Background Primary care spirometry is advocated as a means of facilitating earlier diagnosis of respiratory disease. However spirometry may be normal in the early stages of ILD.

Aim To determine the prevalence of ILD in symptomatic individuals with normal primary care spirometry, and the predictive ability of spirometric parameters to identify ILD.

Methods From a retrospective cohort of 1121 individuals with respiratory symptoms attending primary care clinics for spirometry, 190 had normal spirometry (FVC >-1.64 standardized residuals (SR) and FEV₁:FVC >-1.64 SR according to ERS/ATS 2021 Technical Standard on Interpretative Strategies). Final diagnosis was determined by consensus of respiratory physicians with access to full medical notes and results of further investigations. ROC plots were constructed to compare discriminatory ability of spirometric parameters to identify ILD.

Results Fifty five (29%) were identified as having an ILD. Compared to those without ILD, individuals with ILD were older with higher FEV₁:FVC ratio. FEV₁:FVC SR was the best predictor of ILD, with a FEV1:FVC SR >-0.256 having an AUC of 0.766, sensitivity of 71% and specificity of 71% (Figure 1).

Conclusion In symptomatic individuals, even with normal primary care spirometry, primary care practitioners should have a high index of suspicion for ILD with low threshold for referring for full pulmonary function tests or imaging.