IM3.7-14 | Pneumonia Diagnostic Testing — Summary & Reflection

KEY TAKEAWAYS

Tiered diagnostic approach: All hospitalised patients: CBC, CXR PA, blood cultures ×2 before antibiotics, sputum GS+C before antibiotics, SpO2/pulse oximetry, urea/creatinine. Moderate-severe (CURB-65 ≥3, SpO2 <94%): ABG, pleural aspiration if effusion pH unknown, Legionella urinary antigen, pneumococcal urinary antigen, HIV test. Specialised: HRCT (CXR-inconclusive, IPA halo, PCP ground-glass, COP migratory), BAL (immunocompromised diffuse infiltrates), viral PCR (epidemic/immunocompromised).

CXR key signs: Air bronchogram (alveolar process, patent bronchi) = consolidation confirmed; lobar homogeneous consolidation (pneumococcus); bulging fissure (Klebsiella); bilateral perihilar ground-glass (PCP, pulmonary oedema); cavity with air-fluid level (abscess — anaerobes/Klebsiella).

Sputum quality: ≥25 PMNs + <10 squamous cells per LPF = acceptable; reject if >10 squamous cells. Gram-positive lancet diplococci = pneumococcus; Gram-negative coccobacilli = H. influenzae; Gram-positive clusters = S. aureus.

Blood cultures: Two sets from two sites before antibiotics; S. aureus and S. pneumoniae always true pathogens; CNS in single bottle = likely contaminant (unless prosthetic device or immunocompromised).

ABG interpretation: Type 1 failure (V/Q mismatch): PaO2 low, PaCO2 low, pH high = respiratory alkalosis. Type 2 failure (ventilatory fatigue/COPD): PaO2 low, PaCO2 high, pH low = respiratory acidosis → NIV or mechanical ventilation. Modified Allen's test before every radial ABG.

Pleural fluid: Light's criteria for exudate; pH <7.2 → immediate intercostal tube drainage; turbid/pus = empyema = drain; Gram stain + culture + cytology + AFB for all aspirations in India.

Mantoux: 2 TU PPD intradermal; read at 48–72 hrs (induration, not erythema); ≥10 mm = positive in immunocompetent adult in India; false-negative in severe immunosuppression (anergy).

REFLECT

Return to the opening scenario — the patient who deteriorated on day 2 despite amoxicillin, with blood cultures and sputum not collected before antibiotics were started. With the framework from this module, you can now identify exactly what was missed and why it mattered. More broadly: every investigation you order on a patient should be preceded by two questions — 'what result do I expect, and what will I do differently if the result is different from what I expect?' If the answer to the second question is 'nothing,' the test should not be ordered. If the answer is 'I will change the antibiotic, start drainage, escalate to ICU, or initiate ART,' then the test is essential and must be done correctly. The diagnostic skills in this module are not just knowledge items for examinations — they are the clinical actions that determine whether your patient is treated accurately or empirically for their entire illness.