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Pulmonary Fibrosis

DLCO Testing

03/04/2010

Question:

On August 25, 2006, I had a DLCO test with Predicted value of 44%. The next and following test were performed by another medical clinic: March 24, 2008, DLCO value 38%; July 3, 2008, DLCO predicted value 36%; October 2, 2008, DLCO predicted value 42%; May 18, 2009, DLCO predicted value 44%; December 10, 2009 DLCO predicted value 36%; March 3, 2010, DLCO predicted value 38%. Thus far, I have had no medication or treatment planned, primarily because I have Rheumatoid Arthritis and and 62 years of age. I am taking methotrexate and folic acid and currently enrolled in a clinical study, biologic in nature and involving B-cell depletion and similar to the Rituximab medication. From my research and from my doctor, life expectency is usually less than six years from the onset yet my values have remained constant. I re-visit my doctor in July 2010 and if things remain, will go on an annual visit basis unless things change. From the DLCO predicted values given, is this considered "severe" status? Thanking you in advance -

Answer:

The diffusing capacity (DLCO) is a moderately difficult test to perform and so the values can vary from one pulmonary lab to another. Furthermore, not all labs use the same set of predicted values so, for example, a raw number of 10.0 could be 40% of predicted in one lab and 36% of predicted in another lab. To make things more complicated, the DLCO depends on the blood hemoglobin amount so if there is anemia, the diffusing capacity will appear to be low. There can be minor variations from test to test and therefore, we generally look for consistent trends over time rather than minor fluctuations from test to test. As a general rule, we consider 12-15% change between two tests as statistically significant. Because the diffusing capacity is not a perfect test in every patient, we often look at the overall result of several pulmonary function tests together including the FVC (forced vital capacity), TLC (total lung capacity), and blood oxygen saturation during walking (commonly done during a 6 minute walk test).

 

Patients with rheumatoid arthritis-associated pulmonary fibrosis generally have a better prognosis than patients with idiopathic pulmonary fibrosis (idiopathic means "cause unknown"). We do not know if newer biologic drugs used to treat rheumatoid arthritis have a favorable effect on rheumatoid-associated interstitial disease. Therefore, it is quite possible that the Rituximab could be benefiting the lungs. The decision of whether to use methotrexate is always a difficult one in rheumatoid arthritis since it can cause interstitial lung disease but can also treat some interstitial lung diseases (particularly sarcoidosis) so it is a double edged sword when it comes to treating pulmonary fibrosis.

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Response by:

James N Allen, Jr, MD James N Allen, Jr, MD
Clinical Professor of Pulmonary, Allergy, Critical Care & Sleep Medicine
College of Medicine
The Ohio State University