LAUSR

Dear Editor, Immunoassays (IAs) based on streptavidin-biotin binding are widely used in clinical laboratory testing owing to the high affinity and stable interaction between streptavidin and biotin (vitamin B7) and the development of various biotinylation methods [1]. Biotin is a water-soluble B-complex vitamin and a coenzyme responsible for carboxyl transfer in essential carboxylases. Circulating serum concentrations of biotin in the general population typically range from 0.1 ng/mL to 0.8 ng/mL [2]. Biotin is rapidly absorbed, reaches peak plasma concentrations within 1–2 hours, and has an effective serum half-life of 15 hours [3]. Oral administration of biotin doses of 10 mg resulted in peak plasma concentrations ranging from 53 ng/mL to 141 ng/mL [3, 4]. Considering a maximum dosage of 10 mg once a day (q.d.; 10 mg is >300-fold the adequate daily intake) via over the counter (OTC) biotin products, the serum biotin concentration would drop below the in vitro interference threshold of ≤30 ng/mL after eight hours [3]. A high biotin concentration in the blood can interfere with IAs based on streptavidin-biotin binding and is known to cause false high results in competitive IAs and false low results in sandwich IAs [1]. Although the recommended daily intake of biotin is low (30 μg/day), the use of high-dose biotin supplements (up to 10 mg), which are available OTC, has increased in recent years due to unfounded claims that biotin exerts beneficial effects on hair, nails, and skin [5]. According to one survey, 7.7% (95% confidence interval [CI], 6.6–8.9%) of an outpatient population (N=1,944) indicated biotin use; measuring biotin in plasma samples from emergency department patients (N=1,442) showed that 7.4% (95% CI, 6.2–8.9%) had a concentration at or above the lowest known threshold (10 ng/ mL) for biotin interference in a Roche Diagnostics IA (Indianapolis, IN, USA) [6]. Because the Roche Elecsys assay is based on streptavidin-biotin binding, it may yield incorrect results for samples with high biotin concentration. Measurement of free thyroxine (FT4) using a competitive IA from Roche Diagnostics revealed that high biotin concentrations led to false high results in the Elecsys FT4 II assay. Recently, Roche Diagnostics released the Elecsys FT4 III assay, which is claimed to perform better in the presence of biotin interference. This is the first study to evaluate biotin interference in FT4 assays by comparing the newly released Elecsys FT4 III assay with the previous Elecsys FT4 II assay using a Cobas e602 Analyzer (Roche Diagnostics). The Institutional Review Board of Korea University Anam Hospital, Seoul, Korea, approved this study and waived the need for informed consent (No. 2019AN0400). Pooled serum samples from 10 subjects with three targeted FT4 concentrations (high: >3 ng/dL; normal: 0.89–1.8 ng/dL; low: <0.8 ng/dL) were prepared by mixing three or four serum samples remaining after routine clinical assays of T3 and FT4.