Serology of Human Cryptococcosis

SUMMARY

Sixty-nine patients with cryptococcal meningoencephalitis and six patients with nonmeningeal cryptococcosis were studied serologically. Twenty-nine of the 75 patients had serum anticryptococcal antibody by an indirect fluorescence test (IFA). Titers ranged from 1:1 to 1:32. Forty-seven patients had cryptococcal antigen in serum or cerebrospinal fluid (CSF) detected by a complement fixation (CF) test. Of the 47, 7 also had serum antibody. Antigen titers ranged from 1:1 to 1:2048 in CSF and from 1:4 to 1:512 in serum. Only six patients, four with cryptococcal meningoencephalitis, were negative to both serologic tests.

Among the patients with cryptococcal meningoencephalitis, those with Cryptococcus in india ink smears of CSF usually had cryptococcal antigen in blood, CSF, or both. Patients with negative india ink smears despite positive CSF cultures tended to have serum anticryptococcal antibody. The patients with anticryptococcal antibody were generally less ill than those with antigen and obtained a better response to therapy with amphotericin B.

Although the CF test for antigen has been specific for cryptococcosis, a few false-positive tests for antibody were found. Among 253 control sera, 1.6% were IFA positive, as were sera from 6% of blastomycosis patients and from 12% of histoplasmosis patients. A cryptococcal skin test was also noted to produce false-positive IFA reactions in 31% of 48 patients and could create errors in serologic diagnosis.

The two serologic tests used together have sufficient specificity and sensitivity to be a useful aid in the rapid diagnosis of cryptococcosis.