NOTE: This is similar to the method for diagnosing ocular toxoplasmosis (see 24.07.01) and viral encephalitis (Kennedy et al).
Toxoplasma specific index =
= ((specific CSF antibody level) / (total CSF IgG in g/L)) / ((specific serum antibody level) / (total serum IgG in g/L)) =
= ((specific CSF antibody level) * (total serum IgG)) / ((total CSF IgG) * (specific serum antibody level))
where:
• The specific antibody level in CSF or serum is the inverse of the titer for the Sabin-Feldman dye test. For example, a titer of 1:256 would be 256.
• The units used for total immunoglobulin concentration would cancel out since the index is a ratio.
Interpretation:
• An index > 1 indicates intrathecal production of antibodies to Toxoplasma.
Limitations:
• A patient with defective production of immunoglobulins could have a false negative result.
• Contamination of the CSF sample with blood could cause a false positive index.
