Patients who present with symptoms of central nervous system dysfunction are evaluated with radiographic imaging of the brain to rule out the presence of elevated cerebrospinal fluid pressure. CSF analysis, culture, staining, and immunodiagnostic tests of CSF are the primary diagnostic tests that are performed to diagnose meningitis caused by Cryptococcus. Analysis of the fluid usually reveals a low white blood cell count, low glucose, and elevated protein, but could also be normal in approximately 25-30% of the cases. The culture of the infected fluid yields cream-colored colonies in about 3-7 days, while staining with India Ink permits rapid identification of the Cryptococcus in direct CSF smears. Detection of cryptococcal antigen by immunodiagnostic tests of the serum and the CSF provides a definitive diagnosis of the infection. Different techniques, such as latex agglutination, enzyme-linked immunosorbent assay (ELISA), and lateral flow assays, can be used to detect the cryptococcal antigen.
Disseminated cryptococcosis infection is defined by a positive blood culture or a positive culture from at least two different sites. Disseminated infection is commonly associated with HIV infection or several other immunocompromising conditions.
Cryptococcus can appear as variably-sized budding yeast in clinical specimens. Yeasts range in size from 2 - 20 µm, and a variably sized population of yeast cells are commonly observable in direct and indirect smears. "Soap-bubble" lesions often seen in the brains of cryptococcal meningitis patients are comprised of mucin pools derived from the abundant fungal capsular material present. Regarding stains used in microbiology and histology laboratories:
- Cryptococcus stains poorly using Hematoxylin and Eosin (H&E).
- Cryptococcus is readily highlighted using Periodic acid–Schiff (PAS) and Gomori methenamine silver (GMS) stains.
- The capsule is most commonly demonstrated using the mucicarmine stain.
- The Fontana-Masson stain is positive in metabolically active yeasts and may stain the melanin-like components in the yeast cell walls.
- India ink stain reveals the encapsulated yeast of Cryptococcus; the capsule is readily observable by this staining method as a colorless halo surrounding yeast cells.
The features that are most useful for distinguishing Cryptococcus from other yeasts include the definitive presence of a capsule, narrowed-necked budding, and the size variability of the yeasts.
The top image on the right is a GMS stain demonstrating narrow base budding and characteristic variation in size, while the bottom image is an India ink stain showing the encapsulated yeast of Cryptococcus; the capsule is readily observable by this staining method as a colorless halo surrounding yeast cells.
6. CDC/Ewing, Jr. Image #963. This photomicrograph of a methenamine-stained lung tissue specimen was harvested from an acquired immunodeficiency syndrome (AIDS) patient with cryptococcosis. Histopathology reveals numerous extracellular Cryptococcus neoformans yeast form organisms within the alveolar space. Some of the yeast organisms showed narrow base budding and characteristic variation in size. PHIL public domain. Created 1984. Accessed January 13, 2023. https://phil.cdc.gov/Details.aspx?pid=963 7. CDC/Haley. Image #3771. This photomicrograph of a light India ink-stained specimen revealed some of the ultrastructural morphology exhibited by the encapsulated yeast, Cryptococcus neoformans, one of the causes of the infection known as cryptococcosis. Note the appearance of a halo surrounding each yeast cell represents the observable capsule. PHIL public domain. Created 1969. Accessed January 13, 2023. https://phil.cdc.gov/Details.aspx?pid=3771
