Rapid Plasma Reagin (RPR) test for diagonsis of syphilis

The Rapid Plasma Reagin Test abbreviated RPR  is a non-treponemal macroscopic flocculation test for the serological screening of syphilis.It detects the presence of reagin antibody in the serum of a patent suffering from syphilis.During RPR test method, reagins form visible flocculants upon reacting with the carbon-containing RPR antigen.Rpr test for syphilis has replaced the Venereal Disease Research Laboratory (VDRL) test, as a rapid screening test due to  the following reasons

• There is no need for daily preparation of reagents.
• No microscope is required.
• Heat inactivation of serum is not required

The test uses the VDRL antigen modified with choline chloride to inactivate complement, and charcoal particles to allow the results of the reaction to be read without the use of a  microscope.

It is a reliable, economical, reproducible and rapid test which is easy to read without the help of a microscope. The specificity and sensitivity of the test are similar to those of VDRL test, the sensitivity being 85-90% in primary disease, 100% in secondary, 90-95% in latent disease and 75% in late or tertiary stage.

Principle of Rapid Plasma Reagin Test

RPR antigen suspension is a carbon coated non-treponemal cardiolipin antigen, which detects reaginic antibodies present in serum/plasma of patients suffering from treponemal infections.

These antibodies may be occasionally found in sera of persons with other non treponemal conditions like rheumatoid arthritis, tuberculosis, Kala-azar, 132 etc.

There are two types of antibodies  that are produced in response to treponema pallidum infection which are

• the host forms specific anti-treponemal antibodies to Treponema pallidum
• The anti-lipid antibodies in response to the lipoidal material released from the infected host cell which are referred to as reagins.

When a specimen contains  the anti-lipid antibodies, flocculation occurs due to co-agglutination of the carbon particles of the RPR antigen with the antibodies, which appear as black clumps against the white background of the card, which is read with the naked eye.

The non-reactive specimens show an even light grey colour. This test does not look for antibodies against the actual bacterium, but rather for antibodies against substances released by cells when they are damaged by T. pallidum

Reagent  required for RPR Test (RPR Test Kit)

Materials Required for RPR Test

• RPR Test Kit
• Test Serum Sample
• Micropipettes
• Tips
• Gloves

Rpr test procedure

Please follow the instruction of manual provided with test kits and Allow all reagents to reach room temperature before use if obtained from the fridge.

1. Pipette 25µl of the test sample, positive control and negative control onto three different reaction circle of the disposable slide mostly provided in the RPR test kit.  
2. Mix the RPR reagent and add 25µl of well mixed RPR reagent to test sample, positive control and negative control circles
3. Avoid touching the dropper tip to liquid on the slide.
4. Now,using those mixing stick,thoroughly mix by spreading uniformly over the entire circle of the slide the test sample,positive control and negative control with the  RPR reagent on each slide circle respectively
5. Rotate the slide gently and continuously for 8 mins then  observe for flocculation.

Observation and Result

After mixing the RPR antigen with Positive control, Negative control and Test sample separately observe for visible flocculation as shown below

Interpretation of Test Result

Flocculation in Test sample circle as shown above is considered a positive reaction which indicates the presence of treponemes at a significant and detectable level which is compare with Positive control.

Possible Limitation of RPR Test