In the diagnosis of respiratory diseases, sputum induction has played an important role in the diagnosis of various respiratory diseases over the years. It is also being used increasingly to study pathophysiology of various respiratory diseases. For bronchial asthma diagnosis, Curschmann’s Spirals and Charcot Leyden crystals are popularly used to diagnose the condition. Sputum cytology for presence of malignant cells and various biochemical markers in sputum are also used for the diagnosis as well as a prognostic marker during follow-up cases of suspected lung cancer. In various respiratory infections, isolation of various pathogens e.g., bacteria, fungi, viruses are helpful in diagnoses of respiratory infections in the sputum analysis.

• Respiratory diseases
• Sputum
• Lung cancer
• Sputum analysis

Gaude G, Pujar K (2024) Role of Induced Sputum in Respiratory Diseases. J Chronic Dis Manag 8(2): 1043.

In the diagnosis of respiratory diseases, sputum induction has played an important role in the diagnosis of various respiratory diseases over the years. It is also being used increasingly to study pathophysiology of various respiratory diseases [1]. For bronchial asthma diagnosis, Curschmann’s Spirals and Charcot Leyden crystals are popularly used to diagnose the condition. Sputum cytology for presence of malignant cells and various biochemical markers in sputum are also used for the diagnosis as well as a prognostic marker during follow-up cases of suspected lung cancer. In various respiratory infections, isolation of various pathogens e.g., bacteria, fungi, viruses are helpful in diagnoses of respiratory infections in the sputum analysis.

The inhalation of an aerosol of hypertonic saline to produce sputum was firstly used by Bickerman et al., in 1958 in cytology of lung cancer [2]. Induced sputum was later used for clinical purposes to diagnose various pathogenic microorganisms [3,4]. More than three decades ago, sputum induction was widely used for the diagnosis of pulmonary tuberculosis as it was superior in yield in comparison to gastric lavage. After the invent of fibreoptic bronchoscopy, it was abandoned for few years. But, again revived because of lower risk of TB transmission and low cost [5,6]. Induced sputum has several advantages over other techniques. Bronchoscopy is an invasive  procedure and is not easily applicable on a large scale and in follow- up studies. Sputum analysis serves as an alternative to this, for obtaining airway secretions that may potentially be used for monitoring airway inflammation [4]. Although fibreoptic bronchoscopy with transbronchial biopsy, bronchial brushing and bronchoalveolar lavage are relatively safe procedures they still entail some morbidity and are relatively unpleasant and expensive procedures compared with sputum induction [1]. With sputum induction, samples can be obtained from the lower airways with minimal discomfort to the patient.

In the past decade, various studies favour superior yield of sputum induction over fibreoptic bronchoscopy in diagnosis of pulmonary tuberculosis [7]. Moreover, patient tolerability and safety are good [5,6]. Procedure is also feasible in developing countries setting and in young children [8]. Sputum induction may be particularly helpful in diagnosis of pulmonary tuberculosis in resource limited countries. First attempt to standardize sputum induction was made in early nineties. European Respiratory Society formed the first task Force on sputum induction method for standardizing the procedure [9].

Sputum Induction and Pharmacological Agents Used

Various agents have been used for sputum induction over the years. These includes N-acetyl cysteine, distilled water, normal saline, hypertonic saline, terbutaline, glucose, surfactant active agents like Tyloxapril, Tegemist and β2-agonists. Hypertonic saline is the most commonly used because of its high success rate and safety. For the study of airway inflammation, it is most helpful in sputum production [10]. Beta2 agonist like terbutaline has shown to enhance mucociliary transport in healthy subjects and in patients with chronic obstructive pulmonary diseases and cystic fibrosis [11]. These agents are delivered through ultrasonic nebulizer for the better clinical effects.

There are two methods which are commonly employed for the sputum induction in various clinical conditions. In one method which was proposed by Ireda et al. [12], the inhalation of the same concentration of hypertonic saline (4.5%) is used with increasing time interval is used. In the second method, which was proposed by Pin et al [1], the inhalation of the solution for the same period with increasing concentration of hypertonic saline (3%, 4%, 5%) is used. Some studies have shown that hypertonic saline (3%) is as successful as 5% saline [13].

