Rapid plasma reagin test: High false positivity or important marker of high risk behavior

Sir,

As per the national guidelines, all the donated blood should be mandatorily tested for HIV I and II virus, hepatitis B virus, hepatitis C virus, malaria, and syphilis. Screening of the donated blood for syphilis is carried out either by rapid plasma reagin (RPR) or venereal disease research laboratory in the blood banks. During the period from January to August 2013, a total of 9100 donors were screened for syphilis by RPR (Span Diagnostics Ltd.) at our blood bank. Initial RPR reactive samples were further titrated by doubling dilutions of the samples with physiological saline (0.9%), as per the manufacturer's instructions for semi quantitative RPR. Confirmatory testing by treponema pallidum hemagglutination assay (TPHA) for all initially RPR reactive undiluted samples was done to ascertain the true positivity.

Among 9100 donors screened for syphilis, 20 donors were found to be reactive by RPR. Of 20 reactive donor samples, only 8 were confirmed on the supplementary testing (TPHA) indicating overall 40% true positivity of the initial RPR reactive samples. [Table 1] Of these 12 TPHA nonreactive but RPR reactive, hepatitis B virus (HBV) DNA was incidentally detected in two samples. One of them was both enzyme-linked immunosorbent assay [ELISA] and ID-nucleic acid test [NAT] reactive) while the other was NAT yield of HBV (ELISA-nonreactive, ID-NAT– reactive).

Table 1.

Details of semi quantitative analysis (titration) done on initial RPR reactive samples

Nontreponemal antigen tests (RPR), are designed to test serum for reagin, a heterogeneous group of antibodies, which combine with a cardiolipin-lecithin antigen. This antigen, usually derived from beef heart and is a normal component of human tissue. Because cardiolipin (which mimics treponemal antigens) is used, approximately two-thirds of “reactive” samples during testing can have cross-reactivity and are “biological false-positives.”[1] These BFP can be either due to other type of pathogenic treponemal infections or a strong immunological stimulus[2,3] e.g., acute bacterial or viral infection, vaccination, etc.

One of the study[4] from western region of our country showed 56.4% false positivity of RPR in blood donors as compared with 60% in our study further indicating impact of RPR on the discard rate of the blood bank.

Treponemal tests (TPHA) detect an antibody that is directed toward pathogenic members of the genus treponema. For diagnostic purposes thereby, TPHA is used as the gold standard. In USA, nontreponemal tests such as RPR are recommended for screening and then confirmed with standard treponemal tests such as fluorescent treponemal antibody absorption test or TPHA.

High risk behaviors’, sexually promiscuity, men having sex with men as well as having multiple sexual partners are the risk factors associated with an increased incidence of syphilis infection among the blood donors. Hence, screening for syphilis is a good marker for a high risk activity among the donor population.

Our experience with RPR testing re-emphases the high false positivity of the test and raises the question of retaining or cessation of syphilis donor screening by nontreponemal tests in our setting. Detection of HBV DNA also indicates the importance of screening of syphilis as a surrogate marker for the high risk behavior of a donor.