• Syphilis
• neurosyphilis
• diagnostics
• treponemal antibody titres

Neurosyphilis, like other infections of the central nervous system, results in intrathecal antibody production.1 2 Both treponemal and non-treponemal (lipoidal) antibodies are produced and will continue to be produced even after successful treatment of the infection (this also occurs in the blood at much higher concentrations than the cerebrospinal fluid (CSF)). Therefore, CSF antibodies produced intrathecally are specific for the diagnosis of neurosyphilis. Serum antibodies, particularly IgG (due to its small size), can passively diffuse across the blood–CSF barrier.1 The presence of these antibodies in the CSF does not imply neurosyphilis. Thus, when using CSF IgG antibodies to diagnose neurosyphilis, whether treponemal or nontreponemal, the goal is to identify those antibodies produced intrathecally. During infection with Treponema pallidum, significantly more treponemal antibodies are produced than nontreponemal antibodies in the blood (and intrathecally in neurosyphilis). This results in enhanced passive diffusion of serum treponemal antibodies into the CSF. Consequently, the specificity of CSF treponemal antibody tests is low (due to passive diffusion), but their sensitivity is high (due to high intrathecal production), whereas the specificity of the CSF nontreponemal antibody tests is much higher (less passive diffusion), but the sensitivity is low (lower intrathecal production).3 A reactive CSF non-treponemal test (eg, venereal diseases research laboratory (VDRL)) is diagnostic of neurosyphilis (in the absence of gross blood contamination), but a reactive CSF …