This calculator is provided for informational and educational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare professional before making any medical decisions. The ILD-GAP Index provides group-level mortality probability estimates based on validated cohort data — individual patient outcomes may differ substantially. Results should be used as a reference guide only and not as the sole basis for clinical decisions. Lung transplant referral, palliative care integration, and treatment escalation must be guided by comprehensive multidisciplinary clinical assessment.
ILD-GAP Index Calculator
Calculate the validated ILD-GAP score for interstitial lung disease mortality risk staging. Enter gender, age, FVC percent predicted, DLCO percent predicted, and ILD subtype to receive Stage I through IV classification with 1-year, 2-year, and 3-year all-cause mortality probability estimates based on the Ryerson 2014 ILD-GAP model and Ley 2012 GAP index.
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Very High
| Variable | Category Selected | Points |
|---|---|---|
| Gender (G) | Female | 0 |
| Age (A) | 60 or younger | 0 |
| FVC % Predicted (P1) | Greater than 75% | 0 |
| DLCO % Predicted (P2) | Greater than 55% | 0 |
| Disease Subtype (D) | IPF / Unclassifiable | 0 |
| Total ILD-GAP Score | 0 | |
| Stage | Score | 1-Year | 2-Year | 3-Year | Action |
|---|---|---|---|---|---|
| I — Low Risk | 0–1 | ~6% | ~11% | ~16% | Monitor q6m |
| II — Moderate | 2–3 | ~16% | ~29% | ~42% | ILD centre |
| III — High | 4–5 | ~39% | ~62% | ~76% | Transplant eval |
| IV — Very High | 6–8 | ~57% | ~77% | ~90% | Urgent listing |
This ILD-GAP calculator is for informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. ILD-GAP stage classifications and mortality estimates represent group-level probabilities derived from validation cohorts and do not predict individual patient outcomes. Clinical decisions including lung transplant referral, antifibrotic therapy initiation, and palliative care integration must be made by qualified healthcare professionals in the context of full multidisciplinary clinical assessment. Always consult a respiratory physician or ILD specialist before making any treatment decisions.
About This ILD-GAP Index Calculator
This ILD-GAP index calculator is designed for respiratory physicians, ILD specialists, pulmonologists, and other healthcare professionals who manage patients with chronic interstitial lung disease. It computes the validated ILD-GAP score — a composite of gender, age, FVC percent predicted, DLCO percent predicted, and ILD disease subtype — to classify patients into Stage I through Stage IV mortality risk categories for IPF, CTD-ILD, chronic hypersensitivity pneumonitis, idiopathic nonspecific interstitial pneumonia, and unclassifiable ILD.
The calculator implements the ILD-GAP model derived by Ryerson CJ et al. (CHEST 2014) and the original GAP index by Ley B et al. (Annals of Internal Medicine 2012). Points are assigned to each variable and summed to produce a total ILD-GAP score ranging from 0 to 8, with negative sums reset to 0. The DLCO cannot-perform category scores 3 points, representing severe respiratory limitation. Each stage corresponds to published 1-year, 2-year, and 3-year all-cause mortality probability estimates validated across international cohorts. The model cannot be applied if DLCO was not ordered or was not completed due to non-respiratory reasons.
The three-panel interface combines stacked clickable input cards with a real-time score tally, a horizontal ILD-GAP risk spectrum bar showing the patient's exact position across the Stage I through Stage IV gradient, and a full stage ladder displaying all four risk categories simultaneously for immediate clinical context. Clinical management recommendations by stage — including monitoring frequency, lung transplant referral timing, antifibrotic therapy considerations, and palliative care integration — are updated dynamically with each selection. The complete clinical guide below covers ILD-GAP scoring methodology, validation across diverse populations, limitations, comparison with alternative prognostic tools, and 25 clinical FAQs.
ILD-GAP Index Calculator: A Complete Guide to Predicting Mortality in Interstitial Lung Disease
Interstitial lung disease (ILD) encompasses a diverse group of chronic, progressive pulmonary disorders characterised by varying degrees of inflammation and fibrosis of the lung parenchyma. Accurate prognostication in ILD is essential for guiding treatment decisions, appropriate timing of referrals for lung transplantation, and facilitating informed conversations with patients and their families about disease trajectory. The ILD-GAP Index represents one of the most rigorously validated and widely adopted clinical prediction models for estimating mortality risk across multiple ILD subtypes.
