Serum cyrptococcal antigen: diagnostic value in the diagnosis of AIDS-related cryptococcal meningitis

The incidences of HIV-AIDS patients with opportunistic infections of the central nervous system are increasing. Of these, cryptococcal meningitis is the most important and serious. A simple method for the diagnosis of cryptococcal meningitis is needed despite its variable clinical features and the lack of a capacity in most health facilities in Thailand to exclude it from other diseases especially mass lesions in the brain.

To identify the capability and cut off point of serum cryptococcal antigen for diagnosis and screening of cryptococcal meningitis in HIV-AIDS patients.

One hundred consecutive cases of HIV-AIDS patients suspected of having central nervous system infections were prospectively recruited for the study. The serum of all patients were examined for cryptococcal antigen by latex agglutination test, the Pastorex Cryptococcus manufactured by Sanofi Diagnostic Pasteur, France. If a test was positive, the serum dilution was carried out using 10-fold serial dilution. Every patient went through pre-defined standard investigations to derive at a definite diagnosis. The gold standard for diagnosis of cryptococcal meningitis was the presence of encapsulated yeast forms in the cerebrospinal fluid or a positive culture for cryptococcal neoformans from the cerebrospinal fluid.

Of 100 patients enrolled in this study, 58 patients had cryptococcal meningitis and serum cryptococcal antigen was detectable in 60 patients. If the cut-off point for a positive test was when the serum cryptococcal antigen titer was more than zero, then, the sensitivity of the test was 91.4 per cent, the specificity was 83.3 per cent, likelihood ratio if test positive (LR+) was 5.47, likelihood ratio if test negative (LR-) was 0.1, false positive was 16.7 per cent, false negative was 8.6 per cent.

We conclude that serum cryptococcal antigen is a simple and rapid screening method for diagnosis of cryptococcal meningitis.