Residency · Residency · Respirology

Interstitial Lung Disease - Classification and Approach

Overview and Classification

ATS/ERS Classification of ILD

Interstitial lung diseases encompass a heterogeneous group of disorders characterized by varying degrees of inflammation and fibrosis of the lung parenchyma. The ATS/ERS classification system organizes these disorders into several major categories. The idiopathic interstitial pneumonias (IIPs) include the major entities of idiopathic pulmonary fibrosis (IPF, characterized by the usual interstitial pneumonia or UIP pattern), nonspecific interstitial pneumonia (NSIP), cryptogenic organizing pneumonia (COP), acute interstitial pneumonia (AIP), desquamative interstitial pneumonia (DIP), respiratory bronchiolitis-associated interstitial lung disease (RB-ILD), and lymphoid interstitial pneumonia (LIP). Rare IIPs include idiopathic pleuroparenchymal fibroelastosis (PPFE) and acute fibrinous organizing pneumonia (AFOP). An important category is unclassifiable IIP, which accounts for 10-20% of cases and mandates a multidisciplinary discussion (MDD) approach. ILDs of known cause include connective tissue disease-associated ILD, drug-induced ILD, radiation pneumonitis, and occupational lung diseases such as asbestosis, silicosis, and coal workers' pneumoconiosis. Granulomatous ILDs encompass sarcoidosis and hypersensitivity pneumonitis. Other entities include lymphangioleiomyomatosis (LAM), pulmonary Langerhans cell histiocytosis (PLCH), pulmonary alveolar proteinosis (PAP), and the eosinophilic pneumonias.

Epidemiology

IPF is the most common idiopathic interstitial pneumonia, with an incidence of 3-9 per 100,000 per year and a prevalence that continues to increase with the aging population. Connective tissue disease-associated ILD represents the second most common cause of ILD overall, with rheumatoid arthritis-associated ILD and systemic sclerosis-associated ILD being the most prevalent subtypes. Hypersensitivity pneumonitis is increasingly recognized and may be substantially underdiagnosed, particularly in its chronic fibrotic form, which can closely mimic IPF. Sarcoidosis demonstrates dramatic variation in incidence by race, with rates of 35.5 per 100,000 in Black Americans compared to 10.9 per 100,000 in White Americans.

Clinical Approach to ILD

History - Critical Elements

The temporal course of symptom onset is among the most valuable pieces of diagnostic information when evaluating a patient with suspected ILD. Acute presentations developing over days to weeks suggest AIP, COP, eosinophilic pneumonia, or diffuse alveolar hemorrhage. Subacute presentations evolving over weeks to months raise consideration of hypersensitivity pneumonitis, COP, or drug-induced ILD. Chronic presentations progressing over months to years are characteristic of IPF, NSIP, sarcoidosis, and CTD-associated ILD. A meticulous exposure history is essential and should specifically inquire about birds, molds, hot tubs, and feather duvets or pillows (all relevant to hypersensitivity pneumonitis), as well as asbestos, silica, and coal dust (occupational lung diseases). A detailed lifetime occupational history is critical, as the latency period for conditions such as asbestosis may be 15-30 years. The drug history must capture exposure to agents known to cause ILD, including amiodarone, nitrofurantoin, methotrexate, bleomycin, immune checkpoint inhibitors, nitrosoureas, and busulfan. Smoking history carries specific diagnostic implications: DIP and RB-ILD occur virtually exclusively in smokers, PLCH is strongly associated with smoking, and smoking is a recognized risk factor for IPF but is paradoxically protective against hypersensitivity pneumonitis. Symptoms suggestive of connective tissue disease, including joint pain or swelling, Raynaud phenomenon, skin changes, dysphagia, dry eyes or mouth, and muscle weakness, should be systematically elicited. Family history may reveal familial IPF associated with telomere-related genes (TERT, TERC, RTEL1, PARN) or familial sarcoidosis.