Originally derived from the Gender-Age-Physiology (GAP) model developed specifically for idiopathic pulmonary fibrosis (IPF), the ILD-GAP Index extends this framework to encompass the broader spectrum of chronic fibrosing interstitial lung diseases. By incorporating disease subtype as an additional variable alongside gender, age, and pulmonary function parameters, the ILD-GAP model provides clinically meaningful, disease-specific survival estimates within a single unified staging system.
Origins and Development of the GAP and ILD-GAP Models
The foundation of the ILD-GAP model lies in the seminal work of Ley and colleagues, who published the original GAP Index for IPF in 2012 in the Annals of Internal Medicine. This landmark study derived and validated a multidimensional staging system based on four readily available clinical variables: gender, age, and two pulmonary physiology measures — forced vital capacity (FVC) and diffusing capacity of the lung for carbon monoxide (DLCO). The GAP model was developed using data from 558 patients with IPF across academic centers in the United States and Italy, demonstrating strong predictive performance with a concordance index exceeding 0.74.
Recognising that ILD encompasses many subtypes beyond IPF — including connective tissue disease-associated ILD (CTD-ILD), chronic hypersensitivity pneumonitis (CHP), idiopathic nonspecific interstitial pneumonia (iNSIP), and unclassifiable ILD — Ryerson and colleagues subsequently extended the model in a 2014 publication in CHEST journal. Their study, encompassing over 1,000 patients across five major ILD subtypes, demonstrated that the original GAP model performed well across all ILD diagnoses (c-index 74.6 in the combined cohort). A modified ILD-GAP Index was then developed by adding a disease subtype variable to provide disease-specific survival estimates, accounting for the generally more favourable survival observed in CTD-ILD, CHP, and iNSIP compared with IPF.
Understanding the ILD-GAP Scoring Components
The ILD-GAP Index assigns points across five domains, yielding a total score ranging from 0 to 8. When the mathematical sum of scored variables produces a negative value, the total score is reset to 0. Understanding each component is essential for accurate scoring and clinical interpretation.
Gender Scoring (G Component)
The gender component of the ILD-GAP Index reflects the well-documented sex differences in ILD prognosis. Male sex is associated with worse outcomes in IPF and several other ILD subtypes, consistent with biological differences in fibrotic disease progression, hormonal influences on immune responses, and differences in smoking exposure patterns.
Age Scoring (A Component)
Age is a well-established prognostic factor in ILD, with older age at diagnosis correlating with higher mortality risk. The age component assigns 0 to 2 points based on age thresholds, capturing the non-linear relationship between increasing age and worsening outcomes in chronic fibrosing lung disease.
Physiology Scoring: FVC Component (P1)
Forced vital capacity (FVC), expressed as a percentage of the predicted value, reflects the overall restrictive burden imposed by parenchymal fibrosis. FVC is one of the most widely used clinical endpoints in ILD clinical trials and has strong prognostic validity. Serial FVC decline is a key indicator of disease progression, and baseline FVC percent predicted at diagnosis carries independent mortality risk information.
Physiology Scoring: DLCO Component (P2)
Diffusing capacity of the lung for carbon monoxide (DLCO) — also referred to as the transfer factor for carbon monoxide (TLCO) in some countries — measures the efficiency of gas exchange across the alveolar-capillary membrane. DLCO is often the most sensitive early indicator of physiological impairment in ILD, declining before significant FVC reduction occurs, and is strongly predictive of mortality. When a patient is unable to perform the DLCO maneuver due to respiratory symptoms or severely impaired lung function (i.e., cannot perform rather than chose not to or was not ordered), the highest point value of 3 is assigned. The model cannot be applied if DLCO was not ordered or not performed due to non-respiratory reasons.
Disease Subtype Scoring (D Component)
The disease subtype variable distinguishes the ILD-GAP Index from the original GAP model and represents the key innovation of the 2014 extension. Survival differences across ILD subtypes are substantial. IPF carries the worst median survival among chronic fibrosing ILDs, typically 3 to 5 years from diagnosis. Unclassifiable ILD, whose natural history is highly variable, is grouped with IPF in this scoring system. In contrast, CTD-ILD, CHP, and iNSIP carry generally more favourable survival profiles, largely due to the potential for immunosuppressive therapy response, the younger age at presentation, and the relatively greater proportion of inflammatory (versus fibrotic) disease.
