Step by Step
Pj
Pneumocystis jirovecii — reclassified as a fungus
Pneumocystis jirovecii was once thought to be a protozoan, but is now classified as a fungus. It causes pneumonia specifically in immunocompromised patients, and is considered an AIDS-defining illness when it occurs in a patient with CD4 below 200.
Dx
Diagnosis — cannot be cultured
Pneumocystis cannot be grown in standard culture. Diagnosis instead requires bronchoalveolar lavage (BAL) with GMS (Gomori methenamine silver) staining, which shows characteristic cup-shaped cysts.
Sx
Presentation — progressive dyspnea and bat-wing infiltrates
Patients develop progressive dyspnea, a dry (non-productive) cough, fever, and hypoxia — often more severe than the chest X-ray appearance would suggest. The classic imaging finding is bilateral, diffuse interstitial infiltrates, sometimes described as a "bat-wing" pattern. LDH is typically markedly elevated and correlates with disease severity.
Tx
Treatment — TMP-SMX, plus steroids if hypoxic
First-line treatment is high-dose TMP-SMX for 21 days. If the patient is significantly hypoxic (PaO₂ below 70 mmHg), corticosteroids are added to reduce inflammation and improve survival. TMP-SMX is also used as prophylaxis in any patient with CD4 below 200, preventing the infection before it starts.
A patient with AIDS and a CD4 count of 150 develops progressive shortness of breath over several days along with a dry cough; CXR shows bilateral interstitial infiltrates and labs show a markedly elevated LDH — this is PCP, treated with high-dose TMP-SMX, with steroids added given the degree of hypoxia.
Applied Walkthrough
1
A patient with AIDS and a CD4 count of 150 develops progressive shortness of breath over several days, along with a dry cough and low-grade fever.
2
Chest X-ray shows bilateral, diffuse interstitial infiltrates, and labs show a markedly elevated LDH. Oxygen saturation is significantly reduced, with a PaO₂ of 62 mmHg.
3
Ask: what's the diagnosis, and how aggressively should it be treated? The CD4 count under 200, the bilateral infiltrate pattern, and the elevated LDH all point to PCP.
4
Given the PaO₂ of 62 (below the 70 mmHg threshold), corticosteroids should be added alongside high-dose TMP-SMX — this combination reduces inflammation and has been shown to improve survival specifically in more hypoxic patients.
Exam Application
Exams test the CD4 threshold for PCP as an AIDS-defining illness (below 200), the diagnostic approach (BAL with GMS stain, since it cannot be cultured), the classic presentation (bilateral infiltrates, high LDH), and the treatment structure — TMP-SMX for 21 days, with steroids added if PaO₂ is below 70 mmHg, and prophylactic TMP-SMX for any patient with CD4 below 200.
⚠ Common Trap
The most common trap is forgetting that Pneumocystis jirovecii is now classified as a fungus, not a protozoan (an older classification some materials still reflect) — and forgetting that it cannot be cultured, meaning diagnosis relies specifically on staining a BAL sample rather than growing the organism.
✓ Quick Self-Check
1. What type of organism is Pneumocystis jirovecii currently classified as?
A fungus — it was previously thought to be a protozoan.
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2. At what CD4 count is PCP considered an AIDS-defining illness?
Below 200.
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3. How is PCP diagnosed, given that it cannot be cultured?
Via bronchoalveolar lavage (BAL) with GMS (Gomori methenamine silver) staining, showing cup-shaped cysts.
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4. What is the classic chest X-ray finding in PCP, and what lab value correlates with severity?
Bilateral, diffuse interstitial infiltrates ("bat-wing" pattern); LDH is markedly elevated and correlates with severity.
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5. When are corticosteroids added to PCP treatment, and what is the standard treatment?
Corticosteroids are added if PaO₂ is below 70 mmHg; standard treatment is high-dose TMP-SMX for 21 days.
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