When you develop an IVD calibrator, the choice of matrix is not a procurement decision — it is a scientific and regulatory decision with direct consequences for the assay’s performance across different analytical platforms. This post explains why native human serum without additives is the correct starting matrix for most IVD calibrators, and what “commutability” means in practice.
What is a calibrator matrix?
An IVD calibrator is a material with a known analyte concentration used to establish the relationship between instrument signal and analyte quantity — the calibration curve. The matrix is everything in the calibrator except the analyte: the proteins, lipids, salts, and other biological components that surround the analyte and affect how the assay detects it.
The fundamental principle: the calibrator matrix must behave identically to patient samples in the assay. If the matrix responds differently from real patient samples to the assay chemistry — different protein binding, different viscosity, different antibody access — the calibration curve built from the calibrators will not correctly translate patient sample signals to analyte concentrations. This produces systematic bias.
What is commutability?
Commutability is the property of a reference material or calibrator of producing the same numerical relationship between results from different measurement procedures as that observed for authentic patient samples. In practical terms: a commutable calibrator gives the same result on a Roche cobas platform as on a Siemens ADVIA platform as on an Abbott Architect — because it behaves like a real patient sample on all three systems.
Non-commutable calibrators — typically reconstituted lyophilisates, stabilised preparations, or materials in artificial protein buffers — may work perfectly on the platform they were developed for, but systematically shift results when used as the basis for multi-platform calibration. This is the primary cause of inter-laboratory bias in external quality assessment (EQA) programmes.
Why native human serum maximises commutability
Native human serum — serum collected from human donors without anticoagulant, allowed to clot naturally, and processed by centrifugation without additives — contains the full complement of human proteins in their native state. The protein composition, glycoform distribution, lipid binding, and viscosity match what the assay system was designed to detect in real patient samples.
When you reconstitute lyophilised human serum, add stabilisers (bovine serum albumin, polyethylene glycol, sucrose), or dilute into a buffer matrix, you change the protein environment around the analyte. Antibody-based immunoassays are particularly sensitive to this — the access geometry of the capture and detection antibodies to the analyte changes when the surrounding protein matrix changes. The assay may still produce a signal, but the relationship between signal and concentration will differ from native patient samples.
The specific problem with bovine albumin matrices
BSA-based calibrator diluents are common in research-grade assay development — they are cheap, consistent, and well-characterised. But bovine albumin has different drug and protein binding characteristics from human albumin for many analytes. For any assay where the analyte is protein-bound in patient serum (most hormones, many drugs, some protein biomarkers), using a BSA matrix changes the free/bound fraction of the analyte — directly affecting immunoassay signal.
For commercial IVD calibrators, BSA-based matrices are generally unacceptable for CE-marked assays precisely because they compromise commutability. The IVDR and ISO 17511 (metrological traceability in laboratory medicine) both emphasise the use of matrices that are “as close as possible to the native human sample”.
Which human serum format is correct for IVD calibrators?
The right choice depends on the analyte and the assay format:
| Application | Recommended matrix | Reason |
|---|---|---|
| Most protein biomarker assays (CRP, PCT, troponin) | Human serum standard (mixed donors, pooled) | Average protein background, high commutability |
| Testosterone, PSA, DHT assays | Human AB serum Male only | Physiological hormone background — female serum has different steroid levels |
| hCG pregnancy assays | 1st trimester pregnancy serum or hCG-depleted serum | Native hCG glycoform distribution in real pregnancy matrix |
| Vitamin D (25-OH-D) calibrators | Vitamin D depleted human serum | Zero-calibrator without endogenous 25-OH-D background |
| Steroid hormone assays (estradiol, progesterone) | Steroid-depleted human serum | Removes endogenous steroids for zero calibrator |
| Low-abundance biomarker assays | IgG-depleted human serum | Reduces IgG background for assays where IgG interferes |
| Infectious disease assays (HIV, HBV, HCV) | Native serum from confirmed-positive donors | Only native positive samples meet CTS requirements |
Pooling — why large donor pools matter
A single-donor serum lot for IVD calibration introduces individual biological variability as a systematic factor. One donor may have unusually high or low concentrations of cross-reactive proteins, different glycosylation patterns, or atypical antibody backgrounds. Pooled serum from 20–100+ donors averages out individual variability — producing a more stable, representative matrix that is closer to the true population mean for all protein components simultaneously.
Pooled materials also allow larger production volumes — essential for long-running IVD programmes where the same calibrator lot must perform consistently for 2–5 years. This requires batch reservation arrangements with the supplier to hold the validated lot.
What to specify when sourcing IVD calibrator serum
- Donor origin: EU or US — confirmed screened-negative for HIV, HBV, HCV (confirmed by NAT)
- Collection method: Serum OTC (off-the-clot) or standard serum — specify based on whether platelet-derived growth factors matter for your assay
- Pool size: Minimum 20 donors for research-grade, 50–100+ donors for commercial IVD calibrators
- No additives: Native material — no BSA, no glycerol, no stabilisers at the raw material stage
- Lot volume: Calculate your full programme volume including development, validation, production, and stability — then add 20% and reserve the entire lot
- Documentation: CoA (total protein, IgG, haemoglobin, endotoxin, sterility), donor screening results, IRB documentation
SeamlessBio supplies pooled human serum in all required variants — Standard, Type AB, AB Male, OTC, Delipidated, Vitamin D Depleted, IgG-Depleted — with batch reservation up to 2 years and full regulatory documentation.
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