ILD-GAP Staging and Mortality Estimates
Once the total ILD-GAP score is calculated (range 0–8, with negative values reset to 0), patients are classified into one of four stages (I–IV), each associated with published estimates for 1-year, 2-year, and 3-year mortality risk. These estimates differ by ILD subtype, providing disease-specific prognostic information within a single model.
Stage I (0-1 points): Approximately 6% 1-year mortality. Stage II (2-3 points): Approximately 16% 1-year mortality. Stage III (4-5 points): Approximately 39% 1-year mortality. Stage IV (6-8 points): Approximately 57% 1-year mortality. These estimates are derived from the original validation cohort and may differ in specific populations.
Clinical Implications by Stage
The ILD-GAP staging system was designed not only to quantify mortality risk but also to guide clinical decision-making at each stage of disease. Stage I patients generally do not yet require urgent intervention beyond disease monitoring, optimisation of medical therapy, pulmonary rehabilitation, and regular reassessment. Stage II patients warrant consideration of referral to an ILD centre with specialised expertise, and assessment for antifibrotic therapy in eligible IPF patients. Stage III patients should be evaluated for lung transplantation candidacy; their high short-term mortality risk makes early referral essential given prolonged transplant waitlist times in many centres globally. Stage IV patients who are transplant candidates require urgent listing, while goals of care discussions and palliative support become increasingly important for those who are not candidates.
Validation Across Diverse Populations
The ILD-GAP model has been validated across multiple independent cohorts worldwide. The original derivation cohort from Ley et al. (2012) used patients from US and Italian academic centres. The subsequent Ryerson et al. (2014) extension included a multinational cohort. Additional validation studies have been performed in North American, European, and Asian populations with generally consistent prognostic performance, supporting the model's broad clinical applicability.
Some studies have explored modifications or extensions of the ILD-GAP model. The ILD-GAPC model incorporates the Charlson Comorbidity Index Score (CCIS) alongside the original ILD-GAP variables, potentially improving risk stratification in patients with multiple comorbidities. A modified ILD-NSCLC-GAP system has been developed for patients with concurrent non-small cell lung cancer and ILD. These extensions reflect ongoing efforts to refine prognostication in clinically complex populations.
Population-specific considerations are important. Some studies have suggested that the model may perform differently in East Asian cohorts compared to Western populations, partly reflecting differences in ILD subtype distribution and genetic risk profiles. Studies in Korean IPF patients demonstrated comparable predictive value between the GAP model and the composite physiologic index (CPI). Validation in South Asian populations is more limited. Clinicians applying the ILD-GAP model in diverse ethnic populations should be aware of these limitations and consider population-specific calibration where available.
Limitations of the ILD-GAP Model
Several important limitations of the ILD-GAP Index should be recognised. First, the model was derived from patients seen at academic ILD centres, which may not fully represent community-based or primary care ILD populations. Second, the DLCO requirement means the model cannot be applied when DLCO data are unavailable due to reasons unrelated to respiratory capacity — a practical limitation in some clinical settings. Third, the model does not incorporate emerging biomarkers (such as KL-6, SP-D, or serum CA-19-9), radiological extent of disease on high-resolution computed tomography (HRCT), or genetic variants associated with ILD prognosis. Fourth, as noted by several studies, the ILD-GAP model performs less well in myositis-associated ILD (MA-ILD), a subtype with a distinct and often more unpredictable clinical trajectory. Fifth, the model provides group-level risk estimates rather than individual patient predictions, and all staging systems should be interpreted within the full clinical context.
The ILD-GAP Index cannot be used when DLCO was not ordered, or when DLCO testing was not completed due to non-respiratory reasons (e.g., musculoskeletal limitations, patient refusal unrelated to dyspnoea). In these situations, alternative prognostic approaches or clinical judgment must be used.
