Makito Tanaka, Shingo Tanaka, Kayoko Ochi, Ryo Kobayashi, Ryosei Murai, Satoshi Takahashi
Background/Objectives: Steatotic liver disease (SLD) is a global health priority, increasing the demand for accessible circulating biomarkers. Carboxylesterase 1 (CES1) plays a key role in hepatic lipid metabolism, but its clinical utility as a plasma biomarker remains unclear. This study aimed to validate an automated immunoassay for plasma CES1 and perform a preliminary evaluation of its diagnostic performance for hepatic steatosis quantified using advanced ultrasound. Methods: An automated ELISA for plasma CES1 was analytically validated. A preliminary clinical evaluation was performed in 95 consecutive outpatients structured to approximate a health-screening population, using ultrasound-derived attenuation coefficients (AC) to quantify steatosis. Results: The assay demonstrated robust analytical performance with intra- and inter-assay coefficients of variation < 10% and excellent sample stability. Plasma CES1 significantly correlated with AC (r = 0.585, p < 0.001), yielding an area under the receiver operating characteristic curve of 0.82 for diagnosing ultrasound-defined steatotic livers. In multivariable analysis, CES1 was independently associated with hepatic steatosis (adjusted odds ratio: 1.03 per 10 pg/mL increase, p < 0.001). Conclusions: Plasma CES1 shows strong diagnostic potential for ultrasound-defined hepatic steatosis. However, this preliminary evaluation is subject to a restricted cohort spectrum; further prospective validation in broader, unselected real-world populations is required before clinical implementation.