Page 88 - AIDSBK23C

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Both the direct fluorescence antibody (DFA) and immunohistochemical stains, as well as
the polymerase chain reaction (PCR), are available for diagnosis of Pneumocystis. The direct
fluorescent antibody (DFA) stains available for diagnosis of Pneumocystis in cytologic
specimens are usually obtained from induced sputum or bronchoalveolar lavage. Sensitivity
with the DFA technique is good, but specificity requires skill in interpretation. PCR on BAL and
biopsy material for P jiroveci (carinii) has a sensitivity of 100% but a specificity around 80%
and should be considered in cases of atypical PCP where few organisms are present.[408,409]
PCR applied to oropharyngeal washings in HIV-positive patients has a low sensitivity and
specificity for the diagnosis of PCP.[410] Real time PCR has a diagnostic sensitivity of 100%
with specificity of 96%.[397]
Fluorescence microscopy may aid in screening of Papanicolaou stained organisms that
will demonstrate a bright yellow autofluorescence (from the eosin component of the stain) of the
P jiroveci (carinii) cell wall. Immunofluorescence procedures are not technically difficult and
allow diagnosis in less than 1 hour.[411]
All of these methods can be highly sensitive and specific when performed routinely.
Immunohistochemical staining is slightly more sensitive than GMS stains in tissues and
fluids.[412] The DFA is more sensitive than the calcofluor white method in cytologic
preparations.[408] The Diff-Quik and DFA are the most cost-effective.[413]
Serum markers have been sought for diagnosis of PCP. Beta-D-glucan (BDG) is the
major fungal cell wall component and has a diagnostic sensitivity near 100% and specificity of
88% for PCP in HIV infection. BDG has also been used as a serologic marker for the diagnosis
of candidiasis and aspergillosis. KL-6 antigen is a high-molecular-weight mucin-like
glycoprotein antigen that is strongly expressed on type 2 alveolar pneumocytes and bronchiolar
epithelial cells. KL-6 elevation has a sensitivity of 88% and specificity of 63% for PCP with
HIV. Both of these tests do not perform as well for PCP detection when the patients do not have
HIV infection.[414]
The clinical, gross, and microscopic appearances of P jiroveci (carinii) pneumonia (PCP)
are described fully in the section on respiratory tract pathology. Dissemination of Pneumocystis
outside of the lungs is uncommon (less than 5% of cases). The use of aerosolized pentamidine
isethionate, without systemic therapy, as a prophylaxis against Pneumocystis pneumonia has
been suggested as a possible etiologic factor for this phenomenon. Only hilar lymph nodes, or
another single organ, may be involved, while in rare cases multiple organs are affected (Table
5).[415]
The extrapulmonary microscopic appearance of Pneumocystis is often similar to that of
the alveoli, with a foamy to granular pink exudate on hematoxylin-eosin stain. However, in
widely disseminated cases, P jiroveci (carinii) can produce numerous small 0.1 to 0.3 cm
calcified granulomas that give cut surfaces of parenchymal organs the gross appearance of rough
sandpaper. These calcifications can demonstrate a stippled appearance on roentgenography, as
in the pointillist style of painting. In the spleen, multiple nonenhancing, low-density masses with
necrosis, hemorrhage, or peripheral to punctate calcification may be seen with computed
tomographic scans. Ultrasound may reveal small echogenic foci in liver parenchyma.
Microscopically, foamy to granular pink exudate may be present with extensive calcium
deposition. P jiroveci (carinii) may coexist with other infectious agents, particularly
mycobacteria, at disseminated sites. A Gomori methenamine silver stain reveals the organisms,
even in densely calcified areas, but immunoperoxidase staining with monoclonal antibody to P
jiroveci (carinii) can be helpful when cysts are not readily identified.[415,416]