Area under the receiver operating characteristic curves for detection of rheumatoid arthritis by reference to the level of serum soluble interleukin-18 receptor α complex, immunoglobulin M rheumatoid factor, and anticyclic citrullinated peptide antibody. The area under the receiver operating characteristic curve (ROC-AUC) for the soluble interleukin-18 receptor α (sIL-18Rα) complex was 0.826. At a cutoff point of 63.1 ng/ml, corresponding to the greatest sum of specificity and sensitivity, the specificity was 0.876 and the sensitivity was 0.662 for detection of rheumatoid arthritis. The ROC-AUC for the serum level of immunoglobulin M rheumatoid factor (RF) was 0.902. At a cutoff point of 19 U/mL, the specificity was 0.955 and the sensitivity was 0.752. The ROC-AUC for the serum level of anticyclic citrullinated peptide (CCP) antibody was 0.921. At a cutoff point of 6.5 U/mL, the specificity was 0.992 and the sensitivity was 0.828.