Intrathecal immunoglobulin synthesis can be assessed by different approaches. It can be measured quantitatively by the IgG index or qualitatively by isoelectric focusing (IEF) to detect oligoclonal bands (OCB). In this study we investigated if there is a correlation between the frequency of OCB and the IgG index and if the IgG index predicts the diagnosis of a demyelinating CNS disease (DMD). We found a positive correlation between the IgG index and the frequency of OCB as well as the probability of DMD. We conclude that quantitative assessment of CSF-IgG can be useful because it is easier and quicker to perform than IEF but cannot replace IEF in general because this is the most sensitive method to detect abnormal IgG in CSF.