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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 (qd; 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 …