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Measuring Neutralizing Antibodies in Serum
Antibody levels tell you how much is there. A blocking assay on the P4SPR tells you whether those antibodies stop a virus from binding its receptor, without live virus.
The need
What protects against a virus is not the amount of antibody but its ability to block infection. The reference test for that uses live virus and cultured cells, which means a BSL-3 laboratory and days of work. ELISA is fast and scales well, but it measures how much antibody binds, not whether it neutralizes. Vaccine studies, cohort follow-up and animal work all need something in between.
How it works
The spike protein is attached to an AffiCoat sensor by amine coupling. Diluted serum flows over it, and any antibodies against the spike bind. ACE2 is then injected: where antibodies cover the spike's receptor site, ACE2 cannot bind. The less ACE2 binds, the more neutralizing the serum (Figure 1).
The four channels of the P4SPR, read in turn, make each run self-contained: two channels measure the sample in duplicate, one receives pre-pandemic serum then ACE2 (full binding, the positive control), and one receives the sample then buffer only (the negative control). The blocked fraction comes from the three curves of the same sensor (Figure 2).
What the study showed
| Result | |
|---|---|
| Scale | Nearly 2,000 measurements on more than 1,000 serum samples, against the original and Omicron spike proteins |
| Agreement with live virus | Good correlation with the live-virus neutralization assay (r = 0.74), closer than with ELISA antibody levels |
| Biosafety | Run in a biosafety cabinet in a BSL-2 lab, with no virus, instead of a BSL-3 lab |
| Population trends | Followed vaccine doses, infection waves and the waning of protection over time across the cohort |
Full method and data: Coutu et al., Sensors & Diagnostics 2024 (reference below).
Why the P4SPR fits
| What the lab needs | What the P4SPR brings |
|---|---|
| Function, not just quantity | Direct, label-free measurement of whether serum blocks a receptor interaction |
| No live virus | Only the viral protein and its receptor; works in a BSL-2 lab |
| Fits in a biosafety cabinet | Small benchtop footprint, manual injection, no pump |
| Serum with little dilution | AffiCoat low-fouling surface: serum diluted only 1:5 |
| Controls in every run | Four channels, read in turn: duplicate sample, positive and negative controls on one sensor |
| Many samples per sensor | The surface is regenerated between samples, up to 10 samples per sensor |
Beyond SARS-CoV-2
Any virus or toxin that acts through a known receptor can be tested the same way: immobilize the viral protein, add serum, then the receptor. The assay suits vaccine development in animal models, where samples are few and a BSL-3 lab is not always at hand, and the screening of therapeutic antibodies for blocking activity. For the protein reagents themselves, see our note on checking the activity of protein reference materials.
What you need
| Item | Source |
|---|---|
| P4SPR | Affinité Instruments |
| AffiCoat Sensor (SAMAC-01) and Amine Coupling Kit (AMIN-KIT) | Affinité Instruments |
| Viral protein to immobilize, and its receptor (here: spike protein and ACE2) | Commercial or reference materials |
| Pre-pandemic or naive serum for the positive control | Commercial |
| Running buffer with BSA and surfactant; glycine for regeneration | Standard reagents |
Reference
- J. Coutu et al., "Large-scale validation of a plasmonic sensor for SARS-CoV-2 pseudo-neutralization with a cohort of food and retail workers," Sensors & Diagnostics, vol. 3, pp. 850–862, 2024. doi:10.1039/d3sd00333g (open access, CC BY 3.0).
Illustrations by Affinité Instruments. Figure 2 illustrates the assay design described in the reference above; it is not measured data.