Pneumocystis pneumonia, a (major) opportunistic infection in immunocompromised patients, is caused by the fungus Pneumocystis jirovecii (P. jirovecii). The incidence of Pneumocystis pneumonia, which increased dramatically with the advent of the HIV/AIDS pandemic, has decreased in the industrialized world owing to the widespread use of sulfa drug prophylaxis and the introduction of highly active antiretroviral therapy (HAART). However, P. jirovecii remains an important cause of morbidity and mortality in HIV/AIDS patients, as well as in non-HIV immunocompromised patients, in whom its diagnosis is difficult.
Current diagnosis of P. jirovecii is mostly done by direct microscopic examination of bronchoalveolar lavage (BAL) samples. The fungus is visualized using standard staining techniques or immunofluorescence staining. The drawback of these methods is that diagnosis is difficult and requires specific skills, particularly when the fungal burden is low, which is mainly observed in non-HIV infected patients. Nowadays, (real-time) PCR is increasingly used for the diagnosis of Pneumocystis pneumonia as this technique has an increased sensitivity compared to direct microscopic examination.