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The page below is a sample from the LabCE course Fungal Infections in Humans. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Cryptococcus - Identification

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. 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 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 µ m, 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. Cryptococcus stains poorly using Haemotoxylin and Eosin (H&E) but 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.
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 encapsulated yeast of Cryptococcus; the capsule is readily observable by this staining method as a colorless halo surrounding yeast cells.
Images courtesy of the CDC.