Question

Lab Activity: Testing for Syphilis with the Arlington Scientific (ASI) RPR test kit

Discuss limitations of the procedure with regards to conditions resulting in false positives.

Answer 1

The Arlington Scientific (ASI) rapid plasma reagin (RPR) test is a quantitative rapid diagnostic test, based on the nontreponemal test for the detection of reagin antibodies in human serum and plasma as a sreeing test for the presence of syphilis. Treponema pallidum, the causative agent of syphilis produce two types of antibodies in human body: one is anti-treponemal antibodies that is detected by FTA-ABS antigen and the other is anti-nontreponemal antibodies (reagin) detected by RPR antigen. The reagin antibodies via RPR test does not look for antibodies against the bacterium itself, but rather for non-specific antibodies against substances released by cells (cardiolipins, lecithin) when attacked by the pathogen. The reagents required for this test includes carbon antigen, cardiolipin, lecithin, activated charcoal, etc. The reagin antibody binds with the antigen composed of a complex of cardiolipin, lecithin in presence of activated charcoal, which results in antigen-antibody reaction.

But there are several limitations with respect to this test method, as it can induce false positives. The major limitations of the procedure are as follows:

(i) In patients with secondary syphilis, prozone reaction may occurs in which false negative nontreponemal test results arising due to prozone reaction (ineffectiveness of antibodies to form immune complexes resulting from higher concentration of antibody or antigen). This conditions are also remain active in primary and late syphilis.

(ii) Cases of false positive reactions also occurs with the carbon antigen required for this procedure. Such reactions may take place in samples from individuals addicted to drugs, pregnancy, or with diseases like malaria, mononucleosis, leprosy, pneumonia, etc.

(iii) There are many tropical infectious diseases like pinta, yaws, bejel and other non-venereal treponematoses which may produce false positive results in this test.

(iv) Due to drying effect, the specified time required, if takes longer may induce false positive results.

(v) Tempaerature of the reagents and samples is a key factor for a perfect outcome of the test. If the optimum temperature is exceeded above 30⁰C, then it may also induce false positive test results.