P265 Value of CLIA seroconversion with negative RPR and immunoblot for the diagnosis of early syphilis

Introduction An isolated CLIA seroconversion (i.e. CLIA-reactive, immunoblot non-reactive, RPR non-reactive, with a previous negative CLIA) could indicate a false positive result or early incubating syphilis. To confirm early syphilis, follow-up appointments are often needed. We wanted to evaluate the diagnostic value of such seroconversions. Methods We included every patient with a positive CLIA (Liaison) and a negative RPR and immunoblot visiting the STI clinic (To) between January 2014 to April 2020, and a preceding visit with a negative CLIA in the 6 months prior to the initial consultation (T-1). If available, a follow-up appointment in the 2 months after the initial consultation (T1) was included. If darkfield microscopy (DFM) or PCR for Treponema pallidum was positive at T0, diagnosis of syphilis was confirmed. This also applied to a positive RPR and/or immunoblot in the T1 consultation. Results We included 91 participants with an isolated CLIA seroconversion. The value of the CLIA seroconversion in 19/91 (21%) of the study population could not be established, since they had no positive PCR or DFM ulcur sample at To and had no T1 consultation. Of the remaining 72 patients, 54 (75%) the CLIA seroconversion was confirmed. 28/54 persons (52%) had a PCR or DFM confirmation in the initial consultation and 26/54 persons (48%) had a serologic confirmation in the follow-up appointment. In 18/72 (25%) persons the CLIA seroconversion was a regarded as false positive reaction since no seroconversion in RPR or immunoblot was seen at T1. Conclusion Of the evaluable patients with a CLIA seroconversion, 75% had an early incubating syphilis infection. 48% of these patients would benefit from presumptive treatment, since they had no signs of primary syphilis at the moment of the CLIA seroconversion. However, 25% had a false positive result, thus would receive unnecessary presumptive treatment.