Physical Examination

The physical examination in ILD provides diagnostic and prognostic information. Bibasilar inspiratory crackles with a Velcro-like quality are characteristic of IPF, NSIP, and asbestosis. Digital clubbing is present in approximately 50% of IPF patients and is less common in other ILDs. Skin findings may provide critical diagnostic clues: erythema nodosum suggests sarcoidosis, a heliotrope rash points toward dermatomyositis, sclerodactyly indicates systemic sclerosis, and mechanic's hands are characteristic of antisynthetase syndrome. Joint examination may reveal synovitis consistent with rheumatoid arthritis or proximal muscle weakness suggestive of inflammatory myopathy. Oxygen saturation should be measured at rest and with exertion, using a six-minute walk test or similar assessment.

<image>A comprehensive diagnostic approach algorithm for interstitial lung disease. Start with clinical presentation (dyspnea, cough, crackles, abnormal imaging). First tier: detailed history (exposures, drugs, CTD symptoms, smoking, family history) and physical examination. Second tier: investigations panel showing HRCT pattern recognition (UIP, NSIP, HP, sarcoidosis), PFTs (restrictive pattern, reduced DLCO), serologies (ANA, RF, anti-CCP, myositis panel, ANCA, KL-6), and BAL. Third tier: multidisciplinary discussion (MDD) with radiologist, pathologist, pulmonologist. Decision node: confident diagnosis vs. surgical lung biopsy needed. Show specific diagnostic pathways for IPF (UIP pattern on HRCT may avoid biopsy), HP (exposure + imaging + BAL lymphocytosis), CTD-ILD (autoimmune serologies + clinical features), sarcoidosis (granulomas + exclusion of other causes). Use color coding for each ILD category.</image>

Investigations

High-Resolution CT (HRCT) - Pattern Recognition

HRCT is the single most important diagnostic investigation in the evaluation of ILD, and pattern recognition on imaging drives the diagnostic pathway. The UIP pattern is characterized by basal-predominant, subpleural, heterogeneous fibrosis with honeycombing, with or without traction bronchiectasis, demonstrating temporal and spatial heterogeneity in which areas of normal lung are interspersed with areas of established fibrosis. A definite UIP pattern requires honeycombing with or without traction bronchiectasis in a basal-predominant, subpleural distribution. A probable UIP pattern demonstrates traction bronchiectasis without honeycombing in the same distribution. An indeterminate pattern shows features of fibrosis that do not fit UIP criteria, while an alternative diagnosis pattern suggests a different ILD. The NSIP pattern demonstrates basal-predominant ground glass opacity with or without reticulation, with characteristic subpleural sparing and relative homogeneity. The HP pattern typically shows upper and mid-lung predominance with mosaic attenuation on expiratory images reflecting air trapping, centrilobular nodules, and in chronic fibrotic HP, features that may mimic UIP. Sarcoidosis produces an upper-lobe predominant pattern with perilymphatic nodules and bilateral hilar lymphadenopathy, progressing to fibrotic changes in advanced disease. COP manifests as peripheral or peribronchial consolidation that may be migratory, sometimes with the reverse halo sign (atoll sign). Diffuse alveolar hemorrhage appears as diffuse ground glass opacity with dependent predominance. PLCH produces upper-lobe predominant cysts and nodules with characteristic sparing of the costophrenic angles. LAM is recognized by diffuse thin-walled cysts distributed throughout the lung parenchyma, associated with tuberous sclerosis complex or occurring sporadically.

Pulmonary Function Tests

The classic pulmonary function pattern in ILD is restrictive, with reduced total lung capacity and forced vital capacity and a preserved or elevated FEV1/FVC ratio. The diffusing capacity for carbon monoxide (DLCO) is the most sensitive PFT abnormality in ILD and may be reduced before lung volumes become abnormal. An important diagnostic pitfall is the combined pulmonary fibrosis and emphysema (CPFE) syndrome, in which the opposing effects of fibrosis (restriction) and emphysema (hyperinflation) produce deceptively pseudonormal spirometry and lung volumes, while the DLCO is severely reduced, reflecting the additive destructive effect of both processes on gas exchange. This syndrome carries a particularly high risk of pulmonary hypertension. The six-minute walk test provides prognostic information, with both the distance walked and the desaturation pattern being clinically relevant: a nadir SpO2 below 88% or desaturation exceeding 4% carries prognostic significance.

