The IgM antibodies against Borrelia burgdorferi in cerebrospinal fluid (CSF) are primarily used to assess suspected cases of early Lyme neuroborreliosis, which represents the neurological involvement of Lyme disease due to infection of the central nervous system. It is most valuable during the early stages of central nervous system invasion, typically within the first few weeks of infection, when IgM antibodies may be present before IgG seroconversion occurs. The detection of specific IgM in the CSF provides strong evidence of an active immune response to Borrelia within the central nervous system.
Following a tick bite from an Ixodes species carrying Borrelia burgdorferi, the pathogen may spread through the bloodstream and penetrate the central nervous system, causing inflammation of the meninges, cranial nerves, or spinal roots. As part of the host immune defense, B lymphocytes within the intrathecal compartment produce pathogen-specific antibodies. IgM antibodies are the first class to appear after exposure and typically indicate a recent or active infection. Their presence in cerebrospinal fluid, strongly supports the diagnosis of acute or subacute Lyme neuroborreliosis, particularly when neurological symptoms such as headache, neck stiffness, radicular pain, or cranial neuropathies are evident.
Elevated levels of specific IgM antibodies against Borrelia burgdorferi in CSF suggest ongoing immune recognition of the pathogen within the central nervous system. Such elevation is typically observed within two to six weeks after initial infection. These antibodies, however, should be interpreted in the context of intrathecal antibody synthesis, often evaluated through the antibody index or comparative serum testing. High IgM titers in the absence of corresponding serum IgM may indicate localized production, whereas matched findings in both compartments support systemic and central nervous system infection. Persistent IgM positivity beyond the early infection phase may be observed, but it is less specific and should be interpreted with caution.
Low or absent levels of IgM antibodies in CSF may indicate either the absence of neuroborreliosis or a stage of infection where IgM production has ceased and class-switching to IgG has occurred. False-negative results can also occur in immunocompromised individuals or in those treated early with antibiotics, which may suppress antibody production.
