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Background: As the saturation of transferrin by iron in the serum is approximately 30%, iron loaded to the blood can bind to transferrin not bearing iron. Nevertheless, prolonged iron influx finally results in full transferrin saturation, and iron not bound to transferrin will appear in the serum; this iron is known as non-transferrin-bound iron (NTBI). NTBI damages organs through the production of free radicals. Previously, we established an automated quantification system for NTBI; however, measuring labile plasma iron, which is considered as a highly redox-active component of NTBI, should be a better prognostic factor in iron-overloaded patients. Methods: We designed and developed a novel system for evaluating labile plasma iron utilizing the Trinder reaction. Automated system was utilized because the previously reported methods for labile plasma iron are intricate and the introduction to the clinical stage has been challenging. Validations such as the contribution of serum proteins and metal ions for this system were evaluated using human serum samples. Results: We confirmed that our novel system can evaluate labile plasma iron utilizing Trinder reaction and the oxidative potential of ceruloplasmin in the serum. This system was also confirmed to be clinically practical. Metals other than iron did not influence this system. We observed that samples with high NTBI did not always exhibit high labile plasma iron and vice versa, highlighting the necessity of labile plasma iron quantification in evaluating the toxicity of NTBI. Conclusions: Our novel system should contribute to fundamental and clinical research because it can measure labile plasma iron using the high-throughput automated analyser. [ABSTRACT FROM AUTHOR]