Serologic Testing

A comprehensive serologic panel should be performed in all patients with newly diagnosed ILD. Antinuclear antibody (ANA), rheumatoid factor (RF), and anti-cyclic citrullinated peptide (anti-CCP) antibodies screen for the most common CTD associations. Myositis-specific antibodies, including anti-Jo-1, anti-PL-7, anti-PL-12, anti-MDA5, anti-Mi-2, anti-NXP2, anti-TIF1-gamma, and anti-SRP, are essential for identifying antisynthetase syndrome and dermatomyositis, conditions in which ILD may dominate the clinical picture. Anti-Scl-70 (anti-topoisomerase I) and anti-centromere antibodies screen for systemic sclerosis. ANCA testing evaluates for granulomatosis with polyangiitis, microscopic polyangiitis, and eosinophilic granulomatosis with polyangiitis. KL-6 (Krebs von den Lungen-6) has emerged as a biomarker for ILD activity, particularly utilized in Japan, with rising global utility. Surfactant protein D (SP-D) is elevated in ILD and carries prognostic significance in IPF.

Bronchoalveolar Lavage (BAL)

BAL provides valuable diagnostic information through cellular analysis. Lymphocytosis exceeding 40% suggests hypersensitivity pneumonitis, sarcoidosis, COP, drug-induced ILD, or NSIP. Eosinophilia exceeding 25% is characteristic of eosinophilic pneumonia. Neutrophilia predominates in IPF/UIP and aspiration pneumonitis. Lipid-laden macrophages suggest aspiration or lipoid pneumonia. A milky appearance with PAS-positive material is diagnostic of pulmonary alveolar proteinosis. Hemosiderin-laden macrophages exceeding 20% of total macrophages confirm diffuse alveolar hemorrhage. The CD4/CD8 ratio provides additional diagnostic discrimination: an elevated ratio exceeding 3.5 is suggestive of sarcoidosis, while a reduced ratio below 1.0 is more typical of hypersensitivity pneumonitis, though considerable overlap exists between conditions.

Surgical Lung Biopsy (SLB)

Surgical lung biopsy remains the gold standard for obtaining a histologic diagnosis when clinical and radiologic data are insufficient to establish a confident diagnosis. Video-assisted thoracoscopic surgery (VATS) is preferred over open thoracotomy, and samples should be obtained from at least two lobes while avoiding the most severely affected areas to ensure representative tissue. The elective mortality risk is 1-2%, but rises substantially in acute presentations and in patients requiring mechanical ventilation. Transbronchial lung cryobiopsy (TBLC) has emerged as an alternative that provides larger tissue samples than conventional transbronchial biopsy, with a lower complication rate than surgical lung biopsy. TBLC achieves a diagnostic sensitivity of 70-80% for ILD diagnosis, with a pneumothorax risk of approximately 10% and significant bleeding risk of approximately 5%.

<image>A comparative HRCT pattern recognition panel showing six classic ILD patterns. Each panel should include a representative axial CT image illustration and key diagnostic features: (1) UIP - basal honeycombing with traction bronchiectasis and subpleural predominance; (2) NSIP - ground glass opacity with subpleural sparing; (3) Chronic HP - upper-lobe mosaic attenuation with air trapping and centrilobular nodules; (4) Sarcoidosis - perilymphatic nodules with bilateral hilar lymphadenopathy; (5) COP - peripheral consolidation with reverse halo sign; (6) LAM - diffuse thin-walled cysts. Label each pattern clearly with distinguishing radiographic features and common clinical associations.</image>

Multidisciplinary Discussion (MDD)

Composition and Process

The multidisciplinary discussion represents the standard of care in ILD evaluation and typically involves a respirologist or pulmonologist, a chest radiologist, and a lung pathologist when biopsy material is available. MDD has been shown to improve diagnostic accuracy by 10-20% over individual clinician assessment, a finding that underscores its fundamental importance. The diagnostic process is inherently dynamic, and the working diagnosis may change as new clinical data emerge, disease behavior evolves, or treatment response provides additional diagnostic information. Studies examining agreement between individual clinician assessment and MDD consensus have shown concordance of only approximately 70%, further emphasizing that MDD is not merely a formality but a substantive contribution to diagnostic accuracy.

