streptococcus pyogenes
) are used for direct diagnosis of streptococcal pharyngitis. Patients present with symptoms including sore throat, fever, headache, and abdominal pain.

   Method: