Quick Definition
What is Off-the-Clot (OTC) Serum?: Off-the-clot (OTC) serum is human serum produced by allowing whole blood to clot naturally at room temperature without anticoagulant. During natural clotting, platelets fully activate and degranulate, releasing growth factors (PDGF, EGF, TGF-β) into the serum. OTC serum therefore has higher growth factor content than standard plasma-derived serum.
OTC serum vs standard serum — what is the difference?
Standard human serum is produced from plasma collected with anticoagulant — clotting is then triggered artificially with calcium chloride. Platelets remain largely intact and do not fully degranulate, so their intracellular growth factor content stays trapped. The result is a serum with a more controlled, reproducible protein profile but lower mitogenic activity.
In OTC serum, natural clotting causes complete platelet activation. All intracellular growth factors are released, producing a significantly richer growth factor environment — better for cell proliferation, worse for assay consistency.
Key applications for OTC serum
- Malaria culture (P. falciparum): requires human AB OTC serum — not substitutable with FBS or standard serum
- Hybridoma cloning: PDGF from platelet degranulation supports B lymphocyte-derived hybridoma colony formation
- Primary human cell culture: higher EGF and PDGF content improves proliferation of fibroblasts, keratinocytes and endothelial cells
- Scratch assay migration: higher chemoattractant activity for wound healing assays
Key Facts
- Blood is collected without anticoagulant and allowed to clot naturally for 30–60 minutes at room temperature
- Natural clotting activates platelets via thrombin — full degranulation releases PDGF-AB/BB, EGF, TGF-β1, VEGF, FGF-2 and serotonin
- Growth factor content is significantly higher than standard (plasma-derived) serum from the same donor pool
- Type AB OTC serum is the standard for Plasmodium falciparum malaria culture — one of the few applications where OTC is the only viable option
- Used in hybridoma culture and primary human cell culture where maximum mitogenic activity is needed
- NOT recommended for IVD calibrators, cytokine multiplex assays, or DMPK protein binding assays — higher variability and TGF-β background
- Available from SeamlessBio: Human OTC Serum AB, OTC Mixed — EU and US donors, native and heat inactivated
Further Reading
Human OTC Serum — Available from SeamlessBio
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