New ELISA Screening for Syphilis
The incidence of syphilis declined after the introduction of penicillin 60 years ago but it is still a significant health problem, especially in developing countries. Although in the U.S. at the end of the twentieth century the incidence was probably the lowest ever recorded. The disease is still endemic and has recently surged in some urban areas.
Because the causative agent Treponema pallidum cannot be grown using conventional techniques, serology is the primary method for laboratory diagnosis of syphilis. Syphilis antibodies (and serological tests for syphilis) are divided into two types: non-treponemal and treponemal. In response to infection with T.pallidum, the immune system produces both.
Traditional screening for syphilis infection involves using a “non-treponemal” test, followed by a confirmatory “treponemal” test. We are advocating a reverse of this approach by implementing the treponemal ELISA for screening, followed by the RPR test to determine if the patient has active disease.
“New Approach to Syphilis screening...Reduction in both false-negative and false positive results…Differentiation of prior treated infection and active infection”
Non-Treponemal Antibodies are directed against phospholipids released from the tissue invaded by the bacteria. The traditional serological approach to the diagnosis of syphilis involves identifying these antibodies using flocculation of the phospholipids' cardiolipin in a mixture of cholesterol and lecithin. The original assay (Venereal Disease Research Laboratory, or VDRL) required that this mixture be made up each day and that the tests be examined under a microscope.
Today most laboratories use the Rapid Plasma Reagin (RPR) test in which the lipid suspension is stabilized and charcoal particles are added to allow the flocculation to be observed visually without a microscope. Although this is easier to perform than the VDRL test, it is still relatively labor-intensive, subjective, and prone to false-negative results (if the level of antibody is very high and the flocculation is inhibited in a phenomenon called “pro-zone)”
But the major problem with using the RPR as a screen is that anti-cardiolipin antibodies are not specific for syphilis; up to 50% of positive RPR results may be false-positives (especially in low-risk populations). Therefore, positive screening results using RPR required confirmation using a treponemal-specific antibody assay.
The traditional confirmatory assay employed indirect immunofluorescence, using actual organisms (fluorescent treponemal antibody-absorbed, or FTA-ABS test). Most recently, most laboratories moved to an agglutination assay in which treponemal antigens are coated on either red blood cells (MHA-TP) or colored gelatin particles (TPPA).
When this traditional approach was first introduced, labor costs were relatively low and the costs of the treponemal assays such as FTA-ABS were relatively high. With the introduction of automated treponemal-specific enzyme-linked immunoassays (ELISA”), the reverse is now the case.
New Approach: TREP- SURE ™
· Has better sensitivity; RPR is a difficult test to interpret when low levels of non-treponemal antibody are present and the test may be negative during early or late infection
· Has increased specificity; a lot of positive RPR results are false-positives, especially when screening low-risk populations.
· The new approach is more efficient, results can be reported the same day
· RPR is a manual, labor-intensive test while the EIA is completely automated
· The overall cost of the two approaches is comparable depending on test volume
RPR continues to be used to monitor patients’ response to antibiotic therapy using the RPR titer, a two-fold decrease in titer indicates an appropriate response in patients with primary or secondary infection.