CSF diagnostics

To identify a CNS infection, it is necessary to distinguish between intrathecally produced antibodies and antibodies that have diffused from the blood into the CSF. This is done by measuring the concentrations of pathogen-specific antibodies, the corresponding immunoglobulin classes (total IgG or IgM) and albumin in both the patient’s CSF and serum. When there is an infection of the CNS, pathogen-specific antibodies accumulate in the cerebrospinal fluid. A measure of intrathecal pathogen-specific antibody production is the antibody index AI (also: relative CSF/serum quotient CSQrel). It is calculated from the ratio of pathogen-specific antibodies against total IgG or IgM in the CSF in proportion to the ratio of pathogen-specific antibodies against total IgG or IgM in the serum. An AI >1.5 indicates intrathecal synthesis of pathogen-specific antibodies.  

In addition to the detection of specific antibodies, the determination of the chemokine CXCL13 in the CSF is useful in neuroborreliosis diagnostics. Patients with acute neuroborreliosis often have high concentrations of CXCL13 in early stages of the disease, in many cases even before antibodies against Borrelia are detectable. CXCL13 determination may help to close the gap between infection and positive antibody test and to diagnose neuroborreliosis at an earlier stage. Moreover, CXCL13 can be used as an activity marker to differentiate between acute and past neuroborreliosis. CXCL13 is also suitable as a marker for the disease course after treatment. Its concentration in the CSF decreases rapidly after successful treatment. However, elevated CXCL13 levels can also be observed in other diseases, particularly in CNS lymphoma, HIV infection and neurosyphilis.