The SARS-CoV-2 Immunoglobulin G (IgG) antibody test is a blood test used to determine if an individual has developed an immune response to the virus that causes COVID-19. This test specifically looks for IgG antibodies, markers of past infection or vaccination. The primary goal of an IgG test is not to diagnose an active, current infection; instead, it provides retrospective information about a person’s immunological history. This serology test is distinct from diagnostic tests like PCR or rapid antigen tests, which detect the presence of the virus itself. The results offer insight into past exposure, informing public health surveillance and personal health decisions.
The Specificity of IgG Antibodies
Immunoglobulin G (IgG) is the most abundant type of antibody found in the body’s circulation and plays a significant role in long-term immune memory. When the immune system first encounters a new pathogen, it generates a rapid response, but IgG antibodies are part of the delayed, more targeted “secondary immune response.” These proteins are produced by B cells and circulate in the blood, recognizing and neutralizing the SARS-CoV-2 virus if it is encountered again.
The presence of IgG is a biological record of prior exposure to the virus’s specific antigens, whether through natural infection or vaccination. Because IgG production requires time for the immune system to mature its response, these antibodies typically become detectable in the blood approximately 10 to 14 days after the onset of symptoms or vaccination. This delay is why the test is ineffective for diagnosing an acute infection. IgG molecules also have a long half-life, meaning they persist in the bloodstream for extended periods, providing prolonged immunity.
Decoding Positive, Negative, and Equivocal Results
The interpretation of a SARS-CoV-2 IgG test result falls into one of three categories, each signaling a distinct immune status. A Positive or Reactive result means detectable levels of IgG antibodies against SARS-CoV-2 were found in the blood sample. This outcome confirms that the person has previously been infected with the virus or has successfully responded to a vaccine. The test alone cannot differentiate between infection-induced and vaccine-induced antibodies, though specialized tests targeting different viral proteins can offer clues.
A Negative or Non-Reactive result indicates that the test did not detect IgG antibodies above the assay’s cutoff threshold. This suggests the person has not been previously infected with SARS-CoV-2 and has not developed an immune response from vaccination. However, a negative result can occur if the test was performed too early, before the 10-to-14-day window required for IgG to develop, resulting in a false negative. Individuals with compromised immune systems may also test negative despite prior exposure because they mounted a weak antibody response.
An Equivocal or Indeterminate result means the antibody level falls close to the test’s positive-negative threshold, making a definitive call impossible. These borderline results can arise from a very early or waning immune response, or sometimes from cross-reactivity with antibodies produced against other related coronaviruses, such as those that cause the common cold. A healthcare provider may recommend repeating the test after one to two weeks to see if the antibody level has clarified. For some assays, the result may also be reported as a quantitative value, such as a concentration in International Units per milliliter (IU/mL), which requires interpretation against the laboratory’s specific reference range.
Factors Influencing Test Accuracy
Several factors can affect the reliability of SARS-CoV-2 IgG testing, so results must be interpreted within a clinical context. One concern is the possibility of a false negative, where the test fails to detect antibodies despite a prior infection. This may happen if the individual was tested too soon after exposure or if they had a mild infection that produced a very low level of antibodies. The waning of antibody levels over many months can also cause a previously positive person to eventually test negative.
Conversely, a false positive result occurs when the test incorrectly indicates the presence of SARS-CoV-2 IgG antibodies. While less common with modern, highly specific assays, this can be caused by cross-reactivity, where antibodies created in response to other non-SARS-CoV-2 infections mistakenly bind to the test components. Other non-specific interferences, such as rheumatoid factor or certain autoimmune conditions, can also lead to a false positive reading. Test kits are either qualitative (providing a simple positive/negative answer) or quantitative (giving a numerical value that measures the concentration of antibodies), which influences accuracy and interpretation.
What Antibody Presence Means for Future Protection
The presence of SARS-CoV-2 IgG antibodies indicates that the immune system has established a memory of the virus, but it does not guarantee protection against future infection. The antibodies are a measure of the body’s humoral response; they function to neutralize the virus, preventing it from attaching to and entering host cells. While a positive result suggests a degree of protection, the exact level of antibody required to achieve full immunity is not precisely defined and varies among individuals.
The durability of this protection is a subject of ongoing research, as antibody levels are known to wane over time following both infection and vaccination. For those with natural immunity, protection against reinfection generally diminishes over several months, though protection against severe disease tends to remain strong for a longer period. The emergence of new variants can challenge the protective capacity of existing antibodies, as the immune memory may be less effective against a mutated version of the virus. A positive IgG result serves as confirmation of prior immune engagement, but it should not be mistaken for a lifelong immunity certificate.