Laboratory Diagnosis of Histoplasmosis:

Clinical material:  Skin scrapings, sputum and bronchial washings, cerebrospinal fluid, pleural fluid and blood, bone marrow, urine and tissue biopsies from a variety of visceral organs. 

Direct Microscopy:  (a) Skin scrapings should be examined using 10% KOH  and Parker ink or calcofluor white mounts; (b) Exudates and body fluids  be supposed  to be centrifuged and  the sediment examined using either 10% KOH and Parker ink  or calcofluor white mounts, (c) Tissue sections should be stained using PAS digest, Grocott's methenamine silver (GMS)  or Gram stain.
Histopathology  is particularly useful and is one  of the most significant ways  of alerting  the laboratory  that they  may be dealing  with a potential pathogen. 

Tissue morphology  of H. capsulatum var.  capsulatum (left) showing numerous small narrow base budding yeast cells (1-5um diam) inside macrophages and H. capsulatum var. duboisii (right) showing larger sized budding yeast cells (5-12 um in diam). 

Interpretation:  As a rule, an optimistic direct microscopy demonstrating characteristic yeast-like cells from any specimen should be careful significant. 

Culture:  Clinical specimens should be inoculate onto primary isolation media, like Sabouraud's dextrose agar and Brain heart infusion agar supplement with 5% sheep blood. 

Interpretation: A positive culture from any of the above specimens should be considered significant. 

WARNING:  Cultures of H. capsulatum represent a severe biohazard  to laboratory personnel  and must be handling with extreme caution in an suitable pathogen handling cabinet. 

Serology: Immunodiffusion and/or balance fixation tests for the detection of antibody have proven to  be useful in the analysis of Histoplasmosis,  especially in immunocompetent patients. However, detection of antibodies in immunosuppressed patients is often complicated, with between 20-50% of patients testing negative. 

Identification:  Two variety of H. capsulatum are recognized, depending on the clinical disease: var. capsulatum is the common histoplasmosis, and var. duboisii is the African type. Histoplasma isolates  may also look like species  of Sepedonium and Chrysosporium. Traditionally, optimistic identification required conversion of  the mould form to the yeast phase  by growth at 37C on enriched media; however culture identification by the exoantigen test is now the method of choice.