Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1341 page)

   False-negative reactions may occur, especially due to a prozone effect in serum samples. (Pronase treatment of serum samples decreases the incidence of the prozone phenomenon.) Some isolates from profoundly immunocompromised patients may produce very little polysaccharide capsular material, resulting in false-negative tests.

   There are several sources of false-positive reactions. Positive reactions caused by rheumatoid factor (RF) may be reduced by pretreatment of the specimen with pronase, EDTA, or reducing agents. The syneresis fluid from agar media can cause false-positive results; an aliquot of the specimen for cryptococcal antigen testing should be removed before medium inoculation. Finally, several uncommon pathogens, including
Trichosporon beigelii
and
Capnocytophaga canimorsus
, can cause false-positive cryptococcal agglutination reactions.

   
Common pitfalls:

   Positive cryptococcal antigen titers should be confirmed by culture to document active infection and rule out false-positive reactions. Some infected patients may have very low antigen titers. All specimens submitted for cryptococcal antigen testing should be accompanied by cultures of spinal fluid, blood, or other potentially infected material for fungal isolation.

   Other Considerations

   Antigen titers are usually higher in patients with AIDS compared to those seen in HIV-negative patients with cryptococcal infection. In patients with AIDS, baseline CSF antigen titers <1:2,048 are associated with improved prognosis. Antigen titers should fall with effective antifungal therapy. Steady or increasing cryptococcal antigen titers, even with sterilization of cultures, are an indication of likely treatment failure and recurrence of infection.

CRYPTOSPORIDIUM
ANTIGEN DETECTION

   Use

   This test is used to evaluate diarrheal disease in patients at risk for cryptosporidiosis, specifically for the identification of
Cryptosporidium parvum
in stool specimens.

   
Method:

   Enzyme immunoassays are used. EIAs have very high sensitivity (near 100%) and specificity (near 100%) compared with a series of stool O & P examinations. For information about microscopic
Cryptosporidium
detection, see the test
Ova and Parasite Examination
,
Stool
.

   Different EIA assays for fecal parasite detection have different specimen requirements (fresh vs. preserved) and transport conditions. Laboratories should provide assay-specific information for their providers.

   
Turnaround time:
24–48 hours