Regional Variations and Alternative Prognostic Tools
Multiple complementary prognostic tools exist for ILD, each with distinct strengths. The Composite Physiologic Index (CPI), developed in the United Kingdom by Wells et al., uses FVC, FEV1, and DLCO without clinical variables and has been validated particularly in IPF. The TORVAN score includes additional variables including 6-minute walk distance. The MUC5B promoter variant (rs35705950) has strong genetic associations with IPF prognosis but requires molecular testing. The European Respiratory Society (ERS) and American Thoracic Society (ATS) guidelines recommend serial monitoring with FVC and DLCO alongside clinical assessment, with prognostic tools used to supplement rather than replace individualised clinical judgment. In centres with high volumes of CTD-ILD patients, myositis-specific antibody panels (particularly anti-MDA5) carry important independent prognostic information not captured by the ILD-GAP model.
How to Use the ILD-GAP Calculator
Using the ILD-GAP calculator requires five data points, all of which should be available from a standard clinical assessment at an ILD centre. The patient's biological sex at birth is used for the gender variable. Age at the time of assessment is entered in years. FVC percent predicted and DLCO percent predicted are obtained from recent pulmonary function testing (ideally within 3-6 months). If DLCO cannot be performed because respiratory impairment prevents the test, select "Cannot perform" — this scores 3 points. Finally, the ILD subtype diagnosis (IPF, CTD-ILD, CHP, iNSIP, or unclassifiable ILD) should reflect the consensus multidisciplinary team diagnosis.
The ILD-GAP model can be applied both at initial diagnosis and serially over time to track disease progression and risk reclassification. Using the most recent FVC and DLCO values is recommended for the most current prognostic estimate. Stage migration (increasing stage over time) is a clinically meaningful sign of disease progression.
ILD Subtypes Covered by the ILD-GAP Model
The ILD-GAP model was validated in five major chronic ILD subtypes:
Idiopathic Pulmonary Fibrosis (IPF): The most common and most fatal of the idiopathic interstitial pneumonias, IPF is characterised by progressive fibrosis with a usual interstitial pneumonia (UIP) pattern on HRCT or histology. IPF accounts for the 0-point disease subtype score in the ILD-GAP Index, reflecting its uniformly poor prognosis. Antifibrotic agents (pirfenidone and nintedanib) are now standard first-line therapy in most international guidelines.
Connective Tissue Disease-Associated ILD (CTD-ILD): ILD complicating systemic autoimmune conditions including systemic sclerosis (SSc-ILD), rheumatoid arthritis (RA-ILD), polymyositis/dermatomyositis (PM/DM-ILD), Sjogren's syndrome, systemic lupus erythematosus, and mixed connective tissue disease. CTD-ILD carries a generally more favourable prognosis than IPF, particularly in younger women with SSc-ILD or RA-ILD, and scores -1 point for disease subtype.
Chronic Hypersensitivity Pneumonitis (CHP): Resulting from sensitisation to inhaled organic antigens or chemical compounds, CHP can follow a progressive fibrotic course when antigen avoidance is incomplete or fibrosing pathology is established. CHP scores -1 point in the ILD-GAP model, acknowledging its relatively better prognosis compared to IPF in many cohorts.
Idiopathic Nonspecific Interstitial Pneumonia (iNSIP): A rare idiopathic interstitial pneumonia with a predominantly cellular or fibrotic pattern, iNSIP is diagnosed only after careful exclusion of CTD. Its more favourable prognosis compared to IPF is captured by the -1 subtype point.
Unclassifiable ILD: When a confident diagnosis cannot be reached despite multidisciplinary discussion, the case is classified as unclassifiable ILD. Given the uncertainty regarding natural history, it is grouped with IPF (0 points) for disease subtype scoring.
The Role of the Multidisciplinary Team in ILD Diagnosis and Staging
Accurate ILD diagnosis — a prerequisite for correct ILD-GAP subtype scoring — requires multidisciplinary discussion integrating clinical history, serological evaluation, HRCT findings, and where appropriate, histopathological results from surgical lung biopsy or transbronchial cryobiopsy. The multidisciplinary team (MDT) approach, involving collaboration between respiratory physicians, radiologists, and pathologists with ILD expertise, is the internationally recommended standard for ILD diagnosis. Misclassification of ILD subtype directly affects ILD-GAP scoring accuracy and may lead to inappropriate staging. This underscores the importance of expert MDT-based diagnosis before applying the ILD-GAP model in clinical practice.