Physiologic Principle of Induced Sputum Production

Mist of hypertonic saline droplets are produced by the ultrasonic nebuliser. The smaller droplets are deposited peripherally in the lung. Due to the hypertonicity of the deposited saline, interstitial fluid is drawn into the lower airways by osmosis. There is also bronchial irritation and this stimulates bronchial secretions due to the hypertonic saline. After 10- 20 minutes of nebulization, the fluid produced mobilizes the material in the lower airways. Due to the repeated coughing by the patient due to irritation of the airways, it helps in movement of this material into trachea to produce expectoration [7].

Induced Sputum Characteristics

The induced sputum is a complex structure which is rich in mucin (protein) and also contains DNA degradation products. It contains mediators which are more than bronchoalveolar lavage fluid. If the concentration of the hypertonic saline is increased, there is no difference in the sputum cell composition, but there is difference in the differential effect on mediator concentration in sputum. Duration of the inhalation changes the cell composition in the sputum. It has been observed that there is decrease in neutrophils count and macrophages count increases if the duration of inhalation is increased, thus reflecting that the sample is derived from the distal air base. As the expectorated sputum is derived from the distal airspaces, the quality of induced sputum is better as compared to the normal sputum. Also, it has been observed that the induced sputum contains higher concentration of fluid phase components such as eosinophil cationic proteins (ECP), mucin likes glycoproteins and albumin as compared with bronchoalveolar lavage fluid. Most of these mediator substances are present in the airways and not in the alveolar spaces [13].

Safety and Success of Sputum Induction

Sputum induction is a relatively safe procedure and can be performed as an outpatient department level with proper precautions in safe environment. It was observed that there was no reduction in the lung functions after the procedure thus indicating its safety. The overall success has been observed to be 93% [7]. The procedure is safe even among those patients whose overall lung volumes and lung capacities are reduced due to the underlying diseases, and even whose FEV1 is less than 60% of predicted value or less than 1L. Thus, overall this sputum induction which is a standardized procedure is a safe and successful procedure in clinical practice, and can be performed in patients with moderate to severe airflow limitation in asthma and COPD patients. The procedure of sputum induction using hypertonic saline is a safe procedure even in children and it is very useful to study airway inflammation [14].

Tuberculosis

Pulmonary Tuberculosis: For the clinical diagnosis of pulmonary tuberculosis high index of suspicion is required and to be confirmed by sputum testing by AFB smear and CBNAAT testing, and chest X-ray. Many a times sputum production is not possible in many patients, hence the testing becomes difficult. In these cases, sputum induction plays an important role in producing sputum. Recently sputum induction has shown good results as additional yield in diagnosis of smear negative pulmonary tuberculosis [15]. For the diagnosis of smear negative pulmonary tuberculosis, it has been observed that the diagnostic yield of single induced sputum is similar to fibreoptic bronchoscopy and yield of repeated induction was better favouring this tool over bronchoscopy. It was also observed that in one study from India favours sputum induction procedure by using β2 agonists for sputum induction [16]. Sputum induction was first used by Hensler et al. [17], in 1961 in the diagnosis of active tuberculosis. Hypertonic saline irritates the airways causing the patient to cough and increases the osmolarity of the airway lining fluid, which produces increased vascular permeability, and induces mucus production by submucosal glands [18,19]. Sputum induction has performed well both in resource-poor and resource-rich countries [20,21]. In these studies, it provided adequate samples for the diagnosis of pulmonary tuberculosis which was cost- effective, and about 25% to 42% patients were smear positive on sputum induced samples. However, some studies in developed countries showed that sputum induction added little to overall diagnosis and was deemed costly [22]. Overall, sputum induction has several advantages, which includes less invasiveness, high diagnostic yield, greater patient comfort and safety, low-cost, no age restriction, no need of fasting, out-patient procedure, no need for expert for performance and less time consuming [23]. In different studies adequate sputum sample was obtained in 93-97% of the cases [23]. The yield of sputum smear positivity varied between 26% and 38% [24]. The yield can be increased by multiple induced sputum samples and by using CBNAAT testing [25]. If the relative yield of Sputum induction for previously non-productive cough and previously smear negative sputum is compared, it is seen that the yield is higher in the former group [24]. In HIV infection and in children also sputum induction was observed to be a safe and effective procedure [26].

