2mL serum from a plain red top tube. Allow sample to clot in an upright position for at least 60 minutes, then centrifuge sample and transfer serum to a plastic transport tube within 2 hours of collection. Clearly label tube as serum from a plain red top tube. Refrigerate.
Note: Do not collect samples from patients receiving therapy with high biotin doses (>5 mg/day) until at least 8 hours following the last biotin administration.