Off-the-clot serum is collected from blood that is allowed to coagulate naturally after collection. It has not been exposed to any anticoagulant. Plasma-derived serum is produced by defibrinating pooled human plasma collected in the presence of an anticoagulant, such as sodium citrate. Plasma-derived serum, such as Gemini’s human serum AB (cat# 100-512), is generally more economical and consistent than off-the-clot product. Off-the-clot serum is, however, available in original transfer packs from individual donors, a feature attractive for certain metabolic studies.