A 41-year-old man presents at the Emergency department with a one month history of dry cough, shortness of breath and a six month history of weight loss

A 41-year-old man presents at the Emergency department with a one month
history of dry cough, shortness of breath and a six month history of weight loss. He is
a non-smoker.
He was discharged from hospital two months ago following an episode of
pneumonia. At this time blood cultures had grownStreptococcus pneumoniae. He
recovered fully following discharge. His past medical history includes a case of
shingles treated by his GP a year ago but nil else of note.
On examination he appears breathless but is able to complete sentences. His
respiratory rate is 22/min, and chest expansion, percussion and auscultation are
normal. Heart rate is 100/min with normal heart sounds, JVP is not raised and there
is no oedema. Abdominal and neurological examinations are unremarkable.
His temperature is 37.5°C and blood pressure 110/76 mmHg. O2 saturations are 97%
at rest but drop to 92% on exercise.
Chest x ray shows full resolution of his previous consolidation and no abnormalities
are seen.
Blood tests are taken and reveal:
Haemoglobin 139 g/L (115-165)
White cell count 9.11 ×109
/L (4.00-11.00)
Platelets 132 ×109
/L (150-400)
Sodium 135 mmol/L (135-145)
Potassium 3.7 mmol/L (3.5-5.0)
Chloride 104 mmol/L (98-108)
Urea 6.6 mmol/L (2.5-7.5)
Creatinine 105 µmol/L (40-130)
Albumin 32 g/L (32-45)
Bilirubin 15 µmol/L (<20)
Alanine transaminase 45 U/L (<50)
Aspartate transaminase 30 U/L (<40)
Alkaline phosphatase 123 U/L (40-150)
CRP 130 mg/L (0-10)
What is the most likely investigation to confirm the diagnosis?

CT chest
Induced sputum
Urinary Legionella antigen
Serum angiotensin-converting enzyme (ACE)

This man has Pneumocystis jirovecii formerly known asPneumocystis carinii (PCP)
pneumonia due to HIV infection.
Diagnosis can be made by induced sputum (sensitivity 50-90%) or bronchoscopy +
bronchoalveolar lavage (sensitivity 90-100%) sent for polymerase chain reaction
(PCR) and/or immunofluorescence (or toluidine blue, silver stain or periodic acidSchiff) to show characteristic cysts.
This is a common presentation of late stage HIV infection. Many new presentations
of HIV such as this will not have obvious risk factors for the infection at presentation.
PCP is more common in patients with CD4 count less than 200 and in those with oral
thrush, weight loss or recent (within three months) bacterial pneumonia.
The classic pattern of PCP is a decreased oxygen transfer as evidenced by the drop
in exercise saturations. It typically has a more insidious course than bacterial
pneumonia. Chest x ray may reveal bilateral fluffy infiltrates in lower zones or may be
CT chest may show alveolar shadowing but would rule out bronchocarcinoma.
However, drop in exercise saturations would not be typical of bronchocarcinoma.
A one month gradual history is not typical of pulmonary embolism (PE).
Symptomatic infection is unlikely to present with a normal chest x ray.
Serum ACE may make the diagnosis of sarcoid but is not diagnostic.