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Isolated Low DLCO — SCE Rheumatology MCQ

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ModerateMusculoskeletal ImagingIsolated Low DLCOSCE Rheumatology

A 60-year-old woman with seropositive rheumatoid arthritis has taken a stable dose of methotrexate for 5 years. She reports gradually progressive exertional breathlessness without fever or acute cough. She is a lifelong non-smoker and has a normal haemoglobin concentration. Reproducible pulmonary function testing shows a normal FEV1/FVC ratio, FVC 85% predicted, total lung capacity 88% predicted and haemoglobin-corrected DLCO 58% predicted. Which interpretation is most appropriate?

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Correct answer: A — The disproportionately reduced DLCO may indicate occult interstitial lung disease or pulmonary vascular disease and warrants further assessment, including HRCT

The correct answer is A. A reproducible, haemoglobin-corrected DLCO of 58% predicted is abnormal despite preserved lung volumes. In rheumatoid arthritis, this pattern can precede restrictive physiology in occult ILD, but it is not specific: pulmonary vascular disease is another important possibility. HRCT is appropriate to detect and characterise parenchymal disease, with pulmonary vascular assessment guided by the clinical findings. There is no threshold of 40% below which investigation suddenly becomes necessary. Preserved FVC and total lung capacity do not make an isolated low DLCO normal or unreliable. Methotrexate can cause pneumonitis, but exposure alone is not diagnostic; its typical presentation is an acute or subacute respiratory illness, and alternative causes, including infection and RA-associated lung disease, must be investigated.

Reference: Antoniou KM et al. ERS/EULAR clinical practice guidelines for connective tissue disease-associated interstitial lung disease, screening and diagnostic algorithms, 2026. https://www.medicines.org.uk/emc/product/11723/smpc