Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1424 page)

   Extracted antigen is detected by specific antibodies using standard immunologic techniques, such as LA or EIA. The sensitivity of antigen tests varies by technique and specific kit used, ranging from 60% to 95%; the specificity of most tests exceeds 95%. Therefore, throat culture has been recommended to confirm negative antigen tests, but is not needed to confirm positive tests.

   The Group A Streptococcus Direct Test (Gen-Probe, San Diego, CA) is an FDA-approved molecular diagnostic assay for the detection of
S
.
pyogenes
in pharyngeal specimens. Group A streptococci are detected using a specific DNA probe directed against specific
S
.
pyogenes
rRNA sequences. Sensitivity of the assay is 88–95% with specificity of 98–99.7%. The high sensitivity and specificity for this test allow test results to stand without the need for confirmation of positive or negative tests.

   
Special collection and transport instructions:
Throat swab specimens are collected as recommended for throat cultures.

   
Turnaround time:
<4 hours for antigen tests; <24 hours for molecular tests.

   Interpretation

   
Expected results:
Negative.

   
Positive results:
Positive results are diagnostic of group A streptococcal pharyngitis in patients with consistent clinical findings.

   
Negative results:
Negative antigen tests decrease the likelihood of group A streptococcal pharyngitis but must be confirmed by a more sensitive technique, such as throat culture or molecular detection.

   Limitations

   Only swabs specified by the manufacturer may be used for the Gen-Probe assay.

STREPTOCOCCUS PNEUMONIAE
URINE ANTIGEN TEST

   Definition

   Streptococcus
pneumoniae
is a leading bacterial cause of pneumonia globally and is the most common agent leading to hospitalization in all age groups.
S. pneumoniae
are gram-positive, typically lancet-shaped diplococci. It is the most frequently encountered bacterial agent of community-acquired pneumonia (CAP). Because of the significant morbidity and mortality associated with pneumococcal pneumonia, septicemia, and meningitis, it is important to have diagnostic test methods available that can provide a rapid diagnosis.