Source: UptoDate
Two types of tests are available for rapid diagnosis of disseminated histoplasmosis: antigen determination and histopathologic examination of tissue samples. Serological tests for antibody to H. capsulatum and culture of the organism are less rapid methods but can aid in the diagnosis of disseminated histoplasmosis.
Antigen testing — Tests for antigen in urine and serum should be performed in patients with suspected disseminated histoplasmosis. This test is performed in several reference laboratories. Reports from the laboratory that established the enzyme immunoassay showed that Histoplasma antigen can be detected in the urine of approximately 90 percent and serum of approximately 70 percent of cases, and less frequently in other sterile body fluids
(graph 1). Please note that one of the authors (LJW) is owner and Director of MiraVista Diagnostics, a reference laboratory that performs Histoplasma antigen testing.
Urine antigen appears to be the most sensitive rapid assay in all population groups (table 1). False positive results in urine can occur with blastomycosis, coccidioidomycosis, paracoccidioidomycosis, and penicilliosis .
False positive test results in serum have been reported in approximately 16 percent of solid organ transplant recipients due to the formation of human anti-rabbit antibodies after the receipt of rabbit anti-thymocyte globulin (RATG) [8]. Modifications to the assay have eliminated this cross reactivity, improved specificity without compromising sensitivity, and permitted quantitation [9,10].
Other specimens, such as CSF and bronchoalveolar lavage, should be tested in selected patients who have evidence of disease at specific sites [4,11,12].
Histopathology — Biopsy of a skin lesion or other tissue can reveal the typical two to four micron yeast structures of H. capsulatum. Although the organism sometimes can be demonstrated by hematoxylin and eosin stain, it is better visualized using methenamine silver or periodic acid Schiff stains (picture 4).
Antibody tests — Serologic tests for anti-Histoplasma antibodies using the immunodiffusion and complement fixation methods are positive in two-thirds to three-quarters of cases of disseminated infection. However, the results are often falsely-negative in patients who are immunocompromised. (See "Diagnosis and treatment of histoplasmosis in HIV-infected patients".)
Cultures — Blood cultures should be performed in all suspected cases. Using the lysis-centrifugation technique, cultures are positive in 50 to 70 percent [2,3], including those with AIDS [13]. BACTEC methods for isolation of fungi will yield H. capsulatum with comparable sensitivity to the lysis-centrifugation method, but the time to positivity is longer [14].
Bone marrow aspiration and biopsy for fungal stains and culture should be considered in patients who are anemic, leukopenic, or thrombocytopenic. Bone marrow cultures are positive in over 75 percent of cases [2,3,15].
Histoplasma can also be isolated from other specimens:
* In patients with diffuse interstitial or miliary pulmonary infiltrates, H. capsulatum may be recovered from sputum [16], bronchoalveolar lavage specimens [11], and/or lung tissue in up to 70 percent of cases.
* Urine cultures may be positive in up to 10 percent of patients [17].
* Biopsy of the mouth, skin, gastrointestinal lesions, enlarged lymph nodes, adrenal glands, or liver has provided a diagnosis by culture in approximately one-quarter of patients with disseminated disease [1,2,16].
*In the future, please only post links, not entire articles*
Post Edited By Moderator (Nanners) : 6/13/2010 8:10:50 AM (GMT-6)