Pneumocystis pneumonia represents a potentially life-threatening infection caused by the fungal microorganism Pneumocystis jiroveci. Initially observed in premature and malnourished infants, Pneumocystis pneumonia is today one of the most frequent opportunistic infections in adults infected with human immunodeficiency virus (HIV).
As nonspecific symptoms often hinder a timely diagnosis, one of the most important diagnostic tools for Pneumocystis pneumonia is a high level of clinical suspicion. Clinicians should always consider this diagnosis in HIV-infected patients that complain of shortness of breath, nonproductive cough and/or fever. Microbiological and molecular methods are then employed as a step towards a final diagnosis.
Microbiological and molecular methods
As Pneumocystis jiroveci is not amenable to classic culture methods, the gold standard for the diagnosis of Pneumocystis pneumonia has traditionally been the microscopic demonstration of the organism in respiratory specimens. The visualization of this fungal pathogen is, however, highly dependent upon the experience and skill of the observer.
Cysts can be effectively stained with toluidine blue, methenamine-silver or calcoflour white. In contrast, trophic forms, which are more abundant during the development of pneumonia, are usually detected with Diff-Quik and Wright-Giemsa staining procedures. Today monoclonal antibodies are also used due to their improved sensitivity and specificity of diagnosis from sputum samples.
The expansion of molecular techniques – most notably polymerase chain reaction (PCR) assays – has provided a more reliable approach in the clinical diagnosis of this condition. In research studies on respiratory specimens from adult patients, PCR has shown similar specificity and increased sensitivity when compared to standard microscopy.
In addition, real-time PCR can aid in quantification of the organism load, which can further help in differentiating whether Pneumocystis jiroveci is colonizing or infecting the host. This type of PCR can even be run on formalin-fixed samples of bronchoalveolar lavage fluid that are embedded in paraffin, with sensitivities approaching 83%.
On a chest film, Pneumocystis pneumonia typically presents with bilateral or dispersed ground-glass opacities. Some less common patterns have been reported as well, such as pulmonary nodules, lobar infiltrates and pneumatoceles. Chest radiographs may also be normal-appearing in at least one third of cases.
When high-resolution computed tomography is used, diffuse ground-glass opacities with patchy distribution are often seen, suggesting the accumulation of debris, intra-alveolar fibrin and causative microorganisms. A predilection for the upper parts of the lungs has also been described to be a characteristic sign.
Although serum β-D-glucan (which is a part of the cell wall of Pneumocystis and other fungi) is not specific for Pneumocystis infections, its measurement has been used as a useful tool in the diagnosis of Pneumocystis pneumonia, or at least in screening for the disease.
However, it must be noted that false-positive results can arise due to certain factors such as:
- certain antimicrobial drugs
- the administration of immunoglobulin
In addition, the cut-off value for the diagnosis of Pneumocystis pneumonia still needs to be determined.
Krebs von den Lungen-6 (abbreviated as KL-6) is another marker that is expressed on pneumocytes and epithelial cells of the bronchi. It acts as a sensitive indicator of different types of interstitial pneumonitis. Therefore in cases of Pneumocystis pneumonia, this marker is elevated as a result of injury and subsequent regeneration of alveolar epithelial cells.
Likewise, serum lactate dehydrogenase (LDH) can indicate lung tissue injury, but its diagnostic prominence is inferior when compared to β-D-glucan. In short, all these biomarkers are promising, but their diagnostic performance should be appraised further in order to ascertain the predictive values in at-risk patients. These results should be combined with the microbiological assessment.
Reviewed by Liji Thomas, MD