What this test measures
A neutralising antibody (NAb) assay measures antibody that can functionally block the SARS-CoV-2 spike protein from binding to its cellular receptor (ACE2). Most modern Indian labs offer a surrogate Virus Neutralisation Test (sVNT) — a competitive ELISA that quantifies the percentage of inhibition without using live virus.
The result is typically reported as % inhibition, with values above 30% indicating detectable neutralising activity. Unlike total or IgG antibody tests, NAb measures functional protection rather than just binding antibody.
Why it matters
NAb is the closest laboratory correlate of clinical protection against COVID-19. Higher NAb levels are associated with lower risk of symptomatic infection in vaccine trials and observational studies.
In current Indian practice, NAb is used in immunocompromised patients to decide on additional vaccine doses or monoclonal antibody prophylaxis, in research and clinical trials of vaccines and therapeutics, and in select clinical scenarios where functional immunity matters. Routine NAb testing in healthy adults is not recommended — protection depends on antibody, T-cells, memory B-cells, and other factors not captured by NAb alone.
How to prepare
No fasting required. The test can be done at any time of day. Mention vaccination history, prior COVID-19, and any immunosuppressive treatment.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| SARS-CoV-2 Neutralising Antibody (% inhibition or IU/mL)[1][2] | Cut-off varies by assay (typically > 30% inhibition = positive) | Below cut-off — minimal or no detectable neutralising activity. Possible reasons: never exposed, never vaccinated, antibody waned, immunocompromised non-responder, or variant escape. | Above cut-off — functional neutralising antibody present. Higher values correlate with greater protection against the targeted SARS-CoV-2 spike (note: most assays use the original spike, not current variants). |
COVID antibody tests — function vs binding
| Test | What it measures | Correlation with protection |
|---|---|---|
| Total Antibody / IgG (qualitative) | Binding antibody, yes/no | Indirect, variable |
| IgG Quantitative (BAU/mL) | Binding antibody titre | Moderate correlation |
| Neutralising Antibody (sVNT) | Functional protection | Strongest known correlate |
| Live virus neutralisation (PRNT) | Gold-standard functional test | Reference standard — research only |
| RT-PCR | Active virus | N/A (diagnostic, not protective) |
Frequently asked questions
What does "neutralising" mean?
A neutralising antibody binds the virus and blocks it from entering cells. This is the functional protection mechanism, distinct from antibody binding alone.
Should I get this test before a booster?
WHO, CDC, and ICMR do not recommend NAb-guided booster decisions in healthy adults. Vaccination schedules remain population-based.
Will NAb against the original spike protect me from current variants?
Possibly less so. Most assays measure NAb against the original (Wuhan) spike. Variants like Omicron sublineages can partially escape. Specific variant-spike NAb assays exist but are research-only.
I am on immunosuppression. Is NAb useful?
Yes — for some immunocompromised patients (transplant recipients, lymphoma on rituximab), NAb informs decisions about additional vaccine doses or passive immunisation. Discuss with your treating physician.
How does NAb correlate with infection risk?
Higher NAb levels reduce, but do not eliminate, infection risk. T-cell immunity and memory B-cells also matter.
How long does NAb last?
Months to a year after natural infection or vaccination. Levels decline more rapidly than total binding antibody.
Is NAb a yes/no result or a number?
Most sVNT assays report % inhibition. Some provide IU/mL using WHO international standards. Either way, the trend over time matters more than a single number.
Related Infectious Disease tests
Tests commonly ordered alongside COVID NEUTRALIZING ANTIBODY, or that help interpret an unexpected result.
Sources & references
- WHO — Neutralising Antibody International Standard · accessed 2026-05-30T00:00:00.000Z
- CDC — COVID-19 Antibody Testing · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — COVID-19 Antibody Test · accessed 2026-05-30T00:00:00.000Z
- NIV Pune — COVID-19 Neutralisation Studies · accessed 2026-05-30T00:00:00.000Z
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