A negative diagnosis can be relieving for many individuals, but occasionally in scleroderma, individuals with interstitial lung disease are wrongly diagnosed as healthy according to pulmonary function tests. In a cohort of 102 patients enrolled in a recent study, “Pulmonary Function Tests: High Rate of False Negatives in the Early Detection and Screening of Scleroderma Interstitial Lung Disease,” which was published in Arthritis & Rheumatology, 27 had a low (<80%) forced vital capacity (FVC), while 64 patients had significant interstitial lung disease according to high resolution computer tomography (HRCT) of the chest, indicating a high false negative rate by pulmonary function tests alone.
Dr. Yossra A. Suliman and Dr. Oliver Distler in the Division of Rheumatology at University Hospital Zurich conducted this study with the goal of identifying reliable methods of diagnosing interstitial lung disease in scleroderma patients. Pulmonary function tests are easy to administer, so it would be an attractive diagnostic test. In fact, pulmonary function tests are already the most frequently employed method to detect scleroderma-associated interstitial lung disease. However, as identified by Drs. Suliman and Distler, there is a lack of evidence suggesting that pulmonary functional tests can accurately diagnose interstitial lung disease in scleroderma patients.
To provide this much-needed evidence, the research team recruited scleroderma patients attending the authors’ medical facility and conducted HRCT scans and pulmonary function tests. Interstitial lung disease was accurately predicted (using HRCT as a reference) by pulmonary function tests (FVC) in only 57% of the patients. An accurate prediction could have been either a positive or negative diagnosis, with 34% of patients showing normal FVC and HRCT and 23% of patients showing low FVC and fibrosis with HRCT.
On the other hand, the false negative rate was 53%. Forty patients with a normal FVC showed fibrosis on HRCT, indicating the presence of interstitial lung disease. Five patients had severe fibrosis but a normal FVC, one patient had intermediate (~20%) fibrosis, and 34 had mild-to-moderate (<20%) fibrosis. Additionally, false positive rate was 7.9%. Three patients without interstitial lung disease (according to HRCT) had an FVC of less than 80%.
“Taken together, these findings suggest that clinicians might often miss the detection of [scleroderma-associated interstitial lung disease] when relying solely on pulmonary function tests due to the high rate of false negative results,” concluded the authors. To make up for the sometimes invalid interpretation of FVC, it is necessary to look at other factors such as total lung capacity (TLC), diffusing capacity of the lungs for carbon monoxide (DLCO), and six-minute walk distance (6MWD) to make an accurate diagnosis of interstitial lung disease in scleroderma patients.