Pleural Tuberculosis: Tuberculosis of the pleura may develop either in the presence or absence of pulmonary parenchymal tubercular lesions which may be infiltrates or cavitary lesions. These pleural effusions which are secondary due to tuberculosis contains small number of organisms, thus making diagnosis of pleural tuberculosis difficult and often requires invasive procedures like thoracoscopic pleural biopsy, or blind pleural biopsy or bronchoscopy. These patients often do not produce sputum spontaneously; thus sputum specimens are often not examined. Thus, sputum induction plays an important role in the diagnosis of pleural tuberculosis. In one study by Conde et al. [27], it was observed that the yield of sputum AFB cultures which was obtained by sputum induction was high in patients suspected of having pleural tuberculosis, even when there was no evidence of pulmonary parenchymal disease radiographically.

Interstitial Lung Diseases (ILD)

Many of the interstitial lung diseases including pneumoconiosis, sarcoidosis and non-granulomatous interstitial lung diseases have been studied using induced sputum. It has been observed that the results have been found comparable to Bronchoalveolar lavage (BAL) findings for pneumoconiosis and were able to differentiate sarcoidosis patients from healthy subjects and from patients with non-granulomatous lung diseases. Thus, it can be used both for research as well as in clinical monitoring of patients with interstitial lung diseases and can serve as a good complementary tool to BAL [28].

Community Acquired Pneumonia (CAP)

Sputum testing for the gram staining and cultures and sensitivity testing plays an important test in identifying the causative organisms in community acquired pneumonia and nosocomial pneumonias including hospital acquired pneumonia. If the person is unable to produce sputum for the testing, sputum induction is a standard diagnostic procedure to identify pathogens in lower respiratory tract secretions in adults with pneumonia. In one study by Zare et al. [29], it was observed that it was observed to be a safe and useful procedure in infants and children also with community acquired pneumonias including pneumonias in high HIV positive patients.

Infections in Immunocompromised Individuals

Sputum induction is an important investigation for identifying the causative organisms in opportunistic lung infections in HIV positive infections. It is widely used for the diagnosis of Pneumocystic Jerovecii pneumonia (PJP) with HIV infection. Various types of the organisms can be isolated in the induced sputum including fungal pathogens in HIV infection. It has also clinical utility for diagnosing PJP in immunocompromised patients without HIV infection [30].

Bronchial Asthma

In clinical practice, it is difficult to assess airway inflammation in bronchial asthma and the effects of medication on such inflammation. Subjective assessment of symptoms is always difficult and has often been found to be unsatisfactory for monitoring asthma severity [31]. Nonetheless, the regular use of peak flow measurements has been shown to improve asthma control, peak flow rates and diurnal peak flow relapse. Levels of exhaled gases such as nitric oxide can be measured and has been found to be useful, but more data are needed to fully evaluate the importance of such markers in assessing airway inflammation in asthma, especially since nitric oxide can be produced in large amounts in paranasal sinuses and the stomach [31].

Sputum eosinophilia is usually observed in bronchial asthma. Up to 80% of corticosteroid- naive subjects and more than 50% of corticosteroid- treated subjects with currently symptomatic asthma have a sputum eosinophil count that is outside of the normal range. The validity of a high sputum eosinophil count for the identification of asthma is better than peak expiratory flow (PEFR) measurement [10]. The clinical response to inhaled corticosteroids differs markedly according to the sputum eosinophil count, with little evidence of improvement in symptoms and airway responsiveness in subjects with a baseline sputum eosinophil count of less than 3%. Higher eosinophil count patients have better inhaled corticosteroid response during the follow up of these cases. These findings suggest that measuring the underlying airway inflammation might provide a better guide as to the need for corticosteroid treatment than assessment of functional abnormality [10]. Here sputum induction can be used as a prognostic marker for monitoring the response to the inhaled corticosteroids therapy.

