Indian Journal of Rheumatology

REVIEW ARTICLE
Year
: 2021  |  Volume : 16  |  Issue : 5  |  Page : 69--78

Assessment and follow-up of interstitial lung disease


Devarasetti Phani Kumar 
 Department of Clinical Immunology and Rheumatology, Nizam's Institute of Medical Sciences, Hyderabad, Telangana, India

Correspondence Address:
Dr. Devarasetti Phani Kumar
Associate Professor, Department of Clinical Immunology and Rheumatology, Nizam's Institute of Medical Sciences, Panjagutta, Hyderabad, Telangana
India

Abstract

Patients with suspected interstitial lung disease (ILD) clinically presenting with cough and breathlessness are initially investigated with a chest radiograph and spirometry. The finding of a normal or increased forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) ratio in the presence of reduced FVC on spirometry should alert for the presence of a restrictive defect and the need to order for complete lung function tests, including lung volumes and diffusion capacity of the lungs for carbon monoxide (DLCO) and 6-minute walk test (6MWT). High-resolution computed tomography (HRCT) is required to identify the specific patterns. To determine a more specific diagnosis, serology for connective tissue diseases, bronchoalveolar lavage, and a lung biopsy are needed. A multidisciplinary approach involving the pathologist, rheumatologist, pulmonologist, and radiologist allows the correlation of the clinical, radiologic, and pathologic findings to arrive at the right diagnosis. Screening guidelines based on expert opinion are available for idiopathic pulmonary fibrosis (IPF) and non-IPF ILDs (sarcoidosis, hypersensitivity pneumonitis, and systemic sclerosis). Follow-up monitoring for progression based on risk factors is mandatory for the early identification and management. Symptom assessment, a decline in pulmonary function tests (forced vital capacity [FVC], DLCO, and 6MWT) and worsening fibrosis on HRCT give clues for progression. The monitoring schedule in progressive fibrosing ILD for repeat pulmonary function tests is between 3 and 6 months and for HRCT is 12–18 months in the appropriate clinical context. An individualized approach to repeat pulmonary function tests and HRCT in stable ILD is required.



How to cite this article:
Kumar DP. Assessment and follow-up of interstitial lung disease.Indian J Rheumatol 2021;16:69-78


How to cite this URL:
Kumar DP. Assessment and follow-up of interstitial lung disease. Indian J Rheumatol [serial online] 2021 [cited 2022 Jun 28 ];16:69-78
Available from: https://www.indianjrheumatol.com/text.asp?2021/16/5/69/332980


Full Text



 Introduction



Interstitial lung disease (ILD) encompasses a spectrum of diffuse parenchymal lung disorders classified together because of shared clinical, radiological, and histopathological features.[1] Although often a definite cause is not identified, underlying etiology is known with connective tissue disease (CTD), drugs, and occupational exposure. Despite the shared similarity, ILD of different causes differs in clinical progression and response to treatment.[2] Early recognition of ILD is crucial to plan the management. Here, we review a general approach for the diagnosis and follow-up monitoring in ILD patients, focusing on CTD-ILD.

 Assessment of Interstitial Lung Disease



The goals of the clinical assessment are arriving at a diagnosis, severity assessment, monitoring and prognostic assessment. A comprehensive evaluation should include symptom assessment, physical examination, pulmonary function tests (PFT), and chest imaging (plain chest radiography/high-resolution computed tomography [HRCT] chest) and histopathology as indicated with multidisciplinary discussion (MDD) between pulmonologists, radiologists, and pathologists.[3],[4] The diagnostic algorithm for a patient with suspected ILD is shown in [Figure 1].{Figure 1}

Clinical assessment

Proper clinical history is necessary focusing on demographics, occupational, environmental exposures, medications, smoking status, and history of CTD. Clinical clues at presentation are progressive dyspnea, dry cough, and velcro crackles on lung auscultation. Clubbing is seen mainly in idiopathic pulmonary fibrosis (IPF) and sarcoidosis, and less commonly in CTD ILD. The symptoms are nonspecific do not aid differentiation between the subtypes of ILD.

