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Year : 2006  |  Volume : 23  |  Issue : 2  |  Page : 82-86 Table of Contents   

Sputum induction - A useful tool in diagnosis of respiratory diseases

Department of Chest and Tuberculosis, Post Graduate Institute of Medical Sciences, Rohtak., India

Correspondence Address:
K B Gupta
Dr. K.B. Gupta, 6J/18, Medical Campus, Rohtak-124001 (Haryana).
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0970-2113.44416

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Sputum induction by inhalation of hypertonic saline is a noninvasive, simple, cost effective and safe procedure to collect respiratory secre­tions from lung airways for diagnosis of various respiratory diseases. Sputum induction has higher yield in comparison to spontaneous sputum, BAL, bronchial washing and gastric lavage. It is widely used to assess airway inflammation in asthma and chronic obstructive pulmonary disease (COPD). It can be used as a complementary tool to BAL both in research and in clinical monitoring of patients with interstitial lung disease (ILD). The cells, recovered from spontaneous coughing can be used to study lung cancer, respiratory infections and in diagnosis of pneumocystis carinii pneumonia in patients infected with human immunodeficiency virus. In developing countries having high prevalence of pulmonary tuberculosis, sputum induction (SI) can increase the diagnostic yield, resulting in better categorization of patients for treatment purposes. It increases case detection rate of smear negative pulmonary tuberculosis specially in area where facili­ties for more invasive and expensive techniques are not available.

Keywords: Sputum induction, Pulmonary tuberculosis, Fibero bronchoscopy

How to cite this article:
Gupta K B, Garg S. Sputum induction - A useful tool in diagnosis of respiratory diseases. Lung India 2006;23:82-6

How to cite this URL:
Gupta K B, Garg S. Sputum induction - A useful tool in diagnosis of respiratory diseases. Lung India [serial online] 2006 [cited 2021 Jun 20];23:82-6. Available from: https://www.lungindia.com/text.asp?2006/23/2/82/44416

   Introduction Top

Sputum induction has been used for many years in the diagnosis of various respiratory diseases. It is being increasingly used to study pathophysiology of various respiratory diseases. [1] Curschmann's Spirals and Charcot Leyden crystals are popularly used to diagnose bronchial asthma. Sputum cytology for presence of malignant cells and various biochemical markers in sputum are also used in patients of suspected lung cancers as well as prognostic marker during follow-up. Isolation of various pathogens e.g., bacteria, fungi, viruses are useful in diagnoses of various respiratory infections.

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] . Later on, induced sputum was used for clinical purposes to detect numerous pathogenic microorganism. [3],[4] More than three decades ago, sputum induction was widely used for the diagnosis of pulmonary tuberculosis as it was superior in yield 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] Various studies over the past decade favours superior yield of sputum induction over fibreoptic bronchoscopy in diagnosis of pulmonary tuberculosis . [7] Moreover patient tolerability and safety are good . [5],[6],[7],[8] Procedure is also feasible in developing countries setting and in young children. [9] In resource poor countries, sputum induction may be particularly helpful in diagnosis is because of high prevalence of tuberculosis in these countries. Though, sputum for AFB is still a gold standard to diagnose pulmonary tuberculosis, the chest symptomatics, who have dry cough and/or scanty sputum, pose a major problem in diagnosis of pulmonary tuberculosis. Adequate sputum is essential for quality microscopy diagnosis and is prerequisite for the DOTS therapy. Younger women mostly have either dry cough or scanty sputum and may have only minimal disease radiologically. A study from Vietnam particularly stress the need for quality sputum microscopy in these women. [10]

In early nineties, first attempt to standardize sputum induction and processing was made. First task force on sputum induction method was sponsored by European Respiratory Society. [11],[12],[13]

   Methodology Top

Various agents like distilled water, normal saline, hypertonic saline, glucose, surfactant active agents like tyloxapril, Tegemist and b2-agonist have been used for sputum induction. Hypertonic saline is most commonly used because of high success rate and safety. It is most helpful in sputum production for studying airway inflammation. [14]β2 agonist like terbutaline has been demonstrated to enhance mucocilliary transport in healthy subjects and in patients with chronic bronchitis and mucovisidosis. [15] These agents are delivered through ultrasonic nebulizer.