Sputum eosinophilia is also observed in occupational asthma. There is some evidence that sputum eosinophil counts increase during workplace exposure in subjects with occupational asthma [10]. One obvious application of sputum induction is to use sputum inflammatory indices to increase our understanding of complex relationships between inflammatory cells, mediators and cytokine mechanisms in asthma. The sputum fluid phase seems to be suitable for measuring eosinophil cationic protein, some cytokines and histamine. It can be also used for assessment of airway inflammation for evaluating the effects of drugs on asthmatic airway inflammation and relating their anti- inflammatory effect to the effects on symptoms and disordered airway function [11].

Chronic Obstructive Pulmonary Disease (COPD)

Chronic obstructive pulmonary disease is a chronic inflammatory disorder characterized by inflammation in airways leading to increased secretions, with the presence of macrophages and lymphocytes on airway tissue. Bronchoscopic investigations are often not possible due to disease severity [32]. Thus, sputum induction can be used as valuable tool for studying pathophysiology studies. Sputum neutrophil count is usually high and the neutrophil count can be correlated with a reduction in forced expiratory volume in one second (FEV1) and the rate of decline in FEV1, thus suggesting that neutrophilic airway inflammation is functionally important. Peleman et al. [33], studied the cellular composition of induced sputum in chronic obstructive pulmonary disease and found marked sputum neutrophilia.

Confalonieri et al. [34], studied the effects of two months of treatment with inhaled Beclomethasone dipropionate (1,500 μg/day) on bronchial inflammation in patients with stable, mild to moderate chronic obstructive pulmonary disease, by using sputum induction. They observed that the number of neutrophils present in induced sputum samples decreased after treatment. Moreover, a short course of oral glucocorticoid therapy has been demonstrated to improve pulmonary function in some patients with chronic obstructive pulmonary disease, but not all patients [35]. In a recent trial, Borbeau et al. [36], showed that, in a group of 140 chronic obstructive pulmonary disease patients, 19 (13.5%), responded to the two-week treatment with 40 mg prednisone daily. Response to treatment was defined as a 15% improvement in FEV1. Also employing FEV1 to assess response, Mendella et al. [37], showed that 17% of chronic obstructive pulmonary disease cases were considered responsive to a course of 32 mg/d methylprednisolone for two weeks. Furthermore, Pizzichini et al. [38], studied sputum eosinophilia in smokers and observed that eosinophilic inflammation can occur in smokers with or without chronic airflow limitation (chronic obstructive pulmonary disease) and that sputum eosinophilia may predict those patients who will benefit from steroid therapy. Nowdays the treatment of COPD depends upon whether there is presence of sputum eosinophilia or not. Here sputum induction can be used for producing sputum if patients cannot expectorate suptum due to the severity of breathlessness or due to the debilitating nature of the disease. Fujimoto et al. [35], investigated the influence of glucocorticoid in the reversibility of eosinophilic inflammation in patients with pulmonary emphysema. They found that the reversibility of airway obstruction following the treatment could be correlated with the eosinophil count in the induced sputum, and that the treatment significantly reduced eosinophil count and eosinophil mediators. In addition, patients who did not show improvement in FEV1, had lower baseline eosinophil counts. Sputum induction can be also used for the follow up of these COPD cases so that the pharmacotherapy can be deescalated to dual therapy if there is reduction in the eosinophils count and if there is good symptomatic control of the disease.

Lung cancer

In suspected patients of lung cancer, especially in those with central pulmonary masses, induced sputum cytology is useful procedure and can be used as the first diagnostic procedure because of its safety and high sensitivity. It can be used for those patients who cannot expectorate the sputum. Here, if the patients are unable to produce sputum, sputum induction can be used as the initial tool in the evaluation of the suspected lung cancer cases. At least three samples should be subjected on three successive days whish can increase the sensitivity of the test to almost 60%. In one study by Fernandez et al. [39], it was observed that about 74% of the patients with lung cancer had high degree of histologic correlation (Kappa index of 0.66). Hidaka et al. [40], found its usefulness in diagnosis of Bronchoalveolar carcinoma specially in patients with invasive nature of the disease.

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