ILD may be the initial presentation of CTD or be asymptomatic with slow progression in established CTDs. Therefore, CTD patients should undergo regular symptom assessment and physical examination to look for ILD, and all ILD patients should be screened for an underlying CTDs. Among CTD, only scleroderma has evidence-based recommendations to screen all patients for ILD.[5] Screening modalities vary among treating specialists in diagnosing CTD-ILD. Screening should include chest auscultation, pulmonary function tests, and imaging. The severity of ILD is graded based on clinical assessment, degree of restriction, and impaired gas exchange seen on PFT, radiography, and histologic criteria.[6]

 Pulmonary Function Tests in Interstitial Lung Diseases



Spirometry, diffusion capacity of the lungs for carbon monoxide (DLCO), and 6-minute walk test (6MWT) are pulmonary function tests to screen for ILD. They are also helpful in assessing disease progression, severity treatment response, and prognostication.

 Spirometry



Spirometry measures the maximal air volume that an individual can inspire and expire with maximal effort. The most relevant measurements are the forced vital capacity (FVC), which is the total volume of air expired after a full inspiration, and forced expiratory volume in one second (FEV1), which is the expiratory volume in the first second of an FVC maneuver and their ratio (FEV1/FVC). The American Thoracic Society guidelines (2019) recommend FVC, FEV1, and FEV1/FVC to be routinely reported.[7] Performance of spirometry before and after bronchodilator is used to determine the reversibility of airflow limitation but is of limited use in ILD unless patients have complaints related to airway disease.

The classical pattern on spirometry in ILD is a restrictive ventilatory abnormality with decreased FVC and normal or increased FEV1/FVC ratio with preserved FEV1. Reduced vital capacity (VC) alone is not sufficient evidence of restriction because air trapping due to severe obstruction can also decrease it. Therefore, the restrictive abnormality cannot be diagnosed with spirometry alone, and should be confirmed by lung volumes. Total lung capacity (TLC) is the crucial lung volume to confirm restriction, defined as a TLC value less than the fifth percentile of predicted value[7] together with a normal or increased FEV1/FVC ratio. In clinical practice, spirometry is used to diagnose restrictive patterns due to the non-availability of plethysmography to measure TLC. Flow volume loops in normal and restrictive patterns with classification of severity based on FVC in ILD, is shown in [Figure 2].{Figure 2}

Baseline FVC predicts the severity of ILD but is not specific to differentiate types of ILDs. A decline in FVC of ≥10% of predicted value has consistently shown an association with mortality in ILDs.[8],[9] The rate of FVC decline is used as the primary endpoint in clinical trials of IPF and scleroderma.[10],[11],[12]

Spirometry plays a significant role in the early diagnosis of CTD-ILD. In CTDs, in the lungs there can be involvement ofairways, vasculature, and respiratory muscles apart from the parenchyma. Hence, spirometry findings should be interpreted with DLCO in dissecting the cause for restrictive pattern from a parenchymal or extraparenchymal cause. In systemic sclerosis (SSc), spirometry is vital in the diagnostic workup for ILD but is not sensitive for detecting early ILD as lung volumes are preserved in the early stages.

 Diffusion capacity of the lungs for Carbon monoxide



The diffusion capacity (DL), also called the transfer factor, measures the capacity to transfer gas from alveolar spaces into alveolar-capillary blood. Diffusion capacity of the lungs for carbon monoxide (DLCO) measures the quantity of carbon monoxide (CO) transferred per minute from alveolar gas to red blood cells in pulmonary capillaries, expressed as mL/min/mm Hg, and represents mL of CO transferred per minute for each mm Hg of pressure difference across the available gas exchange surface. The process of CO uptake can be divided into membrane conductance (DM) and reactive conductance (a chemical reaction between CO and hemoglobin).

CO is used because of its extreme avidity for hemoglobin (200–250 times as oxygen), which exerts negligible backpressure. The factors that affect the amount of CO transferred from the alveoli to the blood capillaries are surface area, the thickness of the alveolar-capillary membrane available for gas exchange, and blood volume in the pulmonary capillary. Therefore, decreased surface area, increased thickness of alveolar capillary membrane, and decreased pulmonary flow affect DLCO.

