So how do you make the laboratory diagnosis of syphilis?
Well, that’s very stage-dependent,
and you use a combination of clinical and lab criteria.
So to make a microbiologic diagnosis,
you can actually directly detect Treponema pallidum in clinical material
by either darkfield microscopy
or by immunohistochemical stains of tissue specimens
showing you special stains shown in this tissue sample.
More commonly, however, we rely upon serologic tests.
And the serologic tests are either treponemal –
in other words, they’re derived from treponemal antigens
or they are non-treponemal.
Let’s talk about these treponemal and non-treponemal tests.
First, start with the non-treponemal tests.
The non-treponemal tests use an antigen
that is derived in part from cow heart and other ingredients.
People with syphilis apparently have antibodies to these features.
And so, we test that with either of the RPR or the TRUST test.
Essentially, we attach that antigen to little particles.
They can be red particles or black particles.
The black particles are the RPR.
The red particles are in the TRUST test.
And so, we attach that antigen to those particles,
and then we expose those particles with antigen on them to the patient’s serum.
And if the patient has syphilis,
they will cause those particles to agglutinate
and they can be read with the naked eye.
I think you can see in the figure here there are some particles
that have precipitated and you can dilute them out –
dilute the patient’s serum way out
before the particles are no longer able to clump.
So they are read grossly for what we call macroflocculation.
The other test that is now essentially only used for the cerebrospinal fluid –
used to be used for screening.
But it’s laborious
because it has to be done with a microscope looking for flocculation.
And because you have to use a microscope,
you’ve got to heat and activate the serum.
This is a problem for screening a lot of individuals.
That’s why we no longer really use the VDRL as a screening technique.
The one problem with the serologic test is
that there are some other conditions which will give you a false-positive test,
and that can be either transient or chronic.
If you’re talking about transient false-positives,
you’re talking about diseases
that have lots of antibodies associated
like malaria, brucellosis, or infectious mononucleosis.
A chronic false-positive test for syphilis may be seen
in various connective tissue diseases,
for example, lupus or in HIV infection
where there’s lots of immunoglobulins circulating.
Patients who have IV drug use frequently
have lots of antibodies
because of all the particles they also inject,
which produce a lot of antibodies.
Leprosy and hepatitis C may result in
chronic biologic false-positive test for syphilis.
Then we turn to the treponemal test,
which use actually antigens derived from Treponema pallidum.
We used to use the FTA absorption test,
the fluorescent treponemal antibody test,
but this is no longer considered the gold standard.
What most laboratories now use is something called the
Treponema pallidum particle agglutination test, or TP-PA test.
And these are gelatin particles
that contain the actual antigens of Treponema pallidum.
And they will cause the gelatin particles with the antigen to clump
when exposed to patient’s serum who have syphilis.
This is read manually.
And now, supplanting that, but not completely,
is the so-called enzyme immunoassay test, or ELISA some people refer to it as,
and these are microtiter wells.
Here we’re showing a microbiologist employing an ELISA test.
This is fully automatable.
And now, it’s often done first in many laboratories
instead of the Treponema pallidum particle agglutination test.
The discussion of the serologic diagnosis of early syphilis,
latent syphilis, either early-latent or late latent congenital syphilis,
and other uses of the treponemal and non-treponemal test
are really beyond the scope of this discussion.
We are going to provide you with some material
that will go through all of these,
whether to establish a definite or probable diagnosis serologically.
But I think it’s preferable to have that available for you
that you can download.
But it’s not probably high-yield for examinations
and I prefer to move on to therapy at this point in time.