What investigations should be done for a patient with shortness of breath, confusion, and generalized weakness, with an initial oxygen saturation (SPO2) of 89% on room air that improved to 100% with supplemental oxygen, bilateral crepitations (rales) present, and a chest x-ray showing right lower lobe consolidation and collapse?

Investigations for Suspected Community-Acquired Pneumonia with Respiratory Compromise

This patient requires immediate pulse oximetry (already done), arterial blood gas analysis, complete blood count with differential, chest radiography (already done), blood cultures prior to antibiotics, and sputum culture if obtainable, with urgent assessment for ICU-level care given the presence of confusion, hypoxemia, and radiographic consolidation. 1

Immediate Severity Assessment

This patient meets criteria for severe community-acquired pneumonia based on:

• Confusion/disorientation (minor criterion for severe CAP) 1
• Respiratory rate assessment needed - if ≥30 breaths/min, this is another minor criterion 1
• Hypoxemia with SpO2 89% on room air (PaO2/FiO2 ratio likely <250) 1
• Bilateral crepitations suggesting multilobar involvement 1

The presence of ≥3 minor criteria warrants ICU admission consideration. 1

Essential Laboratory Investigations

Complete Blood Count with Differential

• Obtain CBC within 12-24 hours of symptom onset to assess for leukocytosis (WBC ≥14,000 cells/mm³) or leukopenia (WBC <4,000 cells/mm³) 1
• Manual differential is preferred to assess band forms and immature neutrophils 1
• Left shift (band neutrophils ≥6% or ≥1,500 cells/mm³) indicates bacterial infection 1
• Leukopenia (WBC <4,000 cells/mm³) is associated with excess mortality and complications including ARDS, particularly in gram-negative pneumonia 1
• Thrombocytopenia (platelet count <100,000 cells/mm³) is associated with worse prognosis 1

Arterial Blood Gas Analysis

• Mandatory given SpO2 <92% on room air to assess PaO2, PaCO2, and acid-base status 1
• Hypoxemia (PaO2 <60 mmHg) is a major predictor of mortality in the Pneumonia Prognosis Index 1
• PaO2/FiO2 ratio <250 is a minor criterion for severe CAP 1
• Helps guide oxygen therapy targets (maintain SpO2 94-98% in most patients, or 88-92% if COPD risk) 1, 2

Metabolic Panel

• Urea/BUN - level ≥20 mg/dL is a minor criterion for severe CAP 1
• Creatinine and electrolytes to assess renal function and detect complications 1, 3
• Liver function tests including lactate dehydrogenase and creatinine kinase 1
• Glucose - hypoglycemia in non-diabetics suggests severe infection 1
• Lactate level - elevated lactate indicates tissue hypoperfusion and worse prognosis 1

Blood Cultures

• Obtain two sets of blood cultures before initiating antibiotics 1
• Particularly important given confusion (possible sepsis/bacteremia) and severe presentation 1
• Yield is low in community settings (5-40 episodes per 100,000 resident days), but critical in severe CAP 1
• Essential for identifying polymicrobial bacteremia and guiding targeted therapy 1

Microbiological Investigations

Sputum Culture

• Obtain expectorated sputum if available before antibiotic initiation 1, 3
• Do not perform nasopharyngeal aspirate as it generates aerosols 1
• Helps identify causative organism and antibiotic susceptibilities 3

Additional Considerations

• Urine sample (20-30 mL) for pneumococcal and Legionella antigen testing if available 1
• Stool sample only if gastrointestinal symptoms present 1

Imaging Studies

Chest Radiography (Already Performed)

• Confirms right lower lobe consolidation and collapse 1, 3
• Assess for complications: multilobar infiltrates, large pleural effusions, congestive heart failure, mass lesions 1
• Multilobar infiltrates are a minor criterion for severe CAP 1

Consider CT Chest if:

• Clinical deterioration despite appropriate therapy 4, 5
• Suspicion of complications (empyema, abscess, pulmonary embolism) 6, 7
• Recurrent pneumonia in same location (to exclude underlying structural abnormality) 8

Risk Stratification Biomarkers

Cardiac Biomarkers

• Troponin levels - elevation associated with increased mortality risk 1
• Natriuretic peptides (BNP/NT-proBNP) - assess for right ventricular dysfunction 1

Inflammatory Markers

• C-reactive protein - helps assess severity and response to therapy 1

Critical Clinical Pitfalls

• Do not delay antibiotics while awaiting culture results - obtain cultures then immediately start empiric therapy 1, 3
• Confusion in elderly or any patient warrants aggressive evaluation - it is a marker of severe disease 1
• Leukopenia is more ominous than leukocytosis - associated with ARDS and septic shock 1
• Hypothermia (core temperature <36°C) carries worse prognosis than fever 1
• Consider pulmonary embolism if sudden deterioration occurs, especially with elevated D-dimer 6, 7

Immediate Management Priorities

• Supplemental oxygen to maintain SpO2 94-98% (avoid high-flow >6 L/min initially to reduce aerosol generation) 1, 2
• Initiate empiric antibiotics immediately after cultures: IV co-amoxiclav 1.2g TDS or cefuroxime 1.5g TDS plus macrolide (erythromycin 500mg QDS or clarithromycin 500mg BD) 1, 3
• Fluid resuscitation if hypotensive, but monitor carefully for fluid overload 1, 2
• ICU consultation if ≥3 minor criteria or either major criterion present 1