Interstitial lung disease (ILD) is frequently associated with collagen diseases, and is designated collagen vascular disease-associated ILD (CVD-ILD) that influences the prognosis of the disease. Acute-onset diffuse ILD (AoDILD) occurs in patients with collagen disease with or without underlying CVD-ILD. The prognosis of AoDILD is quite poor. It has been reported that plasma amino acid profiles are altered in rheumatoid arthritis (RA) patients. Here, we investigated the plasma amino acid profiles to determine whether they may be useful for diagnosing CVDILD or AoDILD in collagen disease.
Plasma amino acid levels were analyzed using liquid chromatography/electrospray ionization tandem mass spectrometry in 64 RA patients with or without CVD-ILD, and 15 collagen disease patients with AoDILD. By using support vector machine (svm) analysis, the svm index (AA) was generated from amino acid profiles and the svm index (AA, KL6) was from amino acid profiles together with Krebs von den lungen-6 (KL-6).
Plasma lysine levels were higher in RA patients with ILD than in those without. The optimized cut-off level of svm index (AA) was determined for CVD-ILD in RA, and the specificity and the sensitivity were 86.8% and 65.4%, respectively. These values for the svm index (AA, KL6) were 81.6% and 88.5%. The plasma methionine and phenylalanine levels were significantly increased in the AoDILD state, whereas Fischer’s ratio was decreased.
This is the first report of plasma amino acid profiles in CVD-ILD and AoDILD in collagen disease. The svm index (AA, KL6) will be a better marker for diagnosing CVD-ILD in RA than KL-6 alone, though svm index (AA) is not. The plasma amino acid profiles could be a better marker for AoDILD in collagen disease patients.