Pneumocystis jirovecii pneumonia (PCP), previously known as Pneumocystis carinii pneumonia, is an opportunistic fungal lung infection occurring almost exclusively in immunocompromised individuals. In 50% of cases, PCP is the first manifestation of AIDS (acquired immune deficiency syndrome), but it may be caused by other immunodeficiency disorders. PCP should be suspected in patients with a history of progressive dyspnea and a dry cough with resistance to standard antibiotic treatment. Signs that support this diagnosis include a CD4 count < 200/μL, an increased beta-D-glucan level, and diffuse bilateral infiltrates on chest x-ray. Management of PCP includes high-dose trimethoprim/sulfamethoxazole (TMP/SMX), treatment of the underlying immunodeficiency disorder, and steroids in the case of severe respiratory insufficiency.
- Pathogen: P. jirovecii (former P. carinii): ubiquitous, yeast-like fungus previously classified as a protozoan
- Route of transmission: airborne 
- Risk factors
- History and clinical examination
- Oxygen saturation
- CD4 count
- Beta-D-glucan level
- Chest x-ray (if CXR is inconclusive, a CT scan should be performed)
The diagnosis should generally be confirmed via microscopic identification of P. jirovecii from respiratory secretions.
- Chest auscultation
- Pulse oximetry: oxygen saturation < 90 % at rest (worsens with exertion)
- CD4 count: typically < 200/μL
- Laboratory measures
- ↑ Beta‑D‑glucan levels 
- ↑ LDH (typically)
- Polymerase chain reaction (PCR) 
- Confirmatory test: microscopic identification of P. jirovecii
- Typically symmetrical, diffuse interstitial infiltrates extending from the perihilar region
- May sometimes be normal
- Indicated if PCP is still suspected in a patient with a normal CXR
- High sensitivity for PCP (a negative scan thus suggests the diagnosis of PCP is unlikely)
- Typical features
Acute infection 
- Treatment of choice: high‑dose TMP/SMX for up to 3 weeks
- Glucocorticoids: add in the case of severe respiratory insufficiency (either PaO2 < 70 mmHg or arterial‑alveolar oxygen gradient ≥ 35 mmHg)
- Alternatives if allergic to TMP/SMX or treatment of choice is ineffective
PCP prophylaxis 
We list the most important complications. The selection is not exhaustive.
PCP was formerly the most common cause of death in HIV‑positive patients during the early years of the HIV epidemic. 
- The introduction of highly active antiretroviral therapy) and TMP/SMX prophylaxis in patients with a CD4 count < 200/μl has significantly improved the long-term outcome of the disease. (
- Despite adequate prophylaxis, PCP infection may lead to death in patients presenting with severe symptoms (e.g., respiratory failure).