Rapid Plasma Reagin (RPR) Test


Syphilis is one of the most common sexually transmitted diseases (STDs) caused by the bacteria Treponema pallidum. It can be transmitted through sexual contact with an infected person or from a pregnant person to their baby. Syphilis usually develops in stages, each with different signs and symptoms that can last for weeks, months, or even years. In the early stages, syphilis may not cause noticeable symptoms, but it can still be contagious and cause serious complications if left untreated. Therefore, it is important to screen for and diagnose syphilis as soon as possible.

Syphilis tests are used to detect antibodies in the blood that are produced by the immune system in response to the infection. There are two types of syphilis tests: nontreponemal tests and treponemal tests. Nontreponemal tests measure nonspecific antibodies that react with lipids (fats) released by damaged cells. These antibodies are not specific to syphilis and may also be present in other conditions, such as autoimmune diseases, other infections, and vaccinations. Therefore, nontreponemal tests are mainly used for screening and monitoring the treatment of syphilis. Treponemal tests measure specific antibodies that react only with Treponema pallidum antigens. These antibodies remain in the blood for life, even after successful treatment of syphilis. Therefore, treponemal tests are mainly used for confirming the diagnosis of syphilis and ruling out false-positive results from nontreponemal tests.

The most common nontreponemal tests are Rapid Plasma Reagin (RPR) and Venereal Disease Research Laboratory (VDRL) tests. Both tests are based on the principle of flocculation, which is the formation of clumps when antibodies bind to antigens. The RPR test uses cardiolipin-coated charcoal particles as antigens, which can be seen as black clumps against a white background without a microscope . The VDRL test uses cardiolipin-lecithin-cholesterol antigen, which requires a microscope to observe the clumps . Both tests require a drop of blood or plasma to be mixed with the antigen on a card or slide and rotated for a few minutes. A positive result indicates the presence of reagin antibodies in the sample, while a negative result indicates their absence.

Both RPR and VDRL tests are simple, fast, and inexpensive methods for screening syphilis. However, they have some limitations and differences. For example:

  • RPR and VDRL tests may give false-negative results in early primary syphilis, late syphilis, or when there is a high concentration of antibodies that interfere with the reaction (prozone phenomenon) .
  • RPR and VDRL tests may give false-positive results in other conditions that cause tissue damage or inflammation, such as viral infections (e.g., hepatitis, HIV), autoimmune diseases (e.g., lupus), pregnancy, malignancies, etc. .
  • RPR and VDRL tests need to be confirmed by a treponemal test to establish a definitive diagnosis of syphilis .
  • RPR test is more sensitive than VDRL test for detecting syphilis.
  • RPR test does not require heating or fresh samples, while VDRL test does.
  • RPR test can be performed without a microscope, while VDRL test requires one.
  • VDRL test can be performed on cerebrospinal fluid (CSF) to diagnose neurosyphilis, while RPR test cannot .

In summary, RPR and VDRL tests are useful tools for screening and monitoring syphilis infection, but they need to be interpreted with caution and confirmed by a treponemal test. In addition, they should be performed along with a thorough medical history, physical examination, and risk assessment of the patient.