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Infections with Blastomyces dermatitidis and Histoplasma capsulatum cause a variety of clinical manifestations ranging from self-limited, mild pulmonary illness to potentially life-threatening, disseminated disease. Patients at risk for disseminated disease include neonates and immunosuppressed individuals, particularly those with AIDS, hematologic malignancies, or a recent transplant. Primary infections are acquired through inhalation of microconidia that are present in the environment. In the United States, most cases of blastomycosis and histoplasmosis occur along the Ohio and Mississippi River valleys.
The gold standard for diagnosis of blastomycosis and histoplasmosis remains isolation of the organisms in culture. Although sensitive, recovery in culture and subsequent identification may require days to weeks. The organisms can be identified after growth in culture using traditional macro- and microscopic morphologic techniques or through the use of nucleic acid hybridization probes. Hybridization probe-based procedures are rapid and demonstrate good sensitivity and specificity from culture, although some cross-reactivity with relatively uncommon fungal organisms has been reported. Additional diagnostic tests that can be utilized for these organisms include stains, histopathology, serology, and antigen detection with each of these methods offering advantages and limitations depending on the stage of the illness and the status of the patient. Fungal stains (eg, calcofluor white) offer a rapid diagnostic approach, but demonstrate poor sensitivity and specificity. Serologic tests such as complement fixation and immunodiffusion are noninvasive, but are laborious, subjective, and may show low sensitivity, especially in immunocompromised hosts. Antigen detection also offers a noninvasive approach, but has been demonstrated to show cross-reactivity with antigens from closely related fungal species.
Molecular techniques have been established as sensitive and specific methods for the diagnosis of infectious diseases and have the added advantage of a rapid turnaround time for results. Due to the limitations of conventional diagnostic methods for blastomycosis and histoplasmosis, a single tube, real-time PCR assay was developed and verified for the detection and differentiation of Blastomyces dermatitidis and Histoplasma capsulatum directly from clinical specimens.
Rapid detection of Histoplasma capsulatum and Blastomyces dermatitidis DNA
A positive result for Histoplasma capsulatum indicates presence of Histoplasma DNA; a positive result for Blastomyces dermatitidis indicates presence of Blastomyces DNA.
A negative result indicates absence of detectable Histoplasma capsulatum and Blastomyces dermatitidis DNA.
For cases of suspected disseminated histoplasmosis, a Histoplasma urine antigen test should be ordered instead of the rapid PCR assay, since the urine antigen test has increased sensitivity.
Blood should only be drawn for suspected cases of disseminated disease where the PCR may be helpful to distinguish between Histoplasma capsulatum and Blastomyces dermatitidis infection. Blood is not an appropriate specimen source for the diagnosis of localized disease (eg, pulmonary histoplasmosis or blastomycosis).
A fungal blood culture must also be performed.
This rapid PCR assay detects Histoplasma capsulatum and Blastomyces dermatitidis nucleic acid and, therefore, does not distinguish between the presence of viable, disease-related organisms and nucleic acid persisting from previous, treated disease. Test results should be correlated with patient symptoms and clinical presentation before a definitive diagnosis is made.
A negative result does not rule out the presence of Histoplasma capsulatum or Blastomyces dermatitidis because the organism may be present at levels below the limit of detection for this assay.
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