Sputum is mucus coughed up from the lower airways of the lungs, and it is different from saliva that originates in the mouth. This mucus can contain various cells, including a type of white blood cell called an eosinophil. While the presence of some sputum eosinophils is normal, their quantity is what matters. An elevated percentage of these cells in sputum is a direct indicator of a specific kind of inflammation within the airways, providing a focused view of immune activity.
The Role of Eosinophils in the Lungs
Eosinophils are a component of the body’s immune system. Their function is associated with fighting off certain types of infections, particularly those caused by parasites, and they are also involved in allergic responses. These cells originate in the bone marrow and migrate into various body tissues, where they release molecules to combat foreign invaders.
In some individuals, this protective mechanism can become dysregulated within the respiratory system. Large numbers of eosinophils accumulate in the lung airways as part of a misdirected inflammatory reaction. Once present in high concentrations, these eosinophils release potent chemicals and proteins. These substances can damage the airway lining, leading to chronic inflammation, increased mucus production, and airway hypersensitivity.
This accumulation and release of inflammatory mediators is known as eosinophilic airway inflammation. The process contributes directly to the narrowing of airways and the respiratory symptoms experienced in certain lung conditions. The activity of these cells explains why the underlying inflammation driven by eosinophils must be addressed to manage the condition effectively, not just the symptoms.
The Sputum Collection and Analysis Process
A sample of lung mucus can be obtained in two primary ways. The simplest method is spontaneous sputum, where a patient produces a sample by coughing deeply. This sample must come from the lower airways, not just from clearing the throat, to be useful for analysis.
A more common method is sputum induction. This procedure involves the patient inhaling a nebulized mist of sterile saline solution. This mist helps to loosen mucus from the lower lungs and makes it easier to cough up. The process is performed under the supervision of a healthcare professional to ensure safety and effectiveness.
Once collected, the sputum sample is sent to a laboratory for processing. Technicians select the thickest portions of the sample and treat them with specific chemical stains that make different cell types visible under a microscope. A trained technician then examines the slide, identifies the eosinophils, and counts them relative to other cells. The result is reported as the percentage of eosinophils.
Clinical Significance of Sputum Eosinophil Levels
The percentage of eosinophils in a sputum sample has direct clinical importance. A level of 2-3% or higher is considered elevated and indicates the presence of eosinophilic airway inflammation. This finding helps classify a patient’s respiratory disease by pointing toward a specific inflammatory pathway. This is useful for distinguishing between conditions that present with similar symptoms, such as a chronic cough or wheezing.
One of the primary conditions identified through this testing is eosinophilic asthma. This is a specific subtype of asthma characterized by high levels of eosinophils in the airways. Identifying a patient as having eosinophilic asthma is important because it suggests their disease is driven by a particular inflammatory mechanism, differentiating it from non-eosinophilic asthma.
Another condition is non-asthmatic eosinophilic bronchitis (NAEB). Patients with NAEB experience a persistent cough due to eosinophilic inflammation but do not show the airway narrowing characteristic of asthma. Sputum analysis is fundamental in diagnosing NAEB. Furthermore, a subset of individuals with Chronic Obstructive Pulmonary Disease (COPD) also exhibit eosinophilic inflammation, a finding that can influence their management plan.
Guiding Treatment Decisions
The sputum eosinophil count directly influences how respiratory conditions are treated. A high count is a strong predictor that a patient will respond favorably to corticosteroid medications. These anti-inflammatory drugs are effective at reducing the number and activity of eosinophils in the airways. Therefore, a high eosinophil level suggests that prescribing inhaled corticosteroids (ICS) will likely lead to symptom improvement.
For patients with severe eosinophilic asthma who do not achieve adequate control with ICS, the sputum eosinophil count is a biomarker for determining eligibility for biologic therapies. These medications are proteins designed to target specific molecules in the immune system. Several of these biologics work by directly targeting the pathways that promote eosinophil survival and activity, such as the IL-5 pathway.
By blocking these specific targets, these therapies can lower eosinophil counts in the blood and tissues, leading to fewer asthma exacerbations and improved lung function. The decision to initiate these targeted treatments often depends on objective evidence of eosinophilic inflammation. A confirmed high sputum eosinophil count provides the justification that a patient’s asthma is driven by the mechanism these drugs inhibit, leading to a more personalized strategy.