The SARS-CoV-2 Spike Antibody (Ab) test is a serology blood test that measures the immune system’s response to the virus’s spike protein. This protein is the target of all major COVID-19 vaccines and is also present on the surface of the virus. The test confirms an immune response following vaccination or prior infection. Understanding the specific numerical value and context of the lab report is essential for interpreting an individual’s immune status.
The Purpose of SARS-CoV-2 Spike Antibody Testing
The primary function of the Spike Antibody test is to confirm that an individual has mounted a humoral immune response to the virus’s spike protein. This response is generated either by receiving a COVID-19 vaccine or by having a past SARS-CoV-2 infection. The results provide a historical record of the body’s reaction to the spike antigen.
This test is distinct from diagnostic tests like the polymerase chain reaction (PCR) or antigen tests, which detect an active infection. A positive spike antibody result indicates a prior exposure event, not that a person is currently sick or contagious. The Spike Ab test specifically looks for antibodies targeting the spike protein, the component used in modern vaccines.
Another serology test measures antibodies against the nucleocapsid protein, which is only present after a natural infection. Since vaccines do not contain this protein, results help differentiate the source of immunity. A positive spike antibody result with a negative nucleocapsid result suggests the immune response came from vaccination alone. Conversely, a positive result for both indicates a past infection.
While documenting a prior infection is a secondary use, the more common reason for testing is to confirm a successful reaction to the vaccine regimen. These tests also help track the prevalence of immunity within a population for public health purposes.
Decoding Quantitative Results and Reporting Units
Spike antibody test results are often quantitative, reporting a specific number rather than simply “positive” or “negative.” This numerical value represents the concentration of circulating antibodies in the blood sample. The exact meaning of this number depends entirely on the reporting units and the laboratory’s specific assay.
The two most common reporting units are AU/mL (Arbitrary Units per milliliter) and BAU/mL (Binding Antibody Units per milliliter). AU/mL is manufacturer-specific, making direct comparison between different lab reports difficult. To harmonize results, the World Health Organization (WHO) established a standard leading to the use of BAU/mL.
Many labs now convert their AU/mL results into the WHO-standardized BAU/mL, which allows for a more consistent comparison of antibody levels across different studies and platforms. A lab report will also include a “cut-off” or “threshold” value, such as 33.8 BAU/mL, which determines if a result is considered positive or negative. Any result above this number confirms the presence of antibodies. A result just above the threshold, for example, 40 BAU/mL, confirms an immune response, but a very high result, perhaps 5,000 BAU/mL, indicates a much stronger humoral response. Many tests have an upper limit of quantification; if a result is reported as “>2500 AU/mL,” it means the antibody concentration exceeded the test’s measurable range. The laboratory’s report should always be consulted for the specific cut-off and unit conversion factor used for that particular assay.
Contextual Interpretation: Antibodies from Vaccination vs. Infection
Interpreting the numerical spike antibody level requires understanding the source of the immune response, as levels vary significantly based on vaccination, natural infection, or both. Post-vaccination, high antibody levels confirm a successful response to the vaccine doses received. Antibody levels are often highest shortly after the second dose or a booster, before declining over several months.
Individuals previously infected who then received a vaccine typically exhibit the strongest and most durable response, known as hybrid immunity. The antigen exposure from both the virus and the vaccine synergistically boosts the antibody concentration. This often results in levels several times higher than those seen after vaccination alone and provides a broader range of protection against variants.
In contrast, an immune response from natural infection alone can be highly variable. Some individuals produce very high levels, while others produce a weak response. For people who experienced mild or asymptomatic infection, the resulting antibody concentration may be considerably lower than that achieved through vaccination. A positive spike antibody test in an unvaccinated person confirms past infection but does not guarantee a high level of protection. The interpretation of a specific numerical value must be viewed through the lens of the patient’s full history of infection and vaccination.
What Spike Antibody Levels Do Not Tell You
While a high spike antibody level is reassuring, it is important to recognize the test’s limitations and what the numerical result cannot predict. The number provides a snapshot of humoral immunity at the moment the blood was drawn, but it does not determine the exact duration of protection. Antibody levels naturally wane over time, and a high reading today does not predict how long those antibodies will persist.
The test also fails to measure T-cell mediated immunity, which is another fundamental component of the body’s defense system. T-cells, or cellular immunity, are responsible for recognizing and destroying infected cells and play a significant role in preventing severe disease. Even as antibody levels drop, T-cell responses often remain robust and long-lasting, but the Spike Antibody test cannot quantify this protection.
A high antibody number does not equate to guaranteed immunity against all future variants of SARS-CoV-2. The test measures the ability of antibodies to bind to the spike protein, but this binding may be less effective against newer variants with multiple mutations. Protection is a complex function of both humoral (antibodies) and cellular (T-cell) immunity, and the quantitative antibody test provides only one piece of the immune picture.