Integration with Lung Transplant Referral Guidelines
The ILD-GAP staging system has direct relevance to lung transplant decision-making. International guidelines from the International Society for Heart and Lung Transplantation (ISHLT) recommend early evaluation of IPF patients for transplantation given the progressive natural history and limited treatment options. While specific ILD-GAP thresholds for referral are not mandated in all guidelines, Stage III disease (ILD-GAP score 4-5) is generally considered an appropriate trigger for transplant evaluation, and Stage IV (score 6-8) warrants urgent assessment. Serial risk stratification using the ILD-GAP model can help identify the optimal window for referral before clinical deterioration renders patients too ill for surgical candidacy.
Practical Worked Example
Patient: 68-year-old male, diagnosis of IPF confirmed by MDT
Pulmonary function tests: FVC 62% predicted, DLCO 48% predicted
Scoring:
Gender (Male) = 1 point
Age (68, over 65) = 2 points
FVC 62% (50-75%) = 1 point
DLCO 48% (36-55%) = 1 point
Disease subtype (IPF) = 0 points
Total ILD-GAP Score = 5 points = Stage III
This corresponds to an estimated 1-year mortality of approximately 39%, indicating high-risk disease warranting urgent assessment for lung transplant candidacy.
Monitoring Disease Progression with Serial ILD-GAP Scoring
One of the most clinically valuable applications of the ILD-GAP model is serial re-scoring over time. Because the model is based on readily available clinical variables obtained at routine follow-up visits, it can be recalculated at each assessment — typically every 3-6 months in patients with progressive ILD. Upward stage migration (increasing total score over time) indicates disease progression and should prompt escalation of treatment or expedited transplant referral. Conversely, in patients with CTD-ILD responding to immunosuppressive therapy, stable or improving FVC and DLCO values may result in stable or improving ILD-GAP scores, providing objective evidence of treatment response.
Palliative Care Integration in High-Stage ILD-GAP Patients
For patients in ILD-GAP Stages III and IV who are not transplant candidates — due to age, comorbidities, or personal preference — proactive integration of palliative and supportive care services is critically important. Symptoms in advanced ILD, particularly dyspnoea and cough, can be profoundly impairing and often undertreated. Opioid-based dyspnoea management, pulmonary rehabilitation, supplemental oxygen therapy, and psychological support are all components of comprehensive palliative care for patients with advanced ILD. Advance care planning conversations, including discussion of resuscitation preferences and surrogate decision-making, should ideally occur before acute deterioration when patients can participate meaningfully.
Future Directions in ILD Risk Prediction
The field of ILD prognostication is evolving rapidly. Machine learning-based approaches incorporating multidimensional data — including HRCT quantitative imaging, genomic biomarkers, wearable sensor data, and electronic health record variables — are being actively developed and show promise for improving individual-level risk prediction beyond what simple clinical scoring tools can achieve. However, the ILD-GAP model's combination of simplicity, clinical interpretability, and validated performance across ILD subtypes ensures that it will remain an important clinical tool in the foreseeable future. Parallel advances in antifibrotic therapy, including emerging agents for progressive pulmonary fibrosis beyond IPF, mean that accurate risk stratification is increasingly linked to treatment selection and personalised disease management.
Frequently Asked Questions
Conclusion
The ILD-GAP Index represents a clinically practical, rigorously validated tool for mortality risk stratification in chronic interstitial lung disease. By building upon the original GAP model for IPF and extending it across major ILD subtypes through the addition of a disease subtype variable, the ILD-GAP framework enables standardised prognostication applicable to the heterogeneous population seen in specialist ILD clinics globally. Its five readily available input variables — gender, age, FVC percent predicted, DLCO percent predicted, and ILD subtype — make it an ideal tool for integration into routine clinical practice.
The staging system's four-tier structure (Stages I–IV) provides actionable categories for clinical decision-making, linking objective prognostic data to specific management recommendations around monitoring intensity, treatment escalation, lung transplant referral timing, and palliative care integration. Serial application of the model at follow-up visits enables objective tracking of disease trajectory, supporting evidence-based discussions about treatment response and disease progression with patients and their families.
Important limitations of the model — including its dependence on DLCO availability, its reduced performance in myositis-associated ILD, and its group-level rather than individual-level predictions — should be kept in mind when interpreting results. The ILD-GAP Index is most powerful when used as one component of a comprehensive, multidisciplinary approach to ILD care, integrating clinical assessment, imaging, biomarkers, and patient-specific factors alongside objective staging to guide individualised management decisions.