Testing
(1) Treponema pallidum hemagglutination assay (TPHA)
(1a) test both serum and CSF
(1b) measure total titer and IgM titer
(2) measure total IgG in CSF and serum, in mg/dL
(3) measure albumin in CSF and serum in mg/dL
TPHA IgG titer in CSF =
= (reciprocal of total CSF TPHA titer) - (reciprocal of CSF TPHA IgM titer)
TPHA IgG titer in serum =
= (reciprocal of total serum TPHA titer) - (reciprocal of serum TPHA IgM titer)
ITPA index =
= ((TPHA IgG titer in CSF) / (total IgG in CSF in mg/dL)) / ((TPHA IgG titer in serum) / (total IgG in serum in mg/dL))
Interpretation:
• If the blood brain barrier is intact (albumin serum-to-CSF ratio > 144), then an ITPA index >= 3.0 indicates active production of antitreponemal antibodies within the central nervous system.
• If the patient is treated early (within 2 years), the ITPA will often return to normal, but if treated late (more than 2 years), then the ITPA may remain increased after therapy.
Limitations:
• A bloody CSF tap results in serum contamination of the CSF sample and makes the index uninterpretable.
