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PubMed This is a summary of 13 peer-reviewed journal articles Updated
Pulmonology

Understanding Prognosis and the GAP Index

At a Glance

The GAP Index is a tool doctors use to estimate interstitial lung disease (ILD) prognosis based on your gender, age, and breathing tests. However, because modern antifibrotic medications slow lung decline, standard GAP scores often overestimate risk. Your prognosis improves with effective treatment.

When you are diagnosed with a chronic condition like Interstitial Lung Disease (ILD), it is natural to want to know what the future holds. To help answer this, doctors use “risk stratification” tools to estimate the likely course of the disease [1]. While these models are helpful for doctors to plan your care, it is vital to remember they are based on averages of thousands of people, not a crystal ball for your specific life [2].

The GAP Index: The Traditional Model

The most common tool used in ILD—specifically for Idiopathic Pulmonary Fibrosis (IPF)—is the GAP Index [1]. The name is an acronym for the four variables it measures:

  • G: Gender (Men often have a slightly higher risk profile in these models) [1].
  • A: Age (Risk naturally increases as we get older) [1].
  • P: Physiology (This refers to two key numbers from your breathing tests: FVC and DLCO) [1][3].

Based on these points, patients are assigned a “Stage” that estimates the 1-year to 3-year survival risk:

GAP Stage Score General Interpretation
Stage I 0–3 points Mild disease with the lowest mortality risk [1].
Stage II 4–5 points Moderate disease requiring closer monitoring [1].
Stage III 6–8 points Severe disease; often a sign to prioritize transplant evaluation [1][4].

Moving Beyond the GAP: Newer Models

While the GAP index is widely used, it has limitations. For example, it doesn’t account for how much you struggle to breathe during exercise or other health conditions you might have [2]. To get a more accurate picture, specialists may use newer models:

  • DO-GAP (Distance-Oxygen GAP): This model adds your performance on a 6-minute walk test and your oxygen levels during activity [5]. Because it measures how you actually function in daily life, it is often more accurate than the standard GAP score [6].
  • CPI (Composite Physiologic Index): This is a mathematical formula that combines multiple breathing test results [3]. It was specifically designed to be more accurate for patients who have both ILD and emphysema, as emphysema can sometimes make your lung volume (FVC) look better than it actually is [7][8].

The Most Important Limitation: The “Antifibrotic Effect”

The most critical thing to know about the GAP index is that much of the data used to create it was gathered before modern treatments like antifibrotics (nintedanib and pirfenidone) were available [2].

Because these medications are proven to slow down the rate of lung function decline, many experts believe the old GAP scores now overestimate the risk for many patients [9][10]. If you are on an effective treatment plan, your individual “statistics” are actively being rewritten [11].

Prognosis is not a single number on a chart; it is a moving target that is influenced by your medications, your participation in pulmonary rehab, and how closely you work with your multidisciplinary team [12][13].

Common questions in this guide

What does the GAP Index measure in interstitial lung disease?
The GAP Index is a risk assessment tool that estimates the course of your disease based on four factors: Gender, Age, and Physiology. The physiology portion uses your forced vital capacity (FVC) and diffusing capacity (DLCO) from your breathing tests.
What do the different GAP stages mean?
Stage I indicates mild disease with the lowest risk, Stage II is moderate disease needing closer monitoring, and Stage III represents severe disease. While these stages help doctors plan your care, they are only estimates based on averages and do not predict your exact future.
Do antifibrotic medications change my GAP Index prognosis?
Yes. The original GAP Index was created before modern antifibrotic medications were widely used. Because these drugs help slow lung scarring and preserve lung function, your actual prognosis may be much better than older GAP statistics suggest.
What is the DO-GAP model?
The DO-GAP model improves upon the standard GAP score by including how far you can walk in six minutes and your oxygen levels during that activity. This provides a more realistic picture of how your lungs function in daily life.
Why might my doctor use the Composite Physiologic Index (CPI)?
The Composite Physiologic Index is a formula that combines multiple breathing test results to give a clearer picture of your lung health. It is especially useful for patients who have both ILD and emphysema, as emphysema can artificially inflate certain breathing test numbers.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is my current GAP stage, and how much weight do you place on this specific model for my individual care?
  2. 2.Do you use the Composite Physiologic Index (CPI) to account for other factors, like emphysema, that might affect my breathing test numbers?
  3. 3.How does my performance on the 6-minute walk test change my risk profile compared to the standard GAP score?
  4. 4.Since I am taking antifibrotic medication, how does that change the 'standard' survival statistics associated with my GAP stage?
  5. 5.Are we monitoring 'longitudinal changes' (the trend over time) rather than just a one-time score to determine my prognosis?

