Forced vital capacity (FVC) is routinely used to quantify the severity and identify the progression of idiopathic pulmonary fibrosis (IPF). Although less commonly used, lung transfer of carbon monoxide (TLCO) correlates better with the severity of IPF than does FVC.

Aiming at studying how FVC behaves in relation to TLCO, we analysed cross-sectional data from 430 IPF patients, of which 221 had at least 2 assessments (performed 2.4 ± 1.9 years apart) available for longitudinal analyses. Thresholds for identifying “abnormal” FVC and TLCO values were the statistically-defined lower limits of normal (LLN). For patients with longitudinal data, mean annual absolute declines of FVC and TLCO were calculated.

The correlation between FVC and TLCO (%predicted) was weak (R²=0.21). FVC was “abnormal” (i.e., <LLN) in 38% of patients while 84% of patients had an “abnormal” TLCO. A large majority of the 268 patients with a “normal” FVC had nevertheless an “abnormal” TLCO (n = 209; 78%). On longitudinal analysis, 67/221 patients had an annual absolute decline in FVC≥5%, 34/221 had an annual absolute decline in TLCO ≥10%, and 22 had both.

In IPF, a “normal” FVC should be viewed with caution as it is most often associated with an “abnormal” TLCO, a parameter that is strongly correlated with the morphological extent of the disease. Only 1/3 of the patients with a FVC-based progression criterion also had a TLCO progression criterion. In contrast, 2/3 of patients with a TLCO progression criterion also had a FVC progression criterion.