SARS-CoV-2 spike (S) antibody
Measures antibodies to the virus's spike protein, which rise after COVID-19 vaccination or infection. No number is a proven sign of protection.
AI-verified against cited sources. Awaiting expert reviewAlso called Anti-S, Spike Ab, anti-SARS-CoV-2 SLOINC 94769-7
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What it is
SARS-CoV-2 is the virus that causes COVID-19. The spike protein sits on its surface, and both infection and COVID-19 vaccines teach your body to make antibodies against it. This blood test measures those spike antibodies and reports a number. At labs that use the Roche assay (Quest's listing names Roche), the unit is U/mL. The unit belongs to each assay, so numbers from different labs or makers can't be compared.
Why it matters
A positive result shows your immune system has responded to the spike protein, through vaccination, infection or both. The test can't tell which. The makers state the result should not be read as a sign of how immune or protected you are. No agreed antibody level means you are protected. The test is used mostly in research, or when a clinician has a specific question about whether someone's immune system has responded.
If it’s low
A result below 0.80 U/mL is reported as negative on the Roche-based assay. It means no spike antibodies were found at a level the test can detect. A negative result does not rule out a recent infection.
Common causes
- Never vaccinated or infected
- A long time since the last vaccine dose or infection
- Testing too soon after an infection
- A weaker antibody response because of a condition or medicine that affects the immune system
What can help
- Share your vaccination and infection history with your clinician
- Ask whether this test, or a different one, would answer your question
If it’s high
A higher number means more spike antibody was measured. On the Roche-based assay, 0.80 U/mL or more is reported as positive. The number depends on how long ago your last vaccine dose or infection was, and on the assay. A higher number has not been shown to mean you are better protected.
Common causes
- COVID-19 vaccination, especially a recent dose
- A past SARS-CoV-2 infection
- Both vaccination and infection
What can help
- Share your vaccination and infection history with your clinician
- Ask what, if anything, the result changes for you
What can affect your result
- Time since your last vaccine dose or infection
- Which assay and unit your lab uses
- Sample type: plasma in liquid-anticoagulant tubes, such as citrate, reads lower
- Immune-suppressing conditions or treatments
- Each assay has its own units and cutoffs (for example U/mL for Roche, AU/mL for Abbott), and the makers state results are not interchangeable, so only compare numbers from the same lab and method.
- Plasma collected in liquid-anticoagulant tubes, such as citrate, is diluted and can read lower than serum (Roche).
- Roche reports no biotin interference up to 1,200 ng/mL and no effect from the bilirubin, hemoglobin or lipid levels it tested.
Antibodies are generally detectable several days to a few weeks after an infection or a vaccine dose. Levels change over the following months.
When to talk to a clinician
This test does not show whether you are sick now. If you have symptoms such as fever, cough or shortness of breath, call a clinician about a test that detects the virus itself. For trouble breathing, chest pain or confusion, call 911 or your local emergency number.
Questions you could ask
- Why was this test ordered, and how will the result change what we do?
- What does this result mean for me, given my vaccination and infection history?
- Could my immune system or a medicine I take explain a low result?
- Would a repeat test, at the same lab, be useful?
Questions people ask
What number means I am protected?
No antibody number has been shown to predict protection. The makers of these tests state that results should not be read as an indication or degree of immunity. Your clinician can help you decide what a result means for you.
Why can't I compare my number with someone else's?
Each assay measures antibody against its own standard and reports its own units, such as U/mL or AU/mL. The makers state results from different assays are not interchangeable. Compare only results from the same lab and method.
Can this test tell me if I had COVID-19?
Not by itself, because vaccination also raises spike antibodies. Mayo suggests the nucleocapsid antibody test to look for prior infection in people who have been vaccinated.
Show the science and sources
For clinicians and researchers
The Roche Elecsys Anti-SARS-CoV-2 S assay is an ECLIA double-antigen sandwich that detects antibody (mostly IgG, also IgA and IgM) to the spike protein receptor-binding domain, authorized under an FDA EUA for qualitative and semi-quantitative use. Its measuring interval is 0.40 to 250 U/mL (up to 2,500 U/mL after a 10-fold dilution). Below 0.80 U/mL is negative and 0.80 U/mL or above is positive. The U/mL unit is defined against an internal Roche standard (1 nM of an equimolar mix of two monoclonal antibodies equals 20 U/mL), the IFU states no international standard was available then, and the unit must not be used interchangeably with units of other assays. Abbott's AdviseDx SARS-CoV-2 IgG II reports AU/mL with a cutoff of 50.0 AU/mL and an interval of 22.0 to 25,000 AU/mL. Quest 39820 lists the same 0.8 U/mL cutoff and names Roche in its aliases. Mayo COVSQ is also an ECLIA semi-quantitative spike antibody test, but the Mayo catalog mirror does not show its reference interval. Both infection and spike-based vaccines raise this antibody, so it can't separate them; Mayo recommends the nucleocapsid antibody test to confirm prior infection in vaccinated people. The labeling states the results should not be interpreted as an indication or degree of immunity or protection, and that the clinical significance of positive or negative results after vaccination has not been established.
LOINC 94769-7 (primary), 94505-5, 99597-7, 94661-6. Specimen: serum.
Sources
- Elecsys Anti-SARS-CoV-2 S Instructions for Use (EUA), Roche Diagnostics (hosted by FDA) (2022)
- AdviseDx SARS-CoV-2 IgG II Instructions for Use (ARCHITECT), Abbott Laboratories (hosted by FDA) (2022)
- SARS-CoV-2 Total Antibody, Spike, Semi-Quantitative (test code 39820), Quest Diagnostics
- SARS-CoV-2 Spike Antibody, Semi-Quantitative, Serum (COVSQ), Mayo Clinic Laboratories (via testcatalog.org)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.