It is advisable that bronchodilatation medication, oxygen supply and resuscitation equipment should be available at procedure place. Although experienced technician can conduct sputum induction, a physician should supervise the procedure. Two methods are commonly employed. One proposed by Ireda et a1 [16] in which the inhalation of the same concentration of hypertonic saline (4.5%) for increasing time interval is used while in second proposed by Pin et all inhalation for the same period of increasing concentration of hypertonic saline (3%, 4%, 5%) is used. Study has shown that hypertonic saline 3% is as successful as 3-5% saline. [17]


The ultrasonic nebuliser produces a mist of hypertonic saline droplets. The smaller droplets are deposited peripherally in the lung. It is suggested that the hypertonicity of the deposited saline draws interstitial fluid into the lower airways by osmosis. The hypertonic fluid also causes bronchial irritation and this stimulates bronchial secretions. After 10-20 minutes of nebulisation the fluid produced mobilize the material in the lower airways. Repeated coughing by the patient help in movement of this material into trachea to facilitate expectoration . [7]

Standard protocol for sputum induction

Patient is pretreated with 200 mg of albuterol or equivalent β2-agonist by inhalation. Pre-induction spirometry 10 minutes before and 10 minutes after β2-agonist is done. After placing a nasal clip, induction is started with 3% hypertonic saline (5 to 7 ml). Patient is asked to expectorate whenever he feels or at every 5 minutes. FEV1 and PEF is also checked at every 5 minutes. Saline induction is continued for 15 minutes (3 times, 5 minute each). If sputum sample is inadequate, induction can be continued for another 5 minutes. Procedure is stopped after 20 minutes if FEV1 dropped by 20%. [7]

Processing of the Induced Sputum

Sputum sample is kept in cold place (temp. 4°C) and processed within two hours. It is solubilized with DTT (dithiothreitol) or DTE (ditheoerythritol) followed by Dulbecco's phosphate buffer saline (D­PBS). Then the mixture is filtered and centrifuged. The supernatant is aspirated and frozen at -80°C for further analysis. [18] Numerous laboratories prefer to process the entire sputum. [19] Other prefer to collect and analyse the more viscid proportion of sputum (plugs). [1],[16]

Characteristics of the Induced Sputum

It is a complex medium rich in mucin (protein) and also contains degradation products of DNA. Sputum is rich in mediators much more than BAL. With increase in the concentration of the hypersonic saline, there is no difference in the cell composition of the sputum, but differential effect on mediator concentration in sputum fluid phase is seen. Cell composition varies with duration of inhalation. Neutrophils decreases and macrophages increases by increasing the duration of inhalation reflecting that the sample is derived from the distal air base. Contamination of sputum with saliva is major concern and problem. It can be decreased by blowing nose, bearing nose clips or by washing the mouth. [19]

The quality of induced sputum is better because it is derived from the peripheral airways. This was confirmed by using a radio labeled aerosol bolus delivery technique (99m Tc Sulphur colloid particles). Induced sputum contains higher concentration of fluid phase components such as eosinophil cationic proteins (ECP), mucinlites glycoproteins and albumin as compared with BAL. These are present in airways secretions as compared with alveolar spaces.

Success and Safety of Sputum Induction

Success was obtained by non-squamous total and differential cell counts containing macrophages and safety by the fall in forced expiratory volume in one second (FEV 1). The overall success has been reported as 93%. [7] The procedure is safe even amongst patients with an FEV1 less than 60% of predicted value or less than one L. Thus carefully standardized sputum induction can be safe and successful procedure in patients with asthma and COPD in clinical practice, even in presence of moderate to severe airflow limitation. Sputum induction using hypersonic saline is a safe procedure even in children and it is useful to study airway inflammation. Sputum induction is performed with b2-agonist pretreatment using ultrasonic nebulizer with 4.5% hypertonic saline gave 98% procedure completion rate. 4% of patients demonstrated 15% decrease in FEV1. An adequate sputum sample could be obtained in 92% of the children. Distressing cough was noticed in 13%. In 1% mucosal irritation was noticed. [20],[21]

