One is
diagnostically. An HIV-infected patient who presents with central nervous system symptoms
such as headache and lethargy and has cryptococcal antigen detectable in his or
her serum is at high risk for having cryptococcal meningitis. In HIV-infected patients
with culture-proven cryptococcal meningitis, >99% have a positive cryptococcal antigen
test. The second way in which the antigen test can be used is to follow response to therapy.
Cryptococcal antigen can be quantitated in both serum and CSF by serially diluting these
body fluids and determining the highest dilution (most dilute) that produces a positive
agglutination reaction. This test can be used to follow the patient, with titers falling with
successful therapy. Increases in titer may herald relapse (see answer to question 5 for further
details). Both C. neoformans and C. gattii are positive in this test. Patients with sepsis
due to Trichosporon asahii and Capnocytophaga canimorsus can have a false-positive cryptococcal
antigen test.