Immunoglobulin G Index

Serum and cerebrospinal fluid (CSF)

1.5 mL serum and 1 mL CSF

0.7 mL serum and 0.5 mL CSF (Note: This volume does not allow for repeat testing.)

Red-top tube or gel-barrier tube and sterile (CSF) container

Detect and measure the level of IgG production by the central nervous system; has been applied to the diagnosis of multiple sclerosis along with CSF IgG synthesis rate and CSF immunoglobulin “oligoclonal” bands

Conditions in which lymphoreticular elements of the CNS produce immunoglobulins will result in false-positive (in relation to multiple sclerosis) elevations. Such conditions include, but are not limited to, aseptic meningitis, lymphoma, neurosyphilis, Guillain-Barré syndrome, and cerebral lupus erythematosus. These conditions, however, are either uncommon or are not frequently associated with an increased IgG index.

CSF IgG, CSF albumin, and serum IgG: immunologic; serum albumin: spectrophotometry; ratio and index by calculation

Index: 0.0−0.7

IgG index, synthesis rate, and oligoclonal bands have shown sensitivity at the 80% to 90% level but are not uniformly specific. CSF IgG may be increased, generally with inflammatory disorders of the CNS (eg, idiopathic polyneuropathy, neurosyphilis). Rents in the interface between blood and CSF will allow increase in CSF IgG. The three tests differentiate between increased barrier permeability and increased CNS immunoglobulin synthesis, each in a different manner. The ratio technique relates CSF IgG level to another protein that hopefully reflects protein present on the basis of increased blood/CSF permeability. CSF IgG/CSF total protein is such a measure. CSF IgG/CSF albumin has been proposed by Tourtellotte et al as equally discriminative for MS as the CSF IgG:CSF total protein ratio.^1,2 The index provides good separation of increased CNS IgG production from protein increases, the result of altered permeability of blood/CNS interface.³

Hershey and Trotter have compared sensitivity/specificity (weighted equally) of these tests (not including IgG synthetic rate) and found the CSF IgG index superior with sensitivity of 91% and specificity of 96.5%. Gambino, however, has emphasized that these results are with cerebral infections and immunologic diseases eliminated so that considerable clinical judgment must be exercised. Results of IgG index testing appear to correlate with those of oligoclonal banding determinations, but the IgG index may have no specific value independent of the presence of oligoclonal bands.