When a person is exposed to a new virus

and after an incubation period

the immune system creates proteins called antibodies

as part of the body’s natural response to fight the infection and to neutralize the pathogen

Antibodies can come in different varieties known as isotypes or classes

IgM antibodies are produced against the viral antigen in the early stages of infection

and are detectable after four to seven days after symptoms develop

Following that, IgG antibodies are produced seven to 14 days after the onset of symptoms

and are detectable for months and even years

depending upon the antigen and the individual

These antibodies can also be detected if a person has an infection without any symptoms

The COVID-19 antibody test is used to detect the presence of the anti-SARS-CoV-2 IgM and IgG antibodies

in human whole blood, serum, or plasma samples

by using a cassette-based systems

To perform a test, the cassette is placed on a level surface at room temperature

then one of the fingers is punctured with a lancet

and a plastic dropper is used to take several drops of blood

then one drop is added into the cassette sample well

Finally, drops of a buffer are added into the sample well to facilitate the test

and the cassette is incubated for 10 minutes

The test Kit is a qualitative lateral flow immunochromatographic assay

and the test device contains

a sample pad, where the combined sample flows down

A conjugation pad, which contains Gold - Covid-19 antigen conjugate

the antigen will bind to the anti-SARS-CoV-2 antibodies in the sample

and two different antigens can be used

one with affinity to IgM and a second with affinity to IgG antibodies

The virus antigen is labelled with gold nanoparticles

which can be used as visual indicator

because of the nanoparticle’s interactions with visible light

So that, the Gold nanoparticles absorb and scatter light

resulting in a red color

In addition, rabbit IgG antibodies labeled with gold nanoparticles exist in the conjugation pad

and they are used for quality control to assess test validity

the test strip also contains a nitrocellulose membrane with three lines

first line coated with anti-human IgM antibodies

second line coated with anti-human IgG antibodies

and third line coated with Anti-Rabbit IgG Antibodies

In addition, an absorption pad is used in the test strip to move the sample across the test

The sample migrates laterally across the membrane by capillary action.

During testing, the antibodies in the sample

react with the SARS-CoV-2 antigen, and its conjugate gold nanoparticles

in the conjugation pad

The mixture then migrates laterally across the membrane by capillary action

and the IgM and IgG antibodies are bound

to the anti-human IgM and anti-human IgG antibodies, respectively

While, the rabbit IgG antibodies are bound to the Anti-Rabbit IgG Antibodies

Only those conjugated with COVID-19 antigen-gold nanoparticle complexes

will produce a visible colored line

The test result is positive

if both IgM and IgG antibodies are detected

if only IgM antibodies are present

and if only IgG antibodies are detected

When IgM is Positive and IgG is Negative

the subject’s immune system is actively producing antibodies

to a recent infection with the target virus

When Both IgM and IgG are Positive

the subject’s immune system is actively producing antibodies

to an ongoing infection that likely began more than 14 days ago

When IgG is Positive and IgM is Negative

the subject’s immune system has produced antibodies to the target viral antigen

A negative test result means that the person is not infected with the virus

or perhaps the person has a recent infection but the antibodies are not yet produced

Also, since the accuracy of the test depends on the amount of the antibodies

some people may not develop detectable antibodies after an infection

Because COVID-19 is a new disease

we don’t know exactly how long antibodies last

so a negative test could mean that the person has been infected in the past

and the antibodies are no longer detectable

The test result is invalid

if the quality control line C does not show a red color

therefore the sample must be retested