Diagnostic Confidence Categories (Fleischner Society / ATS 2018)

The Fleischner Society and ATS 2018 guidelines establish a framework of diagnostic confidence categories. A definite diagnosis reflects high confidence based on clinical-radiologic-pathologic correlation. A probable diagnosis indicates that one diagnosis is favored but alternatives remain possible. A possible diagnosis reflects an inability to distinguish between two or three diagnostic possibilities. An unclassifiable designation is applied when data are insufficient or conflicting, and may require longitudinal follow-up or additional investigation to resolve.

Monitoring and Prognosis

Disease Behavior Classification

The concept of progressive pulmonary fibrosis (PPF) has emerged as an important framework for recognizing that any fibrosing ILD, not only IPF, can exhibit progressive behavior despite standard management. The ATS/ERS/JRS/ALAT 2022 criteria define PPF as the presence of at least two of three features within the preceding year: worsening respiratory symptoms, radiologic progression on imaging, and physiologic decline defined as an FVC decline of 5% or more of predicted or a DLCO decline of 10% or more. Nintedanib is approved for the treatment of PPF based on the INBUILD trial. Stable ILD is characterized by no significant change over 6-12 months of monitoring. Reversible ILD encompasses conditions with the potential for resolution, including COP, cellular NSIP, hypersensitivity pneumonitis when the offending exposure is removed early, and eosinophilic pneumonia.

Key Prognostic Markers

Several prognostic markers guide clinical decision-making and transplant referral in fibrotic ILD. An FVC decline of 10% or more over 6-12 months is associated with significantly increased mortality in both IPF and other fibrotic ILDs. A DLCO decline of 15% or more over the same period carries similarly ominous prognostic implications. On the six-minute walk test, a nadir SpO2 below 88% and a walk distance below 250 meters are both associated with poor prognosis. The GAP index, incorporating gender, age, and physiology (FVC and DLCO), provides a validated staging system for mortality prediction in IPF.

Key Clinical Pearls

• The HRCT pattern is the single most important diagnostic tool in ILD; a definite UIP pattern in the right clinical context can diagnose IPF without surgical biopsy
• Multidisciplinary discussion (MDD) improves diagnostic accuracy by 10-20% and should be the standard of care for ILD evaluation
• Progressive pulmonary fibrosis (PPF) is a new concept recognizing that any fibrotic ILD can develop IPF-like progression; nintedanib is approved for PPF based on the INBUILD trial
• Combined pulmonary fibrosis and emphysema (CPFE) produces deceptively "normal" spirometry and lung volumes while DLCO is severely reduced; this syndrome carries a high risk of pulmonary hypertension
• Always perform a thorough drug and exposure history before pursuing invasive diagnostic procedures; drug-induced ILD and HP are treatable with removal of the offending agent

References

1. Raghu G, Remy-Jardin M, Richeldi L, et al. Idiopathic Pulmonary Fibrosis (an Update) and Progressive Pulmonary Fibrosis in Adults: An Official ATS/ERS/JRS/ALAT Clinical Practice Guideline. Am J Respir Crit Care Med. 2022;205(9):e18-e47.
2. Travis WD, Costabel U, Hansell DM, et al. An official American Thoracic Society/European Respiratory Society statement: Update of the international multidisciplinary classification of the idiopathic interstitial pneumonias. Am J Respir Crit Care Med. 2013;188(6):733-748.
3. Walsh SLF, Wells AU, Desai SR, et al. Multicentre evaluation of multidisciplinary team meeting agreement on diagnosis in diffuse parenchymal lung disease: a case-cohort study. Lancet Respir Med. 2016;4(7):557-565.
4. Flaherty KR, Wells AU, Cottin V, et al. Nintedanib in Progressive Fibrosing Interstitial Lung Diseases. N Engl J Med. 2019;381(18):1718-1727. (INBUILD)
5. Troy LK, Grainge C, Corte TJ, et al. Diagnostic accuracy of transbronchial lung cryobiopsy for interstitial lung disease diagnosis (COLDICE): a prospective, comparative study. Lancet Respir Med. 2020;8(2):171-181.

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