This patient, who has serologic results consistent with syphilis of unknown duration, should undergo cerebrospinal fluid (CSF) analysis to exclude the possibility of neurosyphilis as a contributing factor to her cognitive decline. CSF should be sent for protein, glucose, cell count with differential, and VDRL testing. Neurosyphilis must be confirmed or excluded in this patient because this will determine the appropriate therapeutic approach.

A positive CSF VDRL result in the absence of visible blood contamination or any other unexplained abnormality of the spinal fluid (elevated protein, elevated leukocyte count) should prompt treatment of neurosyphilis with intravenous penicillin. If the CSF analysis yields normal results, treatment for late-latent syphilis with intramuscular benzathine penicillin is appropriate.

Both rapid plasma reagin and VDRL are nonspecific tests. The positive serum rapid plasma reagin result in this case was confirmed by a positive specific test (the fluorescent treponemal antibody absorption test). Therefore, syphilis is confirmed by serologic testing, and it would not be necessary to perform an additional nonspecific test for syphilis. Neurosyphilis must be diagnosed by CSF analysis.