Single breath DLCO is the most widely used method for measuring DLCO.[13] The normal value of DLCO at rest is 25 ml/min/mm Hg. DLCO adjusted for lung volume is more specific than unadjusted DLCO for predicting ILD. DLCO changes with anemia, carboxyhemoglobin levels, altitude and lung volumes, so adjustments for these factors are needed before interpretation. DLCO procedure, severity grading, interpretation, and limitations are summarized in [Table 1].{Table 1}

DLCO is reduced in ILDs. Major factors involved are decreased surface area and increased thickness of the alveolar-capillary membrane by inflammation and fibrosis limiting gas exchange. DLCO helps in detecting mild (early or preclinical) ILD as lung volumes get affected only late in the disease. Pulmonary hypertension should be suspected in ILD if DLCO is very low and there is severe exercise-related desaturation with preserved FVC. Steen and Medsger suggested that a FVC/DLCO ratio >1.6 (percentage predicted) was helpful in the diagnosis of pulmonary vasculopathy in SSc patients with or without ILD.[15]

 Six-minute Walk Test



Six-minute walk test is a simple, standardized measure of the distance walked during a defined period of 6 minutes which assesses the submaximal level of functional capacity.[16] Most patients do not achieve maximal exercise capacity during the 6MWT. This test may better reflect the functional exercise level for daily physical activities similar to daily activities performed at submaximal levels of exertion. 6MWT procedure along with parameters, limitations and contraindications are summarized in [Table 2]. During a 6MWT, healthy participants can typically walk 400 − 700 m. 6MWT provides prognostic information complementary to pulmonary function tests. It is the most widely used exercise test, provides a global measure of functional capacity and has ease of administration and reproducibility. It is an inherently safe test that can be performed in the advanced stages of the disease.{Table 2}

The 6MWT is influenced by organ systems involved in exercise (pulmonary, cardiac, peripheral vascular, neuromuscular unit, and muscles) and by specifics of test conditions. It does not distinguish among organ elements causing exercise limitation, including pulmonary, cardiovascular, and neuromuscular factors.

It is a tool validated in pulmonary arterial hypertension, with limited use in ILD as an outcome measure in interventional clinical trials. This tool is being studied widely in IPF, where restriction is more compared to CTD-ILD. In IPF, a meaningful clinically important difference (MCID) in 6-minute walk distance (6MWD) is between 24 and 45 m.[17] Among connective tissue ILD, SSc-ILD registries and clinical trials have used 6MWT as an outcome measure but MCID is not yet well defined and involvement of musculoskeletal and cardiac systems poses difficulty in the assessment of functional limitation.[18]

 Imaging



Chest radiographs and HRCT chest are the essential tools in the diagnosis of ILD. The chest radiograph is used in the initial diagnostic assessment. Chest radiograph helps rule out left heart failure, infection, or cancer encountered in ILD. Diffuse reticulonodular opacities are a common finding. Chest radiographs can be normal in ILD and is inferior to HRCT for the diagnosis and prognostication.

HRCT chest is the gold standard and sensitive method for ILD diagnosis and disease severity assessment. ATS-ERS-JRS-ALAT guidelines (2018) recommend HRCT chest with proper technique to recognize patterns and distribution of the abnormalities in ILD.[19] Volume scans on multidetector computed tomography (MDCT) are preferable at initial assessment [16 slices or higher].

HRCT chest is more sensitive in detecting early disease than lung function tests and chest radiographs. The pattern of the radiographic abnormalities on HRCT can help in identifying specific ILDs. In the study by Lynch et al., patients with a UIP pattern had a significant risk of mortality, unlike other parameters such as non-worsening of symptoms, a decline of lung function, and worsening of fibrosis. Thus, the study findings emphasize the importance of a specific HRCT pattern.[20] Reticulation, traction bronchiectasis, and honeycombing reflect fibrosis. Fibrosis extent at baseline predicts mortality in ILD. HRCT chest may be used to guide lung biopsy site and treatment decisions. The pattern and distribution of radiographic abnormalities can predict the histopathology. The most common HRCT patterns in ILD are summarized in [Table 3].{Table 3}

CTD-ILDs are often associated with NSIP than UIP patterns as compared to idiopathic interstitial pneumonia. NSIP is seen more frequently in systemic sclerosis, dermatomyositis, and mixed connective tissue diseases, while rheumatoid arthritis is frequently associated with UIP. The consensus on HRCT pattern with the prognosis is conflicting in CTD-ILD. In scleroderma, HRCT is the primary tool to diagnose ILD.[5] Abnormalities in other thoracic structures (e.g., esophagal diameter or pulmonary artery dilatation) should raise the suspicion for the presence of CTD in isolated NSIP patterns.

