Evaluating the diagnostic and prognostic value of lone anti-Sm for autoimmune diseases using Euroimmun line immunoassays.

To investigate the value of lone anti-Smith antibody (anti-Sm) using Euroimmun line immunoassay (LIA) in a Chinese population. One thousand two hundred eight of 39,766 patients who were analyzed for anti-Sm had positive anti-Sm, and were divided into true group (having both positive Sm and nRNP/Sm bands) and lone group (only having Sm band without nRNP/Sm band). The proportions of clinical diagnosis of autoimmune diseases (AIDs), non-autoimmune diseases (NAIDs), concentration of C3, C4, and rheumatoid factor (RF), positive rate of autoantibodies of antinuclear antibody (ANA) profile, and titer of anti-Sm and ANA in systemic lupus erythematosus (SLE) patients were analyzed. Lone anti-Sm was evident in 271/1208 (22.42%) of all positive cases. One hundred seventy-five of them had definitive diagnoses with AIDs being the most prominent (69.71%, 122/175). Compared to the true group, SLE patients in the lone group showed significantly lower ANA and anti-Sm titers (both P < 0.001). There was no difference in frequency of other autoantibodies or C3, C4, and RF levels of SLE patients between the two groups. In NAIDs patients, lone anti-Sm indicates less incidence of kidney injury than true anti-Sm (P = 0.05). Lone anti-Sm has great diagnostic value in AIDs, especially SLE. Lone anti-Sm has relationship with mild kidney impairment. Positive anti-Sm patients with no clinical findings or SLE diagnosis should be submitted to new testing to identify changes in anti-Sm, because turning of lone anti-Sm to true anti-Sm indicates evolving kidney injury.