Clinical significance of serum surfactant protein D in patients with rheumatoid arthritis-associated interstitial lung diseases

BackgroundRheumatoid arthritis (RA) is a chronic inflammatory disease that affects approximately 1% of the population, and pulmonary involvement is common. The most common pulmonary manifestation is interstitial lung disease (ILD) which leads to pulmonary fibrosis. ILD is the only complication of RA reported to be increasing in prevalence and it has been shown to account for around 6% of all RA deaths. Surfactant proteins (SP-A, SP-B, SP-C and SP-D) play various important roles in the lung. Serum SP-D levels reflected the disease activity of pulmonary fibrosis.Aim of the studyTo determine the clinical significance of surfactant protein D (SP-D), a useful marker for evaluating interstitial lung diseases and specifically in rheumatoid arthritis associated interstitial lung disease patients.Patients and methodsOur patients were classified into 3 groups (Group 1: 18 patients had rheumatoid arthritis ILD; Group 2: 12 patients had rheumatoid arthritis without interstitial pulmonary disease; Group 3: 10 patients had idiopathic pulmonary fibrosis with no rheumatoid arthritis) and 20 healthy control subjects. Serum SP-D levels were assayed using a specific enzyme-linked immunosorbent assay for all studied groups.ResultsA highly significant difference was found between patients’ groups and the control group regarding disease duration, serum aCCP, SP-D, serum CRP and RF being higher in patients’ group (p < 0.01). However FVC was significantly lower in patients’ groups compared to the control group (p < 0.05). Assessment of the diagnostic performance of SP-D assay revealed that the best cutoff for discriminating rheumatoid patients with interstitial lung disease from those without interstitial lung disease was 219 ng/mL. At this value, SP-D had a diagnostic sensitivity of 94.2%, specificity 90%, negative predictive value 90%, positive predictive value 94.2% and efficiency 95%.ConclusionThe serum SP-D level may be a useful marker for ILD especially in patients with rheumatoid arthritis.