Serum or plasma
Red-top tube or green-top (heparin) tube. Gel-barrier tubes are not recommended.
Trough levels are most reproducible.
Serum or plasma should be separated from cells within two hours of venipuncture. Submit serum or plasma in a plastic transport tube.
Submission/transport (<3 days): Room temperature. For storage beyond three days, specimen should be refrigerated or frozen. Specimen is stable for four days at room temperature, for four days refrigerated, and for 70 days frozen.