What are the causes of brownish green sputum?

Causes of Brownish Green Sputum

Brownish green sputum is most commonly caused by bacterial respiratory infections, with the color indicating the presence of high bacterial loads and neutrophilic inflammation. 1

Primary Causes

1. Bacterial Respiratory Infections

• Haemophilus influenzae: Particularly common in COPD patients 1
• Pseudomonas aeruginosa: Especially in severe COPD (FEV₁ <30%) and those with risk factors 1
• Streptococcus pneumoniae: Common in milder respiratory infections 1
• Moraxella catarrhalis: Often seen in bronchitis and COPD exacerbations 1
• Enterobacteriaceae: More common in severe lung disease 1

2. Disease-Specific Associations

• COPD Exacerbations: Green purulent sputum is 94.4% sensitive and 77.0% specific for high bacterial loads (10⁷-10⁸ CFU/mL) 1, 2
• Bronchiectasis: Chronic colonization with periodic exacerbations 1
• Pneumonia: Often accompanied by other symptoms like fever and chest pain 1

Pathophysiology of Sputum Color

The brownish-green color results from:

• Myeloperoxidase: Released by neutrophils responding to infection 1
• High bacterial load: Typically 10⁷-10⁸ CFU/mL in purulent sputum vs. 7.5×10⁶ CFU/mL in mucoid sputum 1
• Inflammatory cells: Primarily polymorphonuclear leukocytes 1, 3

Diagnostic Significance

The presence of green purulent sputum:

• Has 81% sensitivity for bacterial infection (95% CI: 70-88%) 4
• Has moderate specificity of 50% (95% CI: 35-65%) 4
• Is significantly more likely to yield positive bacterial cultures (84%) compared to mucoid sputum (38%) 2

Clinical Approach to Brownish Green Sputum

1. Assess for signs of acute respiratory infection:

   • Increased dyspnea
   • Increased sputum volume
   • Fever
   • Chest pain

2. Consider underlying conditions:

   • COPD severity (FEV₁ <50% increases likelihood of H. influenzae and P. aeruginosa) 1
   • Recent hospitalizations or antibiotic use (risk factors for P. aeruginosa) 1
   • History of bronchiectasis or chronic bronchitis

3. Diagnostic workup:

   • Sputum Gram stain and culture for patients with:
     • Severe symptoms
     • Hospitalization
     • Failed outpatient therapy
     • Risk factors for resistant organisms 1, 3
   • Ensure proper specimen collection:
     • Deep-cough specimen before antibiotics
     • ≥25 polymorphonuclear leukocytes per low-power field
     • <10 squamous epithelial cells per low-power field 3, 5

Important Caveats

• Not all brownish green sputum requires antibiotics: Despite the correlation with bacterial infection, sputum color alone should not dictate antibiotic therapy without considering clinical context 6
• False positives: Some viral infections can also cause neutrophilic inflammation and purulent sputum 1
• Specimen quality matters: Poor-quality specimens with oropharyngeal contamination can lead to misleading results 3, 5
• Chronic colonization: In chronic lung diseases, bacteria may be present even during stable periods 1

Special Considerations

• In asthma exacerbations: Antibiotics are generally not recommended unless there is evidence of pneumonia or bacterial sinusitis 1
• In acute bronchitis: Purulent sputum alone does not indicate need for antibiotics in otherwise healthy individuals 1
• In COPD: Consider antibiotics for purulent sputum plus increased dyspnea or sputum volume (Anthonisen Type I or II) 1

Brownish green sputum should prompt consideration of bacterial infection, but treatment decisions should incorporate the overall clinical presentation rather than sputum color alone.