A Low Forced Vital Capacity (FVC)/Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) Ratio Increases Clinical Suspicion for Fibrotic Hypersensitivity Pneumonitis (FHP) Over Idiopathic Pulmonary Fibrosis (IPF)

Background and objective Fibrotic Hypersensitivity Pneumonitis (FHP) and idiopathic pulmonary fibrosis (IPF) are interstitial lung diseases (ILDs) that are challenging to differentiate with prognostic and therapeutic implications. Clinical observations suggest that patients with FHP may have a lower baseline ratio of forced vital capacity (FVC) to the diffusing capacity of the lung for carbon monoxide (DLCO), or FVC/DLCO (F/D) ratio, than patients with IPF. In light of this, we aimed to determine whether patients with FHP have a significantly lower baseline F/D ratio than patients with IPF. Methods A retrospective chart review was performed at a single academic ILD center. Patients with a probable or definite diagnosis of FHP or IPF were considered for inclusion, while patients with poor-quality pulmonary function tests (PFTs) were excluded. The data collected included demographics, diagnosis modality, FVC and DLCO values within six months of diagnosis, as well as hemoglobin levels within three months of PFTs. Baseline F/D ratios were calculated using each patient's FVC percentage of predicted value divided by the DLCO percentage of predicted value adjusted for hemoglobin when available. One-tailed independent two-sample T-tests were performed. Results Eighty-nine patients met the inclusion criteria: 39 (44%) with FHP and 50 (56%) with IPF. The mean baseline F/D ratio was significantly lower for patients with FHP (M = 1.24, 95% CI: 1.14, 1.33) than for patients with IPF (M = 1.44, 95% CI: 1.31, 1.57, T(87) = 2.23, p = 0.014). A secondary analysis excluding patients with pulmonary hypertension and resting hypoxemia was performed, yielding 72 patients: 32 (44%) with FHP and 40 (56%) with IPF. The mean baseline F/D ratio was significantly lower for patients with FHP (M = 1.22, 95% CI: 1.12, 1.31) compared to patients with IPF (M = 1.37, 95% CI: 1.27, 1.46, T (70) = 2.37, p = 0.01). Conclusions In patients with probable to definite FHP versus IPF, the baseline F/D ratio was significantly lower in patients with FHP, even after excluding patients with coexisting pulmonary hypertension and resting hypoxemia. A lower baseline F/D ratio may be a novel, clinic-ready index to heighten clinical suspicion for FHP compared to IPF. Further larger prospective studies are needed to validate our findings.