Pulmonary fibrosis (PF), characterized by scarring (fibrosis) in the lungs that leads to coughing and shortness of breath, is a disease that progresses over time. But there is no way to predict how quickly or not a patient’s disease will progress, and nor is there a formal system for identifying its stages.
Traditionally, general descriptors — “mild,” “moderate,” “severe,” or “very severe” — have been used to describe the severity and staging of PF. These “stages” are based on a combination of lung function tests, the need for supplemental oxygen, and how impactful symptoms are in a person’s daily life.
Lung function tests
Clinical lung function measures may be useful in assessing the severity of PF, as well as for evaluating the disease’s progression over time.
The most common lung function tests use spirometry — a method that can be used to measure how much air is being inhaled and exhaled, and how quickly. The amount of air exhaled in a given period of time is referred to as forced expiratory volume or FEV. As an example, the amount exhaled in one second is known as FEV1. In turn, the total amount of air exhaled after a deep breath is referred to as forced vital capacity, or FVC.
FEV and FVC are assessed in milliliters, but the measurements often are calculated as a percentage based on what is normally seen. Broadly, patients with an FVC of 75% or higher are considered to have mild disease, while those with an FVC ranging from 50% to 75% are considered to have moderate disease. Those with severe PF have an FVC ranging from 25% to 49%, and patients in whom FVC values are less than 25% are deemed to have very severe disease.
Another lung function test that may help assess PF severity is the diffusing capacity for carbon monoxide, often abbreviated to DLCO or TLCO. This test assessed the lungs’ ability to transfer inhaled oxygen to the bloodstream.
Imaging of the lungs to detect scar tissue, using X-rays or CT scans, also may be useful in assessing PF severity.
Physical function tests
In addition to measuring lung function, tests capable of evaluating a person’s physical endurance may be useful in assessing PF severity. The six-minute walk test, or 6MWT, is a common measure of physical function in people who are able to walk. As its name suggests, this test measures the distance a person can walk in six minutes.
The GAP model
Since current methods of staging PF are fairly subjective and arbitrary, scientists are working to develop new approaches that can more accurately and systematically classify disease severity. One such system is the GAP model, which stands for “Gender, Age, and Physiology.”
This model incorporates a person’s gender and age, and two lung function measures — FVC and DLCO — to calculate the disease stage. There are three GAP stages, specifically I, II, and III, with higher stages reflecting an increased risk of short-term mortality.
Last updated: Nov. 2, 2021
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