 Serology



Serology plays a significant role in the evaluation of CTD-related ILD. Recognition of an underlying CTD is particularly challenging when ILD is its first or lone manifestation. Definite discussion between rheumatologists and pulmonologists, especially in young patients with NSIP patterns, helps in diagnosis. European respiratory society (ERS)/American Thoracic Society (ATS)task force (2015) recommends testing for autoantibodies in interstitial pneumonia with autoimmune features (IPAF).[21] ATS/ERS guidelines for the diagnosis of IPF recommend that testing for autoantibodies is mandatory to exclude CTD.[19] Therefore, serology plays a role in discrimination between idiopathic ILD versus CTD-ILD and is essential as they differ in their management and prognosis. An antinuclear antibody is a preliminary screening test in CTD diagnosis with high sensitivity and poor specificity. Based on ANA patterns with nuclear staining (homogenous, speckled, nucleolar) or cytoplasmic staining combined with the clinical context, one can plan for targeted antibody testing to diagnose specific CTD. Autoantibodies play a role in the prediction of progression of ILD in CTD, such as anti-topoisomerase (Scl 70), Th/To antibodies in scleroderma, anti-CCP antibodies in rheumatoid arthritis, and non-Jo 1 antibodies in antisynthetase syndrome. Serologic evaluation in ILD includes autoantibodies, as summarized in [Table 4].{Table 4}

 Bronchoalveolar Lavage



Bronchoalveolar lavage (BAL) nucleated immune cell patterns can provide helpful information for narrowing ILD differentials and lessen the need to proceed to lung biopsy. The clinical utility of BAL cell analysis in the diagnosis of ILD is still debatable. Although BAL can help identify conditions such as infection or malignancy in the course of ILD. According to ATS guideline (2012),[22] following the initial clinical and radiographic evaluation of patients in suspected ILD, BAL cellular analysis may be a helpful adjunct in the diagnostic evaluation. It does not have a role in the prognostication and assessment of treatment response. Specific BAL cellular patterns in ILD are summarized in [Table 5].{Table 5}

 Lung Biopsy



Lung biopsy plays a role in cases where, clinical and radiologic data are insufficient to make a firm diagnosis. Lung biopsies are done either by surgical or transbronchial approach. Surgical lung biopsy is the gold standard but is limited by morbidity and mortality. Transbronchial cryobiopsy of the lung gained interest because of minimal invasiveness and tissue sampling similar to surgical lung biopsy. Histopathologic patterns commonly seen are summarized in [Table 6]. Histopathology clues suggestive of CTD include dense lymphoplasmacytic infiltrate, lymphoid hyperplasia, presence of secondary lymphoid follicles, and chronic fibrosing pleuritis and perivascular collagen.[23] Overlap patterns are seen with noninflammatory ILDs, drug pneumonitis, and infections. Biopsy findings are to be collaborated with proper clinical and radiologic context with MDD to arrive at a diagnosis.{Table 6}

 Follow-Up in Interstitial Lung Disease



Follow-up is essential to assess disease progression and treatment response. Parameters for disease progression are summarized in [Table 7]. Risk factors for disease progression include older age, male sex, and lower baseline FVC, DLCO and UIP pattern on HRCT. Guidelines for monitoring exist for IPF,[19] progressive fibrosing ILD phenotype,[24] and SSc-ILD.[5] Guidelines for other CTD-ILD are sparse. Disease progression should be monitored with symptom assessment, PFT (spirometry, DLCO, and 6MWT) and HRCT scans. On average, experts opine that disease progression is monitored every 3–6 months. The progressive fibrosing phenotype in chronic ILD follows a similar course as its prototype IPF and is characterized by worsening respiratory symptoms and hypoxemia, a decline in lung function, increasing fibrosis on HRCT, and early mortality.[25] Close monitoring in fibrotic ILD is necessary to detect progressive disease for better treatment strategies. The monitoring algorithm in fibrosing ILD is shown in [Figure 3]. In stable ILD frequency of monitoring can be less frequent compared to progressive phenotype.{Table 7}{Figure 3}

