SARS-CoV-2 nucleocapsid (N) antibody
Checks for antibodies to a protein inside the COVID-19 virus. A positive result can be a sign of a past infection.
AI-verified against cited sources. Awaiting expert reviewAlso called N antibody, Anti-N, SARS-CoV-2 N AbLOINC 94762-2
What it is
SARS-CoV-2 is the virus that causes COVID-19. After an infection, your body makes antibodies against several parts of the virus, including the nucleocapsid (N) protein that packages its genetic material. Vaccines that teach the body to recognize only the spike protein do not cause N antibodies. This blood test looks for those N antibodies. Labs report the result as positive or negative, and some also print a number called an index.
Why it matters
A positive result suggests your immune system met the virus at some point, even if you never had symptoms or a test at the time. It can't tell you when the infection happened, and it is not a measure of how protected you are. A negative result does not rule out a past infection, because antibody levels can be too low to detect. Clinicians rarely need this test for treatment decisions. It is used most for research, for surveillance, or when someone wants to know about a past infection.
If it’s low
A negative result means no N antibodies were found. That fits with never having been infected, but it does not prove it. Some people never make detectable N antibodies, and levels may fade over time. A negative result also doesn't show whether you were vaccinated, because spike-based vaccines don't cause N antibodies.
Common causes
- No past SARS-CoV-2 infection
- Testing too soon after an infection, before antibodies appear
- Antibody levels that were low or have fallen below what the test can detect
What can help
- Remember that vaccination alone is not expected to make this test positive
- Ask your clinician whether a different test would answer your question
If it’s high
A positive result means antibodies to the N protein were found. This can be a sign of a past SARS-CoV-2 infection, including one with few or no symptoms. Rarely, the result can be a false positive. The makers of these tests note that a positive result may not mean a past infection, so your clinician may use a second, different test to check.
Common causes
- A past SARS-CoV-2 infection, including a mild one or one with no symptoms
- A recent infection that is still resolving
- Rarely, a false-positive result from cross-reacting antibodies
What can help
- Share your vaccination and infection history with your clinician
- Ask whether any follow-up testing would change your care
What can affect your result
- Time since infection: antibodies take about two weeks or more to appear
- Which assay the lab uses (total antibody or IgG, and each maker's cutoff)
- COVID-19 vaccination with spike-only vaccines does not make this test positive
- Your immune system's health
- Assays differ (total antibody or IgG, and each maker's own cutoff), so a result from one lab can't be compared directly with another lab's.
- Samples drawn within about two weeks of symptom onset may be negative because antibodies haven't risen yet; Roche's labeling says to test people 14 or more days after symptoms began.
- Grossly hemolyzed, grossly icteric or very fatty (lipemic) samples may be rejected (ARUP).
Antibodies usually take a couple of weeks to become detectable after an infection. How long they last is not well established.
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 be used?
- Does this result change anything about my care or vaccine plans?
- Would any follow-up testing be useful?
- Would a different kind of test answer the question I have?
Questions people ask
Can this test show whether my COVID-19 vaccine worked?
No. Spike-based vaccines are not expected to make N antibodies, so a negative result is expected after vaccination alone. ARUP notes there are no current recommendations for assessing vaccine response. A spike antibody test responds to vaccination and to infection.
Does a positive result mean I am immune?
No. The test makers state it is unknown whether antibodies protect against catching the virus again, and the result should not be read as a measure of protection.
What is the index number next to my result?
Some labs print a cutoff index or ratio. On the Roche assay 1.0 or above is reactive, and on the Abbott ARCHITECT assay 1.40 or above is positive. A bigger number does not mean more antibody, and numbers from different makers can't be compared.
Show the science and sources
For clinicians and researchers
These are qualitative immunoassays that detect antibody to a recombinant nucleocapsid antigen. Examples with published labeling: Roche Elecsys Anti-SARS-CoV-2 (ECLIA, N antigen, reports a cutoff index with COI below 1.0 non-reactive and 1.0 or above reactive), Abbott ARCHITECT SARS-CoV-2 IgG (N-directed IgG, index S/C with a cutoff of 1.40), ARUP COVID-19 IgG Qualitative by CIA (N-directed IgG, FDA EUA) and Mayo COVTA (total antibody by CIA). The size of the index above the cutoff does not reflect the amount of antibody, and results from different makers should not be used interchangeably. Roche labeling says to test people 14 or more days after symptom onset, that a positive result may not indicate previous infection, and that a second, different serology test can be considered. The assays were authorized under an FDA Emergency Use Authorization, and the performance of the Roche test was not established in people who had received a COVID-19 vaccine. ARUP states that N antibodies do not develop as a result of COVID-19 vaccination and that there are no current recommendations for assessing vaccine response. Mayo notes that its spike antibody test responds to both infection and vaccination, and recommends the N antibody test to confirm prior infection when someone has been vaccinated.
LOINC 94762-2 (primary), 94563-4, 99596-9. Specimen: serum.
Sources
- Elecsys Anti-SARS-CoV-2 Instructions for Use (EUA), Roche Diagnostics (hosted by FDA) (2021)
- SARS-CoV-2 IgG for ARCHITECT Instructions for Use (EUA), Abbott Laboratories (hosted by FDA) (2022)
- COVID-19 IgG, Qualitative by CIA (test 3002776), ARUP Laboratories
- SARS-CoV-2 Nucleocapsid, Total Antibody, Serum (COVTA), Mayo Clinic Laboratories (via testcatalog.org)
- 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.