Pneumocystis jirovecii pneumonia (PCP), previously known as Pneumocystis carinii pneumonia, is an opportunistic fungal lung infection occurring almost exclusively in immunocompromised individuals. It is an HIV-associated condition but may be caused by other immunodeficiencies, including primary immunodeficiency disorders, immunodeficiency resulting from malignancy, following a solid organ or stem cell transplant, or secondary to the long-term use of immunosuppressants such as high-dose steroids. PCP should be suspected in immunocompromised patients with a history of progressive dyspnea, dry cough, or hypoxia. Other findings that further support the diagnosis include an increase in β-d-Glucan and LDH levels, and diffuse bilateral infiltrates on chest imaging. PCP is confirmed if P. jirovecii is detected on an induced sputum sample, bronchoalveolar lavage, or lung tissue biopsy. Management of PCP includes high-dose trimethoprim/sulfamethoxazole (TMP/SMX), treatment of the underlying immunodeficiency disorder, and, in some cases, adjunctive glucocorticoids. Prophylaxis regimens for PCP should be considered for patients with significant immunosuppression and commonly include the use of long-term, low-dose TMP/SMX.
- Pathogen: P. jirovecii (former P. carinii): ubiquitous, yeast-like fungus previously classified as a protozoan
- Route of transmission: airborne 
- Risk factors
Symptoms of PCP 
- May be asymptomatic initially
- Symptoms can have a gradual onset (days to weeks) and include:
- May progress to fulminant respiratory failure
The clinical course of PCP is usually more acute and severe in HIV-negative patients compared to HIV-positive patients. HIV-positive patients often initially have an indolent course with mild exertional symptoms and no fever or cough. 
Clinical examination 
- Chest auscultation
- Pulse oximetry: ↓ oxygen saturation (e.g., < 90% at rest and worsens with exertion)
Consider PCP in patients with respiratory symptoms or unexplained hypoxia and a history of HIV or other causes of impaired (e.g., organ transplantation, immunosuppressive medications, malignancy). See also “Diagnosis of pneumonia.”
- Order an initial pneumonia workup, including:
- Confirm the presence of PCP.
- Identify the underlying cause of immunosuppression (if unknown). 
Because P. jirovecii cannot be routinely cultured, it requires confirmation via histopathological, cytopathological, or molecular identification of P. jirovecii from respiratory secretions or lung tissue.
Nonspecific markers for PCP
- ↑ β-d-Glucan 
- ↑ LDH 
- Arterial blood gas: ↓ PaO2 and ↑
- ↓ CD4 count (in HIV-positive patients): typically < 200 cells/mm3
Imaging studies 
- X-ray chest
- CT chest without contrast (HRCT may increase diagnostic accuracy)
Diagnostic confirmation 
Histopathology (preferred confirmatory test): identification of P. jirovecii
Method: Staining enables visualization of disc-shaped P. jirovecii cysts with central spores.
- Giemsa, Wright, and Diff-Quik stain the cystic and trophic forms but not the cyst wall.
- Methenamine silver, cresyl violet, and toluidine blue stain the cyst wall.
- Molecular testing: alternative to histopathology or cytopathology for PCP diagnosis
PCP treatment 
- General considerations
- Disease severity
- High-dose TMP/SMX is the treatment of choice.
- A 21-day antibiotic course is recommended for most patients, regardless of the antibiotic regimen. 
- For patients with treatment failure :
- Consider changing to an alternative antibiotic regimen under the guidance of an infectious disease specialist.
- Exclude concomitant infections.
|Antibiotic therapy for PCP |
|Mild to moderate PCP|
|Moderate to severe PCP|
- HIV-positive patients
- HIV-negative patients: decision on a case-by-case basis in consultation with a specialist 
- Supportive care (see also “Supportive therapy” in “Pneumonia”)
PCP prophylaxis 
- Goal: to reduce the probability of opportunistic infections from PCP in at-risk populations (specialist consultation is advised)
Primary prophylaxis; indicated for patients with no history of PCP with either:
- HIV infection with a CD4 cell count < 200 cells/mm3
- Immunosuppression from other causes, e.g:
- Secondary prophylaxis: indicated for patients with a history of PCP with immunosuppression in order to prevent recurrence
- Prophylactic regimens: See also “Prevention of opportunistic infections in HIV” for dosages. 
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).