CSF diagnostics

While carrying out assays on CSF to diagnose various CNS infections, it is necessary to differentiate between antibodies that are produced intrathecally from those that migrate from blood to CSF. This is done by comparing pathogen specific antibodies, corresponding classes of immunoglobulins (total IgG, IgM) and albumin content in patient’s serum and CSF. If there’s an infection of CNS, but it has not yet spread to the brain and the blood-CSF barrier is intact, the pathogen specific antibodies in the CSF and serum are same. However, when there is intrathecal production of pathogen specific antibodies, it is measured by a relative CSF to serum quotient (CSQrel). The quotient is calculated from the specific IgG antibodies in total CSF IgG in proportion to specific IgG antibodies in total serum IgG. A quotient of >1.5 is indicative of intrathecal antibody production.

In addition to antibodies, detection of specific chemokines also plays an important role in diagnosing various CNS conditions for example the chemokine CXCL13 in neuroborreliosis. This chemokine is present in the early stage of the disease even before the antibodies against Borrelia are detectable. Thus, CXCL13 determination helps to reduce the gap between infectivity and positivity antibody test and so helps in earlier diagnosis of the disease. Moreover, it helps to differentiate between an acute and past infection and also allows evaluation of treatment response as its levels decrease in response to successful therapy. One thing however to kept in mind is that CXCL13 is also found in other conditions such as CNS lymphoma, HIV and neuro-leus.