SARS-CoV-2 IgG Antibody: Test & Levels

What Is the SARS-CoV-2 IgG Antibody Test?

I recall a patient, a 45-year-old teacher, who came to my clinic three weeks after recovering from a mild case of COVID-19. She wanted to know if she now had protection against reinfection. That’s where the SARS-CoV-2 IgG antibody test comes in. This blood test measures immunoglobulin G (IgG) antibodies specific to the SARS-CoV-2 virus, which causes COVID-19. IgG antibodies typically appear about two to three weeks after infection or vaccination and can persist for months, providing a memory of past exposure.

In clinical practice, this serology test helps distinguish between a past infection and a recent one when paired with IgM testing. It is also used to assess the immune response to COVID-19 vaccination. The LOINC standard code for this assay is 94534-5 (SARS-CoV-2 IgG Ab [Presence] in Serum or Plasma).

Why Is This Test Ordered?

Your healthcare provider may recommend the SARS-CoV-2 IgG antibody test for several reasons:

• To confirm a past COVID-19 infection, especially if you had symptoms but never tested positive with a PCR or antigen test.
• To evaluate the immune response after COVID-19 vaccination, particularly in immunocompromised individuals.
• As part of epidemiological studies to understand population immunity.
• To assist in diagnosing a rare complication of COVID-19, such as multisystem inflammatory syndrome (MIS-C or MIS-A).

Reference Ranges and Interpretation

Unlike many laboratory tests, the SARS-CoV-2 IgG antibody result is typically reported as positive, negative, or equivocal. However, quantitative assays provide a numerical value, often as a signal-to-cutoff (S/CO) ratio. Different manufacturers use different cutoff values, so always check your report for the lab-specific reference range. The table below shows a common interpretation framework used by many clinical laboratories.

Do Reference Ranges Differ by Age or Gender?

For the SARS-CoV-2 IgG antibody test, reference ranges do not differ significantly by age or gender. Children and adults typically generate similar antibody levels after infection or vaccination, though older adults and immunocompromised individuals may have a lower response. The test is qualitative or semi-quantitative, so age- and gender-specific cutoffs are not routinely used.

Limitations of the SARS-CoV-2 IgG Antibody Test

It’s crucial to understand what this test can and cannot tell us. In my clinic, I often emphasise that a positive IgG antibody result does not guarantee immunity from future infection. Antibody levels can wane over time, and protection also involves cellular immunity (T-cells), which this test does not measure.

• Not for acute diagnosis: IgG appears later than IgM and the virus itself. For early symptoms, a PCR or antigen test is appropriate.
• Cross-reactivity: Some assays may react with antibodies from other coronaviruses (e.g., common cold), causing false positives.
• Variability between assays: Results from different labs may not be directly comparable. Always use the same laboratory for serial monitoring.
• Waning antibodies: Negative results months after infection or vaccination do not necessarily mean you are susceptible; memory B-cells can rapidly produce antibodies upon re-exposure.

How to Prepare for the Test

No special preparation is needed. The test is a standard blood draw from a vein in your arm. You can eat and drink normally beforehand. Inform your doctor about any recent COVID-19 vaccinations or infections, as timing can affect interpretation.

Frequently Asked Questions

Can a positive SARS-CoV-2 IgG test tell me if I am immune?

Not definitively. While the presence of IgG antibodies indicates a prior immune response, the degree of protection depends on antibody levels and other immune components. The World Health Organisation notes that a positive result likely indicates some protection against severe disease, but breakthrough infections can still occur, especially with new variants.

How long do SARS-CoV-2 IgG antibodies last after infection or vaccination?

Studies show that IgG antibodies can persist for at least 6 to 12 months after infection, and often longer after vaccination. However, levels gradually decline. Booster doses are recommended to maintain protection, especially for vulnerable populations.

What does a negative IgG antibody result mean after vaccination?

A negative result after vaccination could indicate that you are a low responder, which is more common in immunocompromised individuals or older adults. It does not necessarily mean the vaccine failed, because T-cell immunity may still be present. Your doctor may consider a booster dose or alternative vaccine strategy.