Written by Dr Fasihul Khan, Consultant Respiratory Physician and ILD specialist, providing expert guidance on interstitial lung disease.
Key points
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“Progression” refers to change over time, not a single result
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Not all fibrosis progresses and many patients remain stable
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Small fluctuations in lung function are common and not always meaningful
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Treatment decisions are based on trends, not isolated measurements
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Progression can be slow, and does not always mean rapid decline
What is progressive pulmonary fibrosis?
Progressive pulmonary fibrosis (PPF) refers to fibrotic interstitial lung disease that continues to worsen despite appropriate management. Fibrosis means scarring within the lung tissue. In some ILD subtypes, inflammation may play a significant role and respond to immunosuppressive therapy. In others, fibrosis predominates and behaves differently.
Idiopathic pulmonary fibrosis (IPF) is inherently progressive by definition. However, other fibrotic ILDs may remain stable for prolonged periods. The term “progressive” therefore describes disease behaviour rather than a specific diagnostic label.
Two patients with the same ILD subtype may follow very different courses. One may remain stable for years, while another shows measurable decline. The distinction lies in trajectory over time.
How is progression assessed?
Progression is identified through structured longitudinal monitoring. Clinicians review serial lung function tests, particularly changes in forced vital capacity (FVC) alongside gas transfer measurements, repeat CT imaging, symptom evolution, and overall functional capacity.
FVC reflects the volume of air that can be forcibly exhaled after a full breath in. In fibrotic lung disease, increasing scarring can reduce lung elasticity and lung volume. A sustained decline in FVC across sequential assessments may therefore indicate worsening fibrosis. Small fluctuations in lung function are common and do not automatically indicate deterioration.
Radiological progression is assessed by comparing CT scans over time. Increasing reticulation, traction bronchiectasis, or honeycombing may suggest advancing scarring. However, interpreting radiological change requires experience. Subtle differences between scans can reflect technique, timing, or interpretation rather than true biological progression.
Symptoms also matter. Increasing breathlessness, reduced exercise tolerance, or declining daily function may support physiological findings, but symptoms alone do not define progression.
Distinguishing true progression from normal variability requires careful comparison with previous measurements and imaging.
NICE criteria and treatment eligibility in the UK
In the UK, NICE guidance defines criteria for progressive pulmonary fibrosis in non-IPF ILD when considering eligibility for anti-fibrotic therapy. These criteria require objective evidence of significant physiological decline over time, often demonstrated by a defined reduction in FVC across sequential measurements. Radiological or symptomatic worsening also contributes to the overall assessment.
The central principle is that progression must be demonstrated across time rather than inferred from a single result. Meeting NICE criteria has practical implications, as it may determine whether a patient qualifies for NHS-funded treatment such as nintedanib.
However, applying progression criteria is not purely mechanical. Borderline cases occur. Lung function results can fluctuate or sit close to numerical thresholds. Imaging interpretation can vary between observers. Symptoms may not align neatly with physiological measurements. Clinical judgement remains central to determining whether observed changes represent meaningful progression or expected variability.
Does progression mean rapid decline?
Not necessarily.
Progression exists on a spectrum. Some patients experience slow, gradual change over many years. Others may show more noticeable decline over shorter periods. A smaller proportion may have more accelerated disease behaviour.
It is also important to distinguish gradual progression from acute exacerbations, that are defined as sudden episodes of worsening that can occur in fibrotic ILDs. Acute exacerbations are unpredictable and distinct from the steady decline described by progressive pulmonary fibrosis. Recognising the difference influences management and prognosis.
The rate and consistency of change are central to treatment decisions. This is why repeated measurement over time is essential before concluding that disease is truly progressive.
Can progression be predicted?
Predicting progression in an individual patient remains challenging. Certain imaging patterns and specific clinical features may be associated with higher risk. However, no single test can reliably forecast future trajectory.
Research in this area is advancing rapidly. Multicentre studies published in leading international respiratory journals have examined how genetic predisposition, blood biomarkers, detailed CT characteristics, and physiological patterns relate to future lung function decline. These studies aim to identify earlier signals of progression and improve risk stratification in fibrotic ILD.
Although these approaches do not replace longitudinal clinical assessment, they contribute to a more refined understanding of disease behaviour. Over time, integrating imaging analysis, genetics, biomarkers, and physiological trends may allow more personalised assessment of progression risk and more timely intervention.
Why identifying progression matters
Recognising progression has therapeutic consequences. Anti-fibrotic therapy is typically considered when there is documented decline over time rather than simply the presence of fibrosis on imaging.
This reflects a broader shift in ILD care: management decisions are increasingly guided by how disease behaves, not just what it is called. Identifying progression allows clinicians to intervene proportionately, avoiding unnecessary treatment in stable disease while addressing decline when it becomes evident.
When is specialist review helpful?
Specialist ILD review can be particularly valuable when it is unclear whether disease truly meets progression criteria, when lung function trends are borderline or difficult to interpret, or when treatment decisions are being considered.
Because progression is defined by patterns rather than isolated findings, small differences in interpretation can influence management strategy. Careful reassessment of imaging, physiology, and clinical context can clarify whether observed change represents meaningful progression, expected variability, or an alternative explanation.
Structured longitudinal evaluation ensures that decisions are guided by disease behaviour in practice and tailored appropriately to the individual.