Aim: To detect the presence of reagin in the serum of patient’s blood as an aid in the serological diagnosis of syphilis infection.
PRINCIPLE: The principle is based on the immunological reaction between reagin antibodies and cardiolipin-cholesterol-lecithin antigen resulting to visible flocculation. The antigen suspension in the RPR test reagent contains charcoal-like particles which allows for the macroscopically visible flocculation on the test card or slide.
MATERIAL/APPARATUS: Blood specimen, VDRL antigen (i.e. cardiolipin-lecithin coated cholesterol particle), control reagents, test cards, centrifuge machine, Pasteur pipette, mixing stick or stirrer, timer.
- Bring the cardiolipin antigen reagent to room temperature (23oC – 29oC). This is done by bringing out the reagent from the refrigerator where it is stored when not in use, in order for it to attain room temperature before its usage. After use, the RPR test reagent should be stored in the refrigerator.
- Centrifuge the blood specimen in the centrifuge machine at 400 rpm for 5 minutes. This is done to sediment the red cell component of the blood specimen in order to obtain the serum component of the blood required to perform this test.
- Label the RPR test card with the laboratory number of the patient’s serum to be tested for pallidum infection. Note: Wells for positive (reactive), weakly positive and negative control sera (non-reactive) should also be labeled accordingly.
- Place a drop of the serum on the test card using the Pasteur’s pipette or disposable dropper. A new disposable dropper should be used for each sample.
- Shake the vial/phial containing the cardiolipin antigen reagent and add a drop of it to each of the test wells on the RPR test card including the test serum well.
- Mix the mixture together using a stirrer, mixer or the paddle end of the Pasteur’s pipette.
- Rock /rotate the test card for about 8 minutes after mixing. An automatic rotating machine could also be used to rotate the RPR test card if available.
- Observe the mixture macroscopically after the rocking, and look out for visible flocculation.
REPORTING OF THE RESULT:
Presence of flocculation after the 8 minutes implies that the patient’s specimen contains the reagin antibody, and this is reported as reactive because it is indicative of T. pallidum infection. While absence of flocculation indicates that the patient’s serum does not contain reagin antibodies, and this is reported as non – reactive.
NOTE: RPR test is a non – specific test used in the laboratory diagnosis of syphilis infection. It is only a screening test, and should be confirmed by carrying out other confirmatory test whenever the RPR test proves reactive. The RPR test could also give false positive result in the case of similar infections or reacting antibodies, but this barely occurs as opposed to the TPHA test where false positive result is more common.