Rheumatoid arthritis (RA) disease activity fluctuates over time. The disease activity score 28 (DAS28ESR) is a widely used and validated scoring system for assessing RA activity; however, it requires time and expertise. This study aimed to develop a new molecular assay capable of rapidly and objectively assessing RA activity. We used a rapid immuno-assay system (FREND™) to measure soluble CD14 (sCD14) levels, which reflect the DAS28ESR. SCD14 concentrations in urine and serum of RA patients were measured, and RA activity and responses to anti-rheumatic drugs were examined at baseline and after 6 months. FREND™ quantified sCD14 levels in a drop of serum and urine accurately and within 5 min. Serum sCD14 concentrations and its changes correlated well with disease activity and treatment responses, and the results were comparable to C-reactive protein. The new composite indices, including the DAS28CD14 and simplified DASCD14, better detected RA activity than a single sCD14 value and correlated strongly with the DAS28ESR. These indices exhibited excellent diagnostic performance for discriminating a good response 6 months after treatment. We developed a new system for assessing RA activity and therapeutic outcome within 5 min. CD14-based composite indices may have utility for accurate and frequent monitoring of RA status.