Assessing the Severity of Idiopathic Pulmonary Fibrosis
An important question that my patients always ask is “How bad is my lung disease”. Patients are eager to understand their prognosis and what to expect. In some lung diseases assessing disease severity is quite straightforward. COPD, for example, can be scored based on a single pulmonary function test result. In contrast, assessing Idiopathic Pulmonary Fibrosis severity requires a more complex approach. Broadly, IPF severity is assessed by evaluating symptoms, Pulmonary Function Tests, exercise capacity, and lung structure using CT scans.
How you feel is an important marker of how your lungs are doing. Studies have confirmed that patients with fewer symptoms of shortness of breath generally have a better prognosis. This is not to say that you can affect your prognosis by being in denial about your breathing symptoms. There are several validated questionnaires that have been used in research studies that quantify your symptoms scores. In clinical practice, we rely on the old fashioned technique of talking to your patients about how they feel.
Stages of Idiopathic Pulmonary Fibrosis
Pulmonary Function Tests
Pulmonary Function Tests (PFTs) are an important tool in assessing IPF severity. The easiest test to perform is spirometry. This test takes about two to three minutes and involves a maximal expiration through a mouthpiece followed by a maximal inspiration. The maneuver is repeated at least three times to ensure reproducibility. The result of greatest interest is the Forced Vital Capacity (FVC). This is the amount of air that is exhaled starting from a maximal inhalation. Your results are compared to age, gender and race matched normals. Results are displayed as a volume of air as well as a percent predicted. Normal is about 80% predicted or greater. There are no single agreed upon cut-offs for staging Idiopathic Pulmonary Fibrosis by FVC but many clinicians use the following:
Severity of IPF FVC Percent Predicted (%)
Very severe <25%
More important than the specific value of the FVC is the change in FVC over time. A decline in FVC of > 5-10% is associated with an increased risk of death. For this reason, your doctor should measure your FVC at most visits.
Diffusion Capacity is another type of pulmonary function test. It is a measure of the way in which gas exchanges across your lungs. It takes longer to perform this test than spirometry. As a result, most doctors will only perform this test twice or three times a year. The results are reported as percent predicted. Lower values indicate more advanced disease. Values less than 40% are associated with worse survival. Declines in Diffusing Capacity are also associated with worse outcomes.
Six Minute Walk Test
The six-minute walk test measures exercise capacity (distance walked, oxygen saturation during exercise and heart rate and blood pressure). A growing body of evidence supports the value of this test in assessing IPF severity. For example, lower oxygen levels during exercise and declining distance walked are associated with increased mortality. Unlike pulmonary function tests that measure an aspect of lung function in isolation, exercise tests measure integrated physiology (many parts of your body’s function all at the same time). For example, if you have new joint pain then your walk distance will be reduced and in no way would that signify that your lung disease has progressed. Walk distances are also highly effort dependent. Even something as minor as the type of shoe worn can affect walk distance. As a result, it is important that the test be evaluated in the context of what is going on with the patient.
High Resolution CT Scan
High resolution CT scanning provides an assessment of the structural extent of fibrosis — how much fibrosis is present. Studies have confirmed that more advanced radiographic fibrosis is associated with worse outcomes. In the research setting, there are some semi-quantitative scoring systems that have been developed. These are not useful in clinical practice yet. Over time, increased extent of fibrosis is also associated with less good outcomes.
Other factors associated with a worse prognosis include advanced age, gender, heavy prior smoking history, being underweight, the development of pulmonary hypertension and exacerbations of your underlying disease. IPF patients are also at increased risk for developing lung cancer, which has a powerful impact on prognosis.
Recent Scoring System for Assessing IPF Severity
A recently developed scoring system is gaining some traction in assessing IPF severity and prognosis. The GAP model (G=gender, A=age, P= physiology—FVC and diffusing capacity). Using a complex statistical analysis, investigators were able to develop a tool that appears to be fairly useful. In a subsequent analysis, the addition of information about recent respiratory hospitalizations and changes in FVC over the past six months performed even better than the original GAP model.
As your doctor helps you evaluate the severity of your lung disease, it is essential to keep in mind that clinical research studies look at groups of patients and not individuals. Results from studies describe how the “average” patient behaves. Any particular patient may or may not behave in an “average” way. I have had patients that have survived a decade with very advanced disease. Furthermore, the entire discussion about assessing the severity of your IPF is based on data that was gathered prior to the approval of Esbriet (pirfenidone) and OFEV (nintedanib). These drugs slow the decline in lung function and reduce the frequency of exacerbations and may improve survival. How they affect our ability to assess disease severity remains incompletely understood.