 Clinical Assessment



Assessment for worsening dyspnea, cough and functional limitation should be performed. Among these, worsening dyspnea and exercise limitation are the key markers of disease progression and significant risk factors for mortality. The most studied scale used in clinical practice in ILD is the Medical Research Council Dyspnea Scale (MRCDS). It is an easy to perform and cost-effective means to assess functional limitations of dyspnea being studied in predicting outcomes in ILD.[26] Longitudinal increases of MRCDS predict poor outcome in chronic ILD, with good accuracy. It is a method for monitoring patients unable to perform spirometry, especially those with advanced disease. Therefore, MRCDS should be included in the symptom assessment of progression in chronic ILD. Cough is associated with disease progression and worse quality of life and may predict death or lung transplantation in patients with fibrotic ILD. The objective questionnaires were not validated for use in the clinical practice.

 Pulmonary Function Tests



Serial measurements of PFTs provide an important approach for the objective assessment for disease progression in ILD. The optimal time interval for the repetition of FVC and DLCO has not been formally investigated. Experts recommend monitoring disease course with FVC and DLCO measurements at 3–6 month intervals. However, the disadvantage of spirometry at this frequency is that noise in FVC measurements makes it challenging to find an early decline in FVC, delaying treatment decisions. A flexible approach is needed with a lower threshold for repetition of FVC and DLCO in the scenario of worsening symptoms for the assessment of disease progression. Evidence from clinical cohorts confirms that change in absolute FVC of 10% (with or without a concomitant change in DLCO) or a change in absolute DLCO of 15% (with or without a concomitant change in FVC) is a surrogate marker of disease progression and mortality.

Home spirometry monitoring helps detect FVC decline and acute exacerbations earlier but its utility has been studied only in IPF. A decline in FVC is a recommended marker for disease progression as it predicts mortality. There are limitations of using FVC alone in assessing disease progression, especially in patients with concomitant emphysema described in IPF and RA-ILD, as FVC are is maintained well even with ongoing progressive fibrosis. The combined use of FVC and DLCO helps in overcoming these limitations. The changes in follow-up values of DLCO are often more sensitive to improvement or worsening than a change in TLC or VC.[27] A change in the DLCO is a good index of disease progression or response to therapy. A change >4 mL/min/mmHg is a clinically significant difference. Evidence suggests that progressive fibrosis leads to a gradual decline in pulmonary function tests.

Six-minute walk test is used in clinical trials as a secondary outcome measure post-intervention and in clinical practice in monitoring progression in IPF.[28] 6MWT variables have significant limitations in rheumatic diseases due to extrapulmonary issues affecting ambulation such as arthritis, myopathy, and fatigue and are not recommended for the monitoring of disease progression. During a 6MWT oxygen desaturation of, 88% is associated with increased mortality, suggesting that the development of exertional hypoxemia is an important indicator of disease progression in fibrotic ILD and is used to prescribe supplemental oxygen.[29] 6MWT should be performed at 6–12-month intervals, although the testing frequency will vary depending on availability, disease severity, and rate of progression.

Oxygen saturation by pulse oximetry should be measured at rest and with exertion regardless of symptoms to assure adequacy of oxygenation and identify the need for supplemental oxygen at baseline and during follow-up evaluation.

 Imaging



Serial imaging helps in the assessment of disease progression. There is no data to support the use of chest radiographs in the follow-up of ILD. Serial HRCT chest can show changes in the extent of honeycombing and reticulation, revealing progressive phenotype associated with poorer survival. Visual along with quantitative analysis on scans may better assess disease progression. In patients with progressive fibrosing ILD, HRCT chest is recommended every 12–18 months.[30] Experts recommend repeat HRCT earlier based on worsening symptoms or decline in lung function for the assessment of progression. No consensus recommendations exist for the frequency of repeat HRCT in monitoring CTD-ILD except for systemic sclerosis, where an annual HRCT is recommended, and apart from parenchymal features, other indicators of disease progression are pulmonary artery to ascending aorta ratio and esophagal diameter.[5] Reduced dose HRCT protocols minimize radiation exposure and still provide reliable information.

