With a severe sore throat, a rapid strep test is commonly performed to evaluate for a throat
or tonsil infection caused by a bacteria known as group A streptococcus.
A rapid strep test is performed by first swabbing the throat and running the test on the swab
using a kit made specifically for this purpose.
To start, 4 drops of Reagant A containing sodium nitrite is placed into the extraction
tube.
4 drops of Reagant B containing acetic acid follows.
The two smaller bottles included in every kit are used only for quality control purposes
and not used for actual testing.
The solution is then mixed.
The swab specimen is then placed into the tube and swirled at least 10 times.
The swab should remain in the tube for 1 minute after which it is removed while squeezing
the swab head to remove as much liquid as possible.
A strep test strip is then removed and immediately placed into the extraction tube for 5 seconds.
The strip is then removed and laid flat for 10 minutes.
Here is a 10 minute rapid time lapse showing a positive strep test result.
The red arrow points to the control to ensure the test is valid.
If this red band is absent, any test results would be considered erroneous and needs to
be repeated.
The blue arrow points to a second band that should appear if strep is present.
As such, if TWO bands are present, the patient has strep and should be treated accordingly.
Here is a 10 minute rapid time lapse showing a negative strep test result.
Note that the red arrow points to the presence of a control band indicating a valid test.
Given the absence of a second band, this test would be considered negative and the patient
would not be considered to have strep throat.
So how does this test work?
It uses a testing technique called lateral flow assay.
In essence, this test runs the liquid sample along the surface of a pad with reactive molecules
that show a visual positive or negative result.
The pads are based on a series of capillary beds which has the capability of transporting
fluid spontaneously via capillary action like
a sponge.
In the rapid strep test, the test pad has been pre-coated with rabbit anti-Strep A antibody
in the test band region and goat anti-rabbit antibody in the control band region.
When the test strip is immersed into the specimen solution, it gets absorbed into the test strip
pad where it mixes with the antibody shown here as white letter Y.
The antibody is attached to a pigment shown here as a yellow circle.
If the specimen contains strep depicted here as a purple star, the antibody binds to the
strep and drags it across the test strip via capillary action.
When the antibody-pigment-strep complex encounters the rabbit anti-strep A antibody immobilized
in the test region, it produces a colored test band and indicates a positive result.
To serve as a test control, a colored band will also appear in the control region if
the test has been properly performed which encompasses any free antibody-pigment complex
not captured in the test region.
Obviously, there is many more antibody-pigment complexes so even if strep is present, there
will always be unattached antibody-pigment complexes to allow for a positive control
band.