Objective: This study evaluated the diagnostic performance of chemiluminescent microparticle immunoassay (CMIA) in screening for syphilis in Asian pregnant women.
Material and Method: This study retrospectively reviewed the CMIA results of pregnant women attending the antenatal care
clinic, King Chulalongkorn Memorial Hospital. Women with reactive CMIA results were extracted from the laboratory database and further analyzed. A reactive/positive result for Treponema pallidum was defined as having a sample/cut-off absorbance ratio of >1.0. Samples were also tested by rapid plasma reagin (RPR) and Treponema pallidum particle agglutination assay (TPPA).
Results: From February 2011 to January 2013, a total of 11,640 pregnant women were tested and 65 women (0.56%) had reactive CMIA results. Among these cases, 58 women (89.2%) had non-reactive RPR results. TPPA were non-reactive in 35 women (60.3%) who had non-reactive RPR results. A total of 23 women (39.7%) with RPR non-reactive and TPPA reactive
results; therefore, the prevalence rate of syphilis in this population was estimated as 1.98 per 1,000 pregnant women. Among this, 7 cases had a history of past, partial treatment for syphilis and 16 cases were considered as untreated, late, latent
syphilis. If RPR tests were used as the screening test, 16/23 cases (69.6%) cases with untreated syphilis would be missed.
Conclusion: Even though CMIA has high false positive results, however it is still recommended that this reverse sequence
screening be used instead of the traditional algorithm. The rate of false positive results can be decreased by adjusting the
sample/cut-off absorbance ratio of CMIA.

Keywords: Chemiluminescent microparticle immunoassay, Pregnancy, Rapid plasma reagin, Syphilis screening, Treponema
pallidum particle agglutination assay