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B5.5 Utility of Syphilis Chemiluminescence Immunoassay Index Values for Distinguishing True Infection Among Subgroups of Patients with Discordant Treponemal Immunoassay Results

Tuesday, March 13, 2012: 3:55 PM
Nicollet Grand Ballroom (A/B)
Ina Park, MD, MS1, Joan Chow, MPH, DrPH2, Jennifer Shieh, PhD3, Mark Stanley, MPH3, Heidi Bauer, MD, MS, MPH4 and Jeffrey Schapiro, MD5, 1Sexually Transmitted Disease (STD) Control Branch, Division of Communicable Disease Control, Center for Infectious Diseases, Cal, California Department of Public Health, Richmond, CA, 2Epidemiology and Surveillance Section, California Department of Public Health, STD Control Branch, Richmond, CA, 3Kaiser Permanente Northern California-Regional Laboratory, Berkeley, California, Berkeley, CA, 4Sexually Transmitted Disease (STD) Control Branch, Division of Communicable Disease Control, Center for Infectious Diseases, California Department of Public Health, Richmond, CA, 5Kaiser Permanente Northern California (KPNC) Regional Laboratory, Berkeley, CA

     

Background: Although qualitative results of treponemal chemiluminescence immunoassays (CIAs) are reported to clinicians, these assays also produce quantitative optical density index (ODI) values to indicate amounts of antigen-antibody conjugate in a specimen.  The clinical utility of these ODI values is unknown.

Objectives: Describe differences in median CIA ODI values according to demographic characteristics, clinical history and subsequent serology results. 

Methods: Cross-sectional analysis of patients screened with a CIA at Kaiser Permanente Northern California from August-October 2007. Median ODI values were compared by gender, pregnancy, HIV status, syphilis history, presence of symptoms, TP-PA status, and repeat serology results using the Kruskall-Wallis test. 

Results: Of 21,623 specimens tested, 439 (2%) were CIA-positive, of those 255/439 (58%) were rapid plasma reagin (RPR)-negative; all discordant specimens were tested with Treponema pallidum particle agglutination assay (TP-PA) and 184/255 (72%) were TP-PA-positive.  Of 255 CIA-positive, RPR-negative patients, median ODI values (range 1.0-70.0), were higher for males versus females (5.8 vs. 3.0, p=0.01), patients with prior syphilis (9.0 vs. 3.4, p <0.0001), and symptomatic patients (asymptomatic-4.9, genital ulcer-8.2, rash-21.0, p=0.01).   No differences were found by HIV or pregnancy status.  TP-PA-positive patients had higher median ODI values than TP-PA-negative patients (9.8 vs 1.6, p<0.0001).  Of 78 patients with ODI values ≥ 12.0, all were TP-PA-positive.  Of 109 patients retested within 12 months, 11 (10%) seroconverted to CIA-positive, RPR-positive, 91 (84%) were unchanged, 7 (6%) seroreverted to CIA-negative. ODI values varied significantly by repeat serologic outcomes (seroconverted-17.6, unchanged-5.6, seroreverted-1.4, p=0.007).

Conclusions: Higher median ODI values were observed among subgroups with epidemiologic risk factors for syphilis.  ODI values were low among patients who seroreverted to CIA-negative.  Performance of a second treponemal test may not be necessary for discordant CIA specimens with high ODI values. 

Implications for Programs, Policy, and Research: Further studies with prospective, systematic collection of serial CIA results are needed to better characterize the clinical utility of ODI values. 

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