Rapid tests for syphilis

Non-treponemal tests

Syphilitic infection leads to the production of nonspecific antibodies that react to cardiolipin. This reaction is the foundation of “non-treponemal” assays such as the VDRL (Venereal Disease Research Laboratory) test and Rapid Plasma Reagin (RPR) test. Both these test are flocculation type tests that use an antigen-antibody interaction. The complexes remain suspended and therefore visible due to the lipid based antigens. With the VDRL a microscope is required while a positive with the RPR can be seen with the naked eye. All non-treponemal tests measure immunoglobulins G (IgG) and M (IgM) anti-lipid antibodies formed by the host in response both to lipoidal material released from damaged host cells early in infection and to lipid from the cell surfaces of the treponeme itself.

These non-treponemal tests are widely used for qualitative syphilis screening. However, their usefulness is limited by decreased sensitivity in early primary syphilis and during late syphilis, when a large number of untreated patients will be negative by these methods.

After adequate treatment of syphilis, non-treponemal tests eventually become non-reactive. However, even with sufficient treatment, patients sometimes have a persistent low-level positive non-treponemal test.

With non-treponemal tests, false-positive reactions can occur for a large number of reasons, the most common of which is other infections, both viral and bacterial. Additionally these tests may show false-negative when the patient’s antibody titer is very high due to a hook effect (also called a prozone effect). Because of the issues with false positives, confirmation with a second treponemal test that is specific for T. pallidum antibodies is recommended.

Non-treponemal tests are relatively simple to perform and interpret, can allow rapid return of results and are very cheap. However, they still require some laboratory equipment (especially the VDRL) and trained personnel to perform and interpret test reactions.

Non-treponemal test types

VDRL

The VDRL  type test was invented before World War I, with its first iteration being that developed by August von Wasserman in 1906. The test, as it is largely still done today, was developed in 1946 by Harris, Rosenberg, and Riedel. The VDRL assay detects an antibody against cardiolipin, a cellular membrane component that is released by cells when they are damaged by T. pallidum or in some other medical conditions. The VDRL, like the RPR, can also give false positive results for a variety of reasons (see below under RPR).

The VDRL test may be done on a serum sample or cerebro-spinal fluid (CSF), however serum samples must be heat inactivated at 56 degrees C for 30 minutes to destroy any compliment. CSF does not need to be heat inactivated. It is the only non-treponemal test that can be used on CSF (To check for possible neurosyphilis). The VDRL test is not very useful for detecting syphilis in very early or advanced stages.

The VDRL test is somewhat equipment intensive over newer tests, including the RPR. It requires a low power microscope to read the results as well as all the materials necessary to process the patient sample especially a centrifuge and a stable hot-water bath. Also to be noted is that there are many platforms and iterations of the VDRL assay, mostly due to the extremely long life span of the test methodology.

RPR

Rapid Plasma Reagin (RPR) is basically an improved version of the VDRL test and uses the same antigen. A large number of iteration of this test are available but one of the most common, easiest and cheapest are those run on a small test card.

In addition to screening for syphilis, once RPR reactivity has been qualitatively established, a quantitative titer can be used to indicate the stage of infection and its response to therapy.  The RPR test can be done on unheated plasma or serum, but is not recommended for use on CSF.

The term "reagin" refers to the anti-cardiolipin antibodies. In the test, a patient’s serum (or plasma) is mixed with a stabilized VDRL cardiolipin antigen suspension that has cholesterol, lecithin, and carbon added in. The antigen is not attached to these particles, but the carbon is trapped in the lattice formed by the antigen-antibody complex that forms when reactive serum is added. The antigen is lipoidal in nature and because of this the antigen-antibody reaction remains suspended, as a visible flocculation.

The RPR test is an effective screening test. However, as with all assays, it has drawbacks.  False positives have been reported in test that use the VDRL antigen for a number of medical conditions such as: viral infections (Epstein-Barr virus, HIV, hepatitis, measles, vaccinia, varicella), lymphoma, tuberculosis, Lyme Disease, leprosy, malaria, endocarditis, lupus erythematosus, pregnancy, intravenous drug abuse. As with the VDRL assay, early syphilis infections may give a false negative result.

Due to the limitations (specifically false positives) these two screening tests should always be followed up by a more specific treponemal test. Unfortunately even in these tests the occasional false positives can still occur.

Run time
approx. 10-15 minutes.

Requires:

• a centrifuge to spin blood tubes for serum,
• A rotator*
• A refrigerator
• pipettes for quantitative/titer testing,
• In the VDRL- a Light Microscope

* These may be optional or even unnecessary depending on the test protocol and format.