Immunoglobulin G Synthesis Rate, Cerebrospinal Fluid

Tourtellotte IgG synthesis rate is theoretically the most definitive approach to determining IgG production by the central nervous system. It is based upon an empirically-derived formula, the validity of which, however, has been verified by radiolabeled IgG experiments.¹ The synthesis rate formula involves a constant described as “a ratio constant that quantitatively determines the proportion of CSF IgG that normally passes by filtration from the serum into the CSF across an intact BBB” (blood brain barrier). The formula is given as:

de novo CNS IgG_syn = ((IgG_csf - IgG_s/369) - (Alb_csf - Alb_s/230) (IgG_s / Alb_s) (0.43)) x 5

A study by Tourtellotte and associates has shown that about 90% of multiple sclerosis patients show evidence of enhanced IgG synthesis.² A study from the Cleveland Clinic indicates a sensitivity and specificity comparable to that for the IgG index in predictive value for MS.³ Sensitivity (96%) and specificity (98%) applied to a group considered as definite multiple sclerosis. A group of “possible multiple sclerosis” patients included a number of cases in which CNS IgG synthesis was normal − just 55% had an elevated rate. In the group with “neurologic diseases other than MS,” 96% of patients had normal CNS IgG synthesis. The study suggests that determination of CSF IgG synthesis rate contributes importantly to the diagnosis of MS. The study also emphasizes that correlations between test results and diagnoses are dependent upon the validity of what must remain a clinical neurologic diagnosis.