Off-the-clot (OTC) serum — the standard process

Whole blood is collected from screened donors into containers without anticoagulant. The blood is allowed to clot naturally at room temperature (typically 30–60 minutes). During clotting, fibrinogen is converted to fibrin by thrombin, forming a solid clot that entraps red cells and platelets. The clot contracts and the remaining liquid — serum — is separated by centrifugation. Crucially, the platelet degranulation during clotting releases growth factors (PDGF, EGF, TGF-β1, IGF-1) from platelet alpha-granules into the serum, giving off-the-clot serum a higher growth factor concentration than plasma or plasma-derived serum.

Plasma-derived (recalcified) serum — the apheresis route

Plasma is first collected via plasmapheresis (apheresis) using an anticoagulant — typically sodium citrate or ACD-A. To produce serum from this plasma, calcium chloride is added to overcome the citrate anticoagulation and force clotting. The resulting serum has similar protein composition to OTC serum but slightly lower growth factor concentrations (platelet activation is less complete) and contains residual citrate. Plasma-derived serum is available in much larger lot volumes — apheresis yields 600–800 mL per session vs. ~200 mL from whole blood — which is an advantage for IVD calibrator manufacturing and bulk research use.

Primary human cell culture

Primary cells derived from human donors — fibroblasts, endothelial cells, keratinocytes, chondrocytes, hepatocytes — have evolved to function in a human biochemical environment. When cultured in media supplemented with FBS, they are exposed to bovine proteins that differ structurally and functionally from their human counterparts: bovine albumin, bovine transferrin, bovine growth factors. For primary human cells where species matching is important for phenotypic fidelity — particularly in drug testing and disease modelling — human serum provides a more physiological matrix that better preserves native cell behaviour.

PBMC isolation and immunoassays

Human peripheral blood mononuclear cells (PBMCs) are the central effector population in immunology assays — ELISPOT, mixed lymphocyte reaction (MLR), cytokine release assays, NK cell cytotoxicity, and T-cell activation studies. FBS is unsuitable for these applications because bovine IgG cross-reacts with anti-human antibodies in detection systems, bovine complement can non-specifically lyse human cells, and xenogeneic proteins trigger background T-cell responses that inflate assay noise. Human AB serum — heat-inactivated, from male donors — eliminates all three sources of interference and is the field standard for PBMC-based immunoassays.

ATMP and cell therapy manufacturing

CAR-T cells, NK cells, TILs, and MSCs intended for clinical use must be manufactured under xeno-free conditions wherever possible — a regulatory expectation that is becoming progressively stronger across EMA, FDA and PMDA frameworks. Human serum Type AB provides a xeno-free alternative to FBS that supports T-cell and NK cell expansion with equivalent or superior fold expansion and phenotype maintenance. For allogeneic ATMP platforms, pooled AB serum from ≥20 donors provides the lot consistency required for reproducible manufacturing campaigns.

Viral vector production (HEK293, Vero)

HEK293 cells — the workhorse of AAV and lentiviral vector production — are human embryonic kidney cells. They perform optimally in a human matrix. Human serum at 5–10% in DMEM or DMEM/F12 supports higher transfection efficiency and better post-transfection cell viability than standard FBS in many AAV8 and AAV9 production protocols, particularly during the 72 h post-transfection harvest window when cell health is most critical.

IVD assay calibrators, controls and matrix-matched validation

Human serum is the biologically correct matrix for immunoassay calibrators because it matches the actual patient sample type — avoiding the matrix mismatch effects (hook effect, non-specific binding, matrix viscosity differences) that occur when calibrators are prepared in buffer or bovine serum. Pooled multi-donor AB serum provides the large, homogeneous lots needed for serial calibrator production. For IVD manufacturers under IVDR, using human serum as the calibrator matrix is increasingly standard practice for assays whose intended clinical sample is human serum.

Hybridoma culture and mAb production

Standard hybridoma culture uses FBS — but at 10–20%, bovine IgG at ~10–12 mg/mL contaminates the supernatant and co-purifies with the target monoclonal antibody on Protein A columns. Switching to Ultra Low IgG FBS (<50 μg/mL IgG) or IgG-depleted human serum eliminates this contamination without changing the culture medium formulation significantly. For clinical hybridoma clones where xeno-free conditions are specified, IgG-depleted human AB serum provides the ideal supplement.

Step 1 — Define the application requirement

Start with the question: does your application involve human immune cells, allogeneic donor material, or a clinical sample matrix? If yes, Type AB is non-negotiable. If the application is basic cell culture or cell line maintenance, blood type is less critical but AB remains the safest default.

Step 2 — Decide on heat inactivation

Use heat-inactivated (HI) serum for: T-cell / NK cell expansion, ELISPOT, MLR, cytotoxicity assays, any application where complement activity could cause background lysis. Use native (non-HI) serum for: applications where complement is part of the assay biology, or where heat-labile growth factors (e.g. PDGF, EGF) are critical for cell performance.

Step 3 — Pooled vs. single donor

Use pooled (≥20 donors) for: reproducible large-scale experiments, calibrator and control manufacturing, any run where lot-to-lot consistency matters more than donor traceability. Use single donor for: autologous studies, research where donor variability is the experimental variable, or applications requiring full chain-of-custody traceability per donor unit.

Step 4 — Confirm additional specifications

For GMP and regulated applications: request full viral screening documentation per lot (not per process), CoA with endotoxin (<5 EU/mL standard; <1 EU/mL for critical applications), mycoplasma result, sterility (Ph. Eur. or USP), and TSE/vCJD risk assessment. For IVD use: specify citrate or EDTA plasma-derived serum format if coagulation factor status is relevant to your assay. For research use: standard off-the-clot AB serum with CoA is typically sufficient.

Storage

Store human serum at −20 °C (long-term) or 2–8 °C for up to 30 days after thawing. Avoid repeated freeze-thaw cycles — each cycle degrades labile growth factors and increases the risk of protein precipitation. Aliquot into working volumes before freezing; do not refreeze thawed material.

Thawing

Thaw at 37 °C water bath with gentle agitation — do not exceed 37 °C. Do not thaw at room temperature for extended periods, as this promotes protein aggregation. Swirl gently after thawing to homogenise — do not vortex.

Lot qualification — what to assess

• Cell proliferation assay: growth rate within ±15% of reference lot
• Functional assay: performance in your actual application (cytokine response, transduction efficiency, antibody titre etc.)
• Endotoxin: LAL test — confirm result matches CoA
• Sterility: run in-house mycoplasma PCR on received material
• Haemoglobin: visual check + CoA value (<20 mg/dL standard)
• Viral screening: review per-lot documentation — HIV, HCV, HBV, syphilis (NAT + serology)

Lot reservation

Once a lot passes qualification, reserve the remaining volume with your supplier before it is sold. In practice, the best-performing lots of human serum are in high demand — especially AB male pooled lots with low endotoxin. Reserve before you need the volume, not when the current lot is running out.