A follow-up assessment for comorbidities, particularly in fibrotic ILD, is needed – the most common being pulmonary vascular disease, heart failure, coronary artery disease, obstructive sleep apnea, and gastroesophageal reflux. Infections that can mimic ILD exacerbation should be diagnosed and treated appropriately. Tolerability and adverse effects with immunosuppressive and anti-fibrotic agents should be monitored. Recommendations are against screening echocardiography in fibrotic ILDs except in patients at high risk for pulmonary hypertension such as systemic sclerosis (where annual screening is recommended) and before lung transplant evaluation.[5],[24]

 Conclusion



In ILD assessment and follow-up, symptom assessment, pulmonary function tests (spirometry, DLCO, and 6MWT) along with CT imaging play a role in management decisions and prognostication. Guidelines for the frequency of PFT and HRCT are not uniform and scarce across ILDs except for IPF and SSc-ILD. An individualized approach with a low threshold for frequent pulmonary function tests and repeat imaging should be undertaken based on the worsening of clinical symptoms.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1Ryu JH, Olson EJ, Midthun DE, Swensen SJ. Diagnostic approach to the patient with diffuse lung disease. Mayo Clin Proc 2002;77:1221-7.
2Collard HR, King TE Jr. Demystifying idiopathic interstitial pneumonia. Arch Intern Med 2003;163:17-29.
3Ageely G, Souza C, De Boer K, Zahra S, Gomes M, Voduc N. The impact of multidisciplinary discussion (MDD) in the diagnosis and management of fibrotic interstitial lung diseases. Can Respir J 2020;2020:9026171.
4Furini F, Carnevale A, Casoni GL, Guerrini G, Cavagna L, Govoni M, et al. The role of the multidisciplinary evaluation of interstitial lung diseases: Systematic literature review of the current evidence and future perspectives. Front Med (Lausanne) 2019;6:246.
5Hoffmann-Vold AM, Maher TM, Philpot EE, Ashrafzadeh A, Barake R, Barsotti S, et al. The identification and management of interstitial lung disease in systemic sclerosis: Evidence-based European consensus statements. Lancet Rheumatol 2020;2:E71-83.
6American Thoracic Society, European Respiratory Society. American Thoracic Society/European Respiratory Society International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias. This joint statement of the American Thoracic Society (ATS), and the European Respiratory Society (ERS) was adopted by the ATS board of directors, June 2001 and by the ERS Executive Committee, June 2001. Am J Respir Crit Care Med 2002;165:277-304.
7Graham BL, Steenbruggen I, Miller MR, Barjaktarevic IZ, Cooper BG, Hall GL, et al. Standardization of spirometry 2019 update. An official American Thoracic Society and European Respiratory Society technical statement. Am J Respir Crit Care Med 2019;200:e70-88.
8Solomon JJ, Chung JH, Cosgrove GP, Demoruelle MK, Fernandez-Perez ER, Fischer A, et al. Predictors of mortality in rheumatoid arthritis-associated interstitial lung disease. Eur Respir J 2016;47:588-96.
9Brown KK, Martinez FJ, Walsh SL, Thannickal VJ, Prasse A, Schlenker-Herceg R, et al. The natural history of progressive fibrosing interstitial lung diseases. Eur Respir J 2020;55:2000085.
10Nathan SD, Albera C, Bradford WZ, Costabel U, du Bois RM, Fagan EA, et al. Effect of continued treatment with pirfenidone following clinically meaningful declines in forced vital capacity: Analysis of data from three phase 3 trials in patients with idiopathic pulmonary fibrosis. Thorax 2016;71:429-35.
11Flaherty KR, Wells AU, Cottin V, Devaraj A, Walsh SL, Inoue Y, et al. Nintedanib in progressive fibrosing interstitial lung diseases. N Engl J Med 2019;381:1718-27.
12Erre GL, Sebastiani M, Fenu MA, Zinellu A, Floris A, Cavagna L, et al. Efficacy, safety, and tolerability of treatments for systemic sclerosis-related interstitial lung disease: A systematic review and network meta-analysis. J Clin Med 2020;9:E2560.
