Background: Immunoassays (IAs) are widely used to measure concentration of serum estradiol (E2) despite some limitations including cross-reactivity. Liquid chromatography-tandem mass spectrometory (LC-MS/MS) for E2 measurement has a theoretically greater specificity and sensitivity than IAs. We report a case with unexpected discrepancy in E2 values measured by IA and LC-MS/MS. Case presentation: A 7-year-old girl was referred because of an ovarian tumor. Physical examinations revealed prepubertal statuses. Serum E2 with ECLIA was 69 pg/mL. GnRH stimulation test revealed a prepubertal response. On imaging studies, the diagnosis was mature teratoma of the right ovary. After tumor enucleation, the diagnosis was pathologically confirmed. E2 with ECLIA decreased to 11 pg/mL. Preoperative E2 with LC-MS/MS was 1.15 pg/mL. Conclusions: We conclude the preoperative E2 with ECLIA was falsely high. We speculate the antibody used in ECLIA had cross-reactivity to endogenous compounds. LC-MS/MS should be considered when high serum E2 measured with IA is inconsistent with physical and/or endocrinological data.
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