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Cerebrospinal fluid treponemal antibody titres: a breakthrough in the diagnosis of neurosyphilis
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    TPPA test accuracy matters

    Although we agree with Ghanem et al. that CSF TPPA titer is a valuable test for the diagnosis of neurosyphilis[1], we would like to emphasize that a cut-off TPPA titer should be recommended with caution as proposed by others [2]. Such semi-quantitative laboratory tests may vary depending on the operator or reagent. Our IQC from a single patient during a 2 years period showed that TPPA inaccuracy is about 2 titers (Table). Moreover, a 2 log2 variation is accepted by organisms providing samples for external quality assessment for syphilis serology [3]. Similarly to what occurs with neuroborreliosis, quantifying anti-treponema pallidum IgG (antiTp- IgG) in CSF, immunoassays in serum and intrathecal antibodies index could be a reliable approach for the diagnosis of neurosyphilis. We found some positive antiTp-IgG index in CSF with TPPA titers below 320, suggesting an intrathecal synthesis of anti-treponema pallidum IgG. The diagnosis of neurosyphilis still lacks a gold standard test and further research is warranted. 1. Ghanem, K.G., Cerebrospinal fluid treponemal antibody titres: a breakthrough in the diagnosis of neurosyphilis. Sex Transm Infect, 2020.
    2. Marra, C.M., et al., Cerebrospinal Fluid Treponema pallidum Particle Agglutination Assay for Neurosyphilis Diagnosis. J Clin Microbiol, 2017. 55(6): p. 1865-1870.
    3. Muller, I., et al., Is serological testing a reliable tool in laboratory diagnosis of syphilis? Meta-analysis of eight external quality control sur...

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    Conflict of Interest:
    None declared.