The CoviD-19 Saga: What are Antibodies detected by Rapid Tests?



RT-PCR is the gold standard test as it detects ongoing CoviD-19 Infection. A positive RT-PCR means an active infection at the time of testing. A negative RT-PCR means CoviD-19 infection was not detected at the time of testing. If a patient was previously positive in the RT-PCR test and subsequently tested negative (generally after days of isolation or treatment) the patient may be considered as recovered from CoviD-19. Sometimes two negative tests, days apart, are needed to be certain.

Rapid Antibody Tests do not detect CoviD-19, instead; it detects antibodies instead. Antibodies generally appear a certain time after an infection. Rapid Antibody Tests may have False Negative and False Positive Results. False Negative results happen when no Antibodies are detected even when a person has ongoing CoviD-19 infection, most especially if the infection is still in its early stages. False positive results happen when Antibodies for a different type of coronavirus (or any infection) were detected and resulted in a positive result even if the person does not have CoviD-19.

Rapid Antibody Tests are used in circumstances where there’s limited availability of RT-PCR and there is a need to be tested immediately such as in emergency situations, contact testing and mass testing in communities suspected of CoviD-19. Rapid Antibody Tests may also be used to know (with varying accuracy) if a person was previously infected with CoviD-19 but was never tested with RT-PCR, since the person will most likely no longer get a positive RT-PCR result if he has recovered.


Rapid Antibody Test kits detect Antibodies.

Antibodies also known as immunoglobulins (Ig) are Y-shaped disease-fighting proteins produced by the immune system, specifically by immune cells called plasma cells (aka B cells) to neutralize pathogens such as viruses, bacteria, parasites and other disease causing substances. The first time a person is infected or otherwise exposed to a foreign substance (known as an antigen), their immune system recognizes the microorganism or substance as “non-self” and stimulates plasma cells to produce specific antibodies that can bind to that antigen and neutralize the threat.

Antibodies are formed in the days and weeks after infection, may protect a person from re-infection, at least for a time, although it has not been proven whether that holds true for CoviD-19. Do note that antibodies differ between different animals, and are divided into Isotypes or classes. Placental mammals including us humans are known to have 5 antibody classes, and further divided into subclasses (IgA having 2, IgG having 4)

A general description of each antibody we have are as follows:

IgA is an antibody that plays a crucial role in the immune function of mucous membranes which line our gastrointestinal and respiratory system, and also line our body openings i.e. eyes, ears, nose, mouth, throat, vagina, urethra and anus. It is also the antibody found in breast milk (which is responsible for the natural passive immunity in infants while their immune system develops)

IgD is an antibody that serves as a receptor in the formation of B cells. Its definite role is not completely understood and it’s rarely measured.

IgE is an antibody that binds to allergens and triggers histamine release. It’s the main antibody involved in allergies. It is also the main antibody involved against parasitic infections.

IgG is the most common / most abundant type of antibody in the body’s circulation. It is also the only antibody capable of crossing the placenta to give passive immunity to the fetus in pregnant mothers. The body retains a catalog of IgG antibodies that can be rapidly reproduced whenever exposed to the same antigen – they form the basis of long term immunity.

IgM is the largest antibody and is the first to appear in response to an infection. IgM antibodies are produced as the body’s first response to a new infection or to a new “non-self” antigen, providing short-term protection. They may increase for several days to weeks and then decline as IgG production begins.

There are two types of Immunity. One is humoral / antibody mediated immunity which is responsible for utilizing B cells and antibodies. The other is cell mediated immunity which involves another type of immune cell, the Killer T cells, but we won’t elaborate these as we are just generally introducing them to give light to how Antibody Tests work.

The two types of antibodies detected in testing are immunoglobulin M (IgM) and immunoglobulin G (IgG).

IgM may show up anywhere from 5 to 10 days from when you first get infected. Then, there is immunoglobulin G, or IgG, which may appear within 10 to 14 days. There are tests which distinguish IgM from IgG which can give information on which phase the infection is currently (Early stage or Later stage, or even Previous infection which designates that the person has recovered – but this status arguably still needs the RT-PCR test kit to be certain)

The test typically looks like this:

"Antibody test kit". by Reuters
“Antibody test kit”. by Reuters

and results of the tests may look like this:

"Test Results" by Biomedomics
“Test Results” by Biomedomics

A total of three detection lines are possible, with the control (C) line appearing when sample has been flowed through the cassette.

Negative Result: If only the quality control line (C) appears and the detection lines G and M are not visible, then no novel coronavirus antibody has been detected and the result is negative.

Positive Result, M only: If both the quality control line (C) and the detection line M appears, then the novel coronavirus IgM antibody has been detected and the result is positive for the IgM antibody, This may suggest the patient is in the early stages of infection.

Positive Result, G only: If both the quality control line (C) and the detection line G appears, then the novel coronavirus IgG antibody has been detected and the result is positive for the IgG antibody. This may suggest that the patient may be in the late stage or has recovered.

Positive Result, G and M: If the quality control line (C) and both detection lines G and M appear, then the novel coronavirus IgG and IgM antibodies have been detected and the result is positive for both the IgG and IgM antibodies. This may suggest that the patient may be in the middle

An IgM positive test may suggest the infection is in its early phase, a positive IgG in its late phase or even recovered, and an IgM and IgG positive test may suggest an ongoing infection but note that there is yet to be a confirmatory test in detecting the exact phase of infection. Again emphasis must be noted that Rapid Antibody Tests can be used to screen patients suspected of having been affected by CoviD-19, However, results of test should not be the only basis for diagnosis as it cannot confirm the presence of CoviD-19. Results should be used in combination with clinical observations and other testing methods such as RT-PCR as Rapid Tests are known to provide False Negative and False Positive results.



  1. […] He cited the case of four OFWs who are among the 28 accommodated in a quarantine facility here who tested positive in a rapid test.  The four are made to undergo confirmatory RT-PCR tests.  Rapid tests detect antibodies and results are inconclusive, unlike the RT-PCR which detects the virus’ genetic material. (READ also The CoviD-19 Saga: What are Antibodies detected by Rapid Tests?) […]

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