Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1340 page)

   
Negative results:
Submission of multiple specimens may be required for
C
.
diphtheriae
isolation.

CRYPTOCOCCUS
ANTIGEN TEST

   Use

   This test may be ordered for the early diagnosis of infections caused by
Cryptococcus neoformans
. It is usually appropriate for immunocompromised patients presenting with clinical signs of meningitis. Testing is most sensitive when testing CSF for cryptococcal meningitis. Testing serum has a lower sensitivity for confirmation of infection at other sites. Determination of antigen titer (testing twofold serial dilutions of the specimen) is recommended for positive CSF specimens to monitor response to treatment.

   Method:

   There are several formats for commercially available cryptococcal antigen tests, most commonly latex agglutination assays. In these assays, latex particles are coated with polyclonal or monoclonal antibodies against
C
.
neoformans
antigens.

   Agglutination at dilutions of 1:8 or greater indicates active disease. Approximately 95% of patients with cryptococcal meningitis are detectable by cryptococcal antigen testing of the CSF.

   The sensitivity for CSF is 93–100%, and for serum, it is 83–97%. Specificity for both specimen types is typically >95%.

   
Turnaround time:
<24 hours

   Interpretation

   
Expected results:
Negative.

   
Positive results:
Cryptococcal infection very likely. Positive results should be confirmed by culture.

   
Negative results:
Cryptococcal infection unlikely. Use fungal culture to definitively rule out cryptococcal infection.

   Limitations