What indicates a lower respiratory tract infection with greenish purulent sputum?

Greenish Purulent Sputum Indicates High Bacterial Load in Lower Respiratory Tract Infections

Greenish purulent sputum in lower respiratory tract infections indicates the presence of myeloperoxidase from neutrophils and correlates with high bacterial loads (≥10^7 CFU/mL), being 94.4% sensitive and 77.0% specific for detecting significant bacterial colonization. 1

Pathophysiology of Green Sputum Color

The green color results from myeloperoxidase, an enzyme released by neutrophils (polymorphonuclear leukocytes) during the inflammatory response. 1 This is not simply "pus" but specifically reflects:

• High neutrophil content: Myeloperoxidase levels are elevated when bacterial loads reach ≥10^7 CFU/mL 1
• Bacterial burden: Purulent sputum has median bacterial loads of 10^7-10^8 CFU/mL, compared to 7.5 × 10^6 CFU/mL in mucoid sputum 1
• Predominant organism: In 92% of high bacterial load specimens, one bacterial species predominates 1

Clinical Significance and Diagnostic Value

In COPD Exacerbations

Green purulent sputum strongly predicts bacterial infection requiring antibiotics. 2 In a study of 121 COPD patients:

• 84% with purulent sputum had positive bacterial cultures versus only 38% with mucoid sputum (p < 0.0001) 2
• C-reactive protein was significantly elevated (median 4.5 mg/L) with purulent sputum 2
• Sensitivity 94.4% and specificity 77.0% for high bacterial load 1, 2

The most common organisms isolated are:

• Haemophilus influenzae (most common) 1
• Streptococcus pneumoniae 1
• Moraxella catarrhalis 1
• Pseudomonas aeruginosa (in severe COPD with FEV1 < 50%) 1

Critical Caveat for Acute Bronchitis

However, purulent sputum does NOT indicate bacterial infection in otherwise healthy patients with acute bronchitis. 1 This is a crucial distinction:

• Purulence results from inflammatory cells or sloughed epithelial cells, which can occur with viral infections 1
• Over 90% of acute bronchitis cases are viral 1
• Antibiotics are not recommended for acute bronchitis regardless of sputum color 3

Diagnostic Algorithm for Greenish Purulent Sputum

Step 1: Assess for Pneumonia First

Check for:

• Tachycardia (≥100 beats/min) 1
• Tachypnea (≥24 breaths/min) 1
• Fever (≥38°C) 1
• Focal consolidation on chest exam (rales, egophony, fremitus) 1

If present → Treat as pneumonia with appropriate antibiotics 4

Step 2: If No Pneumonia, Determine Underlying Condition

For COPD patients:

• Antibiotics indicated if all three Anthonisen criteria present: increased dyspnea, increased sputum volume, AND increased sputum purulence 3
• Consider antibiotics with two criteria if purulent sputum is one of them 3
• In severe COPD (FEV1 < 30%, >4 exacerbations/year), obtain sputum culture to guide therapy 1

For otherwise healthy adults with acute cough:

• Do NOT prescribe antibiotics based on sputum color alone 1, 3
• Green sputum in acute bronchitis does not warrant antibiotics 3, 5

Sputum Quality Assessment for Microbiological Testing

When obtaining sputum for culture and Gram stain, quality is paramount:

Adequate specimen criteria: 6

• <10 squamous epithelial cells per low-power field (100x) 6

• 25 polymorphonuclear cells per low-power field 6

• Purulent appearance (predominantly neutrophils) 6

Gram stain interpretation: 1, 6

• Most useful when showing predominant morphotype (≥90% of organisms) 1, 6
• Sensitivity/specificity for S. pneumoniae: 35.4%/96.7% 1, 6
• Sensitivity/specificity for H. influenzae: 42.8%/99.4% 1, 6

Common Pitfalls to Avoid

1. Do not equate green sputum with bacterial infection in acute bronchitis - This leads to inappropriate antibiotic use 1, 3

2. Do not rely on sputum color alone - Always consider clinical context, vital signs, and underlying lung disease 3

3. Recognize that only 39-40% of pneumonia patients produce adequate sputum - Inability to produce sputum does not rule out bacterial infection 6

4. Prior antibiotics dramatically reduce diagnostic yield - Obtain specimens before starting treatment when possible 6

5. Even with bacteremic pneumococcal pneumonia, sputum cultures are positive in only 40-50% of cases - Negative culture does not exclude bacterial infection 1, 6