What is Induced Sputum?
Induced sputum is a diagnostic procedure where patients inhale nebulized hypertonic saline solution to stimulate sputum production from the lower airways, allowing collection of respiratory samples for analysis when patients cannot naturally produce sputum or when spontaneous sputum is inadequate.
The Procedure
The standard technique involves 1:
- Pre-medication with a short-acting bronchodilator (SABA) to prevent bronchospasm
- Sequential inhalation of increasing concentrations of hypertonic saline (typically 3%, 4%, and 5%) for 5 minutes each
- Delivery via ultrasonic nebulizer
- Total duration of approximately 15 minutes
After inhalation, patients expectorate the induced sputum, which is then processed using a mucolytic agent (dithiothreitol), filtered through mesh gauze, and centrifuged to create cytospin preparations for cellular analysis 1.
Key Distinguishing Features from Expectorated Sputum
Induced sputum differs fundamentally from expectorated (spontaneous) sputum:
- Expectorated sputum = naturally coughed up without intervention
- Induced sputum = actively stimulated through hypertonic saline inhalation
The induced method produces higher cell viability (62-65% vs. 41% in spontaneous samples) and more consistent quality for diagnostic analysis 2. Success rates reach 80-90% in adults when patients cannot produce spontaneous samples 3.
Clinical Applications
Induced sputum serves multiple diagnostic purposes 1, 4:
- Airway inflammation assessment - differential cell counts (eosinophils, neutrophils, macrophages, lymphocytes)
- Infectious disease diagnosis - particularly tuberculosis and bacterial infections
- Chronic respiratory conditions - asthma, COPD, eosinophilic bronchitis, chronic cough evaluation
Safety Considerations
The procedure is generally safe but requires specific precautions 3:
- Contraindications: FEV₁ <1.0 L is a relative contraindication
- Monitoring: Spirometry and oxygen saturation should be checked at 7-minute intervals
- Expected effects: Small decreases in FEV₁ (mean 0.098 L) and oxygen saturation may occur 2
- Adverse events: Occasional excessive coughing leading to vomiting; bronchospasm (prevented by pre-treatment with bronchodilator)
Processing Requirements
Critical limitation: Induced sputum requires same-day processing for cell quantification and viability assessment, unlike routine cytology 1. This necessitates specialized laboratory facilities, typically only available in secondary care and specialist centers 3.
The differential cell count is performed by manually counting 400 non-squamous cells on stained cytospin preparations, with results expressed as percentages 3.