Sputum analysis in diagnosis and management of obstructive airway diseases

Abstract

Induced sputum analysis has recently emerged as a potential new clinical tool in the diagnosis and management of obstructive airway diseases such as asthma, chronic obstructive pulmonary disease, and other disorders including bronchiectasis. Its safety has been demonstrated in numerous studies, and its efficacy is superior to previous techniques for determining airway inflammation. It is a noninvasive and highly reproducible approach in generating a measurable index of inflammatory cells in the airways of the lungs. Recent studies have shown that exacerbations, particularly in patients with moderate to severe asthma, can be reduced by routine analysis of induced sputum samples. We now have the ability to clinically apply sputum measurements to manage asthmatics. Inflammatory markers and cell types in induced sputum can also be investigated using newer technologies with more sensitive qualitative and quantitative features than basic cellular analysis. This review outlines the procedure for sputum induction, characterizes inflammatory cell types in the sputum, and addresses recent advances in the field of sputum analysis.

Figure 1

Example of a procedure for sputum induction. This scheme demonstrates a sputum induction protocol that is currently in use at the University of Alberta Hospital pulmonary function clinic. Patients are subjected to spirometry and given β₂-agonist, followed by a second spirometric analysis. Based on the resulting FEV₁ and FEV₁/FVC ratio, the patient is given either 3% hypertonic saline for standard sputum induction or 0.9% isotonic saline for higher risk patients (to prevent the risk of exacerbations resulting from higher concentrations of saline). Patients who exhibit severe bronchoconstriction or poor lung volume are withdrawn from sputum induction. If the patient's asthma is severe from a clinical perspective, or requires oral prednisone for disease control, the patient should commence with high risk induction. Nebulized saline is administered on a repeated basis with accompanying spirometry to check for falling FEV₁ or patient discomfort. Provided that spirometry results and patient comfort are within acceptable limits, a total of three steps of nebulized saline inhalation is conducted. Sputum is collected at each step following inhalation of saline and processed for analysis.

Figure 2

Eosinophil counts and concentrations of eosinophil cationic protein (ECP) in induced sputum of asthmatic and healthy nonatopic control subjects. Reprinted from Louis and colleagues (2000), with permission obtained from the authors and the American Thoracic Society.

Figure 3

Detection of modified tyrosine residues from activated eosinophils and neutrophils in induced sputum as determined by nuclear magnetic resonance analysis. (a) Spectrum from an induced sputum sample obtained from a cystic fibrosis patient, compared with (b) a spectrum from control sputum. Peaks corresponding to tyrosine and some modified tyrosine residues are indicated. Spectral traces are from Saude et al 2004).

Figure 4

Increased 3-chlorotyrosine production in induced sputum of cystic fibrosis (CF) patients determined by nuclear magnetic resonance (NMR) analysis. Induced sputum samples from seven CF patients were analyzed by NMR to determine their levels of modified tyrosine residues. (a) The production of 3-chlorotyrosine, specific for neutrophil activation, was significantly elevated in CF sputum samples compared with control sputum samples. (b) Sputum levels of 3-chlorotyrosine correlated significantly with the percentage of sputum neutrophils from CF patients. Figures from Saude and et al (2004).