Questions For You

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References

References (13)
  1. 1

    The prognostic role of Gender-Age-Physiology system in idiopathic pulmonary fibrosis patients treated with pirfenidone.

    Harari S, Caminati A, Confalonieri M, et al.

    The clinical respiratory journal 2019; (13(3)):166-173 doi:10.1111/crj.12999.

    PMID: 30675755
  2. 2

    Machine Learning and BMI Improve the Prognostic Value of GAP Index in Treated IPF Patients.

    Lacedonia D, De Pace CC, Rea G, et al.

    Bioengineering (Basel, Switzerland) 2023; (10(2)) doi:10.3390/bioengineering10020251.

    PMID: 36829744
  3. 3

    Comparison of CPI and GAP models in patients with idiopathic pulmonary fibrosis: a nationwide cohort study.

    Lee SH, Park JS, Kim SY, et al.

    Scientific reports 2018; (8(1)):4784 doi:10.1038/s41598-018-23073-3.

    PMID: 29555917
  4. 4

    Prognostic Utility of the GAP Score in Interstitial Lung Disease Patients Evaluated for Lung Transplantation: A Single-Center Study.

    Mora-Cuesta VM, Zuazaga-Fuentes J, Iturbe-Fernández D, et al.

    Clinical transplantation 2025; (39(3)):e70136 doi:10.1111/ctr.70136.

    PMID: 40109152
  5. 5

    External validation and longitudinal application of the DO-GAP index to individualise survival prediction in idiopathic pulmonary fibrosis.

    Chandel A, King CS, Ignacio RV, et al.

    ERJ open research 2023; (9(3)) doi:10.1183/23120541.00124-2023.

    PMID: 37228268
  6. 6

    Derivation and validation of a simple multidimensional index incorporating exercise capacity parameters for survival prediction in idiopathic pulmonary fibrosis.

    Chandel A, Pastre J, Valery S, et al.

    Thorax 2023; (78(4)):368-375 doi:10.1136/thoraxjnl-2021-218440.

    PMID: 35332096
  7. 7

    Staging systems and disease severity assessment in interstitial lung diseases.

    Tomassetti S, Ryu JH, Poletti V

    Current opinion in pulmonary medicine 2015; (21(5)):463-9 doi:10.1097/MCP.0000000000000198.

    PMID: 26176966
  8. 8

    Functional and prognostic effects when emphysema complicates idiopathic pulmonary fibrosis.

    Jacob J, Bartholmai BJ, Rajagopalan S, et al.

    The European respiratory journal 2017; (50(1)) doi:10.1183/13993003.00379-2017.

    PMID: 28679612
  9. 9

    A survival analysis of idiopathic pulmonary fibrosis in the context of antifibrotic therapy in Saudi Arabia.

    Khan MA, Ghamdi BA, Alhamadi M, et al.

    Annals of thoracic medicine 2023; (18(2)):79-85 doi:10.4103/atm.atm_264_22.

    PMID: 37323372
  10. 10

    Efficacy of early antifibrotic treatment for idiopathic pulmonary fibrosis.

    Sugino K, Ono H, Watanabe N, et al.

    BMC pulmonary medicine 2021; (21(1)):218 doi:10.1186/s12890-021-01595-3.

    PMID: 34246227
  11. 11

    Predictive factors of mortality in patients with idiopathic pulmonary fibrosis treated with antifibrotics: a novel prognostic scoring system.

    Polat G, Özdemir Ö, Ermin S, et al.

    Sarcoidosis, vasculitis, and diffuse lung diseases : official journal of WASOG 2024; (41(2)):e2024021 doi:10.36141/svdld.v41i2.13779.

    PMID: 38940720
  12. 12

    Validation of the gender, age, physiology model and other prognostic factors in interstitial lung disease patients with systemic autoimmune rheumatic disease.

    Hwang YJ, Lee JK, Lee JH, et al.

    Scientific reports 2025; (15(1)):24691 doi:10.1038/s41598-025-08484-3.

    PMID: 40634418
  13. 13

    ILD-GAP combined with the monocyte ratio could be a better prognostic prediction model than ILD-GAP in patients with interstitial lung diseases.

    Hirata M, Hara Y, Fujii H, et al.

    BMC pulmonary medicine 2024; (24(1)):16 doi:10.1186/s12890-023-02833-6.

    PMID: 38183005

This page explains the GAP Index and ILD prognosis models for educational purposes only. Always consult your pulmonologist to understand what your specific scores, tests, and stages mean for your individual care plan.

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