13Graham BL, Brusasco V, Burgos F, Cooper BG, Jensen R, Kendrick A, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J 2017;49:1600016.
14MacIntyre N, Crapo RO, Viegi G, Johnson DC, van der Grinten CP, Brusasco V, et al. Standardization of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J 2005;26:720-35.
15Steen V, Medsger TA Jr. Predictors of isolated pulmonary hypertension in patients with systemic sclerosis and limited cutaneous involvement. Arthritis Rheum 2003;48:516-22.
16ATS Committee on Proficiency Standards for Clinical Pulmonary Function Laboratories. ATS statement: Guidelines for the six-minute walk test. Am J Respir Crit Care Med 2002;166:111-7.
17du Bois RM, Weycker D, Albera C, Bradford WZ, Costabel U, Kartashov A, et al. Six-minute-walk test in idiopathic pulmonary fibrosis: Test validation and minimal clinically important difference. Am J Respir Crit Care Med 2011;183:1231-7.
18Impens AJ, Wangkaew S, Seibold JR. The 6-minute walk test in scleroderma – How measuring everything measures nothing. Rheumatology (Oxford) 2008;47 Suppl 5:v68-9.
19Raghu G, Collard HR, Egan JJ, Martinez FJ, Behr J, Brown KK, et al. An official ATS/ERS/JRS/ALAT statement: Idiopathic pulmonary fibrosis: Evidence-based guidelines for diagnosis and management. Am J Respir Crit Care Med 2011;183:788-824.
20Lynch DA, Godwin JD, Safrin S, Starko KM, Hormel P, Brown KK, et al. High-resolution computed tomography in idiopathic pulmonary fibrosis: Diagnosis and prognosis. Am J Respir Crit Care Med 2005;172:488-93.
21Fischer A, Antoniou KM, Brown KK, Cadranel J, Corte TJ, du Bois RM, et al. An official European Respiratory Society/American Thoracic Society research statement: Interstitial pneumonia with autoimmune features. Eur Respir J 2015;46:976-87.
22Meyer KC, Raghu G, Baughman RP, Brown KK, Costabel U, du Bois RM, et al. An official American Thoracic Society clinical practice guideline: The clinical utility of bronchoalveolar lavage cellular analysis in interstitial lung disease. Am J Respir Crit Care Med 2012;185:1004-14.
23Song JW, Do KH, Kim MY, Jang SJ, Colby TV, Kim DS. Pathologic and radiologic differences between idiopathic and collagen vascular disease-related usual interstitial pneumonia. Chest 2009;136:23-30.
24Fisher JH, Johannson KA, Assayag D, Morisset J, Boer KD, Manganas H. et al. Long-term monitoring of patients with fibrotic interstitial lung disease: A Canadian Thoracic Society Position Statement, Canadian Journal of Respiratory,CriticalCare,andSleepMedicine, 2020;4:3,147155. DOI: 10.1080/24745332.2020.1796206.
25Cottin V, Wollin L, Fischer A, Quaresma M, Stowasser S, Harari S. Fibrosing interstitial lung diseases: Knowns and unknowns. Eur Respir Rev 2019;28:180100.
26Khadawardi H, Mura M. A simple dyspnoea scale as part of the assessment to predict outcome across chronic interstitial lung disease. Respirology 2017;22:501-7.
27Jegal Y, Kim DS, Shim TS, Lim CM, Do Lee S, Koh Y, et al. Physiology is a stronger predictor of survival than pathology in fibrotic interstitial pneumonia. Am J Respir Crit Care Med 2005;171:639-44.
28Nathan SD, du Bois RM, Albera C, Bradford WZ, Costabel U, Kartashov A, et al. Validation of test performance characteristics and minimal clinically important difference of the 6-minute walk test in patients with idiopathic pulmonary fibrosis. Respir Med 2015;109:914-22.
29Brown AW, Nathan SD. The Value and Application of the 6-Minute-Walk Test in Idiopathic Pulmonary Fibrosis. Ann Am Thorac Soc. 2018;15:3-10. doi: 10.1513/AnnalsATS.201703-244FR.
30Hwang JH, Misumi S, Curran-Everett D, Brown KK, Sahin H, Lynch DA. Longitudinal follow-up of fibrosing interstitial pneumonia: Relationship between physiologic testing, computed tomography changes, and survival rate. J Thorac Imaging 2011;26:209-17.