What this test measures
This quantitative immunoassay measures IgG antibody against the SARS-CoV-2 spike protein in BAU/mL (binding antibody units per millilitre) — a WHO international standard unit that allows comparison across labs and platforms.
The assay does not distinguish between antibody from past infection and antibody from vaccination, but it does give a numeric titre that can be followed over time to track waning or boosting of the antibody response.
Why it matters
For most healthy Indian adults, quantitative IgG is not needed in routine clinical care — antibody level does not perfectly correlate with clinical protection, and WHO/CDC/ICMR do not recommend antibody-guided vaccination decisions in the general population.
It has a more defined role in immunocompromised patients (transplant, lymphoma, biologics) where antibody response to vaccination is variable and a quantitative measure helps decide on additional doses or passive immunisation. It is also used for research, serosurveys (ICMR national serosurveys), and occasional pre-procedural or insurance assessments.
How to prepare
No fasting required. The test can be done at any time of day. Mention vaccination dates, any prior COVID-19 diagnosis, and any immunosuppressive medications.
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 |
|---|---|---|---|
| Anti-SARS-CoV-2 Spike IgG (BAU/mL)[1][2] | Reference cut-off varies by assay (typically > 7–10 BAU/mL = positive) | Below assay cut-off — minimal or no detectable IgG. Either never exposed, never vaccinated, waned response, or immunocompromised non-responder. Consider additional vaccine dose if eligible. | Above cut-off — IgG present. Higher titres often (but not always) correlate with greater protection. No universally agreed "protective" threshold for SARS-CoV-2. |
COVID IgG quantitative — interpretation context
| Scenario | Typical pattern | Action |
|---|---|---|
| Recently vaccinated (2–4 weeks) | High titre | Expected response |
| 6+ months after last dose | Lower titre | Booster eligibility per local policy |
| Immunocompromised post-vaccination | Variable, often low | Consider additional dose or monoclonal antibody |
| Post-infection, unvaccinated | Variable | Hybrid immunity if also vaccinated is strongest |
| No history of infection or vaccination | Below cut-off | Vaccinate |
Frequently asked questions
What is a "protective" antibody level for COVID-19?
There is no universally agreed protective threshold. Higher titres generally correlate with lower infection risk, but variants, T-cell immunity, and individual factors all contribute. Antibody-guided decisions are not recommended for healthy adults.
Why use BAU/mL instead of IU/mL?
BAU (Binding Antibody Units) is the WHO international standard unit for SARS-CoV-2 IgG, allowing comparison across assays and labs. It replaced earlier non-standard units that varied between manufacturers.
My titre dropped from 800 to 200 BAU/mL. Should I worry?
Some decline is expected over months. Immune memory cells often persist even when measurable antibody falls. Follow your vaccination schedule rather than chasing antibody levels.
I am on rituximab and my titre is very low. What now?
B-cell depleting agents impair antibody response. Discuss additional vaccine doses, monoclonal antibody prophylaxis, and updated boosters with your treating physician.
Can this test diagnose current COVID-19?
No — antibody testing is for past exposure or vaccine response. RT-PCR or rapid antigen tests diagnose current infection.
Is this test useful for long-COVID?
It can support a history of past infection but does not diagnose long-COVID. Symptom-based clinical evaluation is the standard.
Should children be tested?
Routine testing in children is not recommended. Decisions in immunocompromised children should be made with the paediatric team.
Related Infectious Disease tests
Tests commonly ordered alongside COVID ANTIBODY IGG (QUANTITATIVE), or that help interpret an unexpected result.
Sources & references
- WHO — International Standard for Anti-SARS-CoV-2 · accessed 2026-05-30T00:00:00.000Z
- CDC — COVID-19 Antibody Testing · accessed 2026-05-30T00:00:00.000Z
- ICMR India — COVID-19 Guidance · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — COVID-19 Antibody Test · accessed 2026-05-30T00:00:00.000Z
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