Clinical uses of induced sputum

1. Asthma

It is helpful in better understanding of pathology of bronchial asthma, assessment of severity of disease, evaluation of effect of treatment and in confirmation or exclusion of diagnosis. [23],[24],[25] Increased level of eosinophils in sputum allows to forecast the efficiency of glucocorticosteroids in the therapy of asthma. Eosinophilic bronchitis is an important cause of chronic cough and sputum eosinophil, ECP (eosinophilic cationic protein) in induced sputum confirms the diagnosis. [26],[27] Increased eosinophil count above the upper limit of 3% of non-squamous cells in the absence of typical clinical and PFT findings is used as a diagnostic tool. It is also used for better monitoring of drug activity and response to treatment and also to decide minimum dose of inhaled corticosteroids. Induced sputum eosinophil count is used as a diagnostic tool in occupational asthma. [28]


Sputum analysis after sputum induction gives useful information about airways inflammation in COPD. [29] Sputum eosinophilia has been found as characteristic of patients with steroid response and sputum examination might serve as a screening test for continuation of long term steroid treatment in COPD. It can be used to investigate different aspects of airway inflammation. Sputum analysis has given evidence for increased number of macrophages, neutrophils and eosinophils in COPD. Changes in various mediators have been found in sputum supernatant of COPD patients (IL-8, LTB-4 and TNFa). It is also useful in serial monitoring of induced sputum inflammatory markers in COPD [24],[30] It is useful in assessment of the effect of glucocorticosteroids or markers of neutrophilic airway inflammation like activation of neutrophils which signifies clinical improvement in patients of COPD. [31] Thus, sputum induction can also guide treatment also.

3. Lung Cancer

In elderly patients of central pulmonary masses, cytology of induced sputum is useful as the first diagnostic procedure because of its safety and high sensitivity. Fernandez et al [32] found 74% sensitivity in his study with a Kappa index of 0.66 with high degree of histologic correlation. Hidaka et a1 [33] found its usefulness in diagnosis of bronchoalveolar carcinoma.

4. Tuberculosis

a) Pleural tuberculosis:
Pleural tuberculosis may occur in the presence or absence of pulmonary parenchymal lesions on the chest radiograph. [34] Pleural effusion associated with tuberculosis contains relatively small number of organisms making diagnosis of pleural tuberculosis difficult and oftenly requiring invasive procedures like bronchoscopy and pleural biopsy. Sputum specimens are not oftenly evaluated because many patients are not able to produce sputum spontaneously. Conde et al [35] in their study observed that the yield of sputum cultures obtained by sputum induction is high in patients suspected of having pleural tuberculosis, even when there is no radiographic evidence of pulmonary parenchymal disease.

b) Pulmonary tuberculosis: Diagnosis of pulmonary tuberculosis in smear negative cases has been resting mainly upon high index of clinical suspicion, x-ray findings and tuberculin testing. Recently sputum induction has shown good results as additional yield in diagnosis of smear negative pulmonary tuberculosis. [36] Diagnostic yield of single induced sputum is as good as FOB and yield of repeated induction is better favouring this tool over bronchoscopy for diagnosis of smear negative pulmonary tuberculosis. A study from India also favoring it by using β2 agonists for sputum induction. [37]

5. Interstitial Lung Diseases (ILD)

Induced sputum has been used to study ILD, more specifically pneumoconiosis, sarcoidosis and nongranulomatous ILD. Results have been found comparable to BAL findings for occupational lung disease and were able to distinguish sarcoidosis patients from healthy subjects and from patients with nongranulomatous lung disease. Thus it can be used as good complementary tool to BAL both in research as well as in clinical monitoring of patients with ILD. [38]

6. Opportunistic Infection in immunocompromised (IC) host

It is widely used for the diagnosis of pneumocystic pneumonia (PCP) with AIDS. It has also clinical utility for diagnosing PCP in IC without HIV infection. [39]

7. Community acquired pneumonia (CAP)

Sputum induction is a standard diagnostic procedure to identify pathogens in lower respiratory tract secretions in adults with pneumonia. Zare et al [40] found it as safe and useful procedure in infants and children also with CAP from a high HIV prevalence area.

8. Others

Cough due to gastroesophageal reflex disease (GRD) has shown increased macrophages laden with lipid in induced sputum. [41]

Sputum induction is a safe, simple noninvasive available tool which can be used for diagnosis and management of various respiratory illness like bronchial asthma, COPD, tuberculosis, lung cancer, ILD etc. It can be a good research tool for several lung diseases.

   References Top

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