Intrathecal immunoglobulin synthesis. Comparison of various formulae with the 'gold standard' of isoelectric focusing.

The recent introduction of a further new formula for calculating intrathecal Ig synthesis led us to compare the results of calculations obtained when using it and certain other older formulae with those of isoelectric focusing (IEF), since only the latter proves the intrathecal synthesis of oligoclonal proteins. On comparison, it was found that the results produced by the usual formulae were, without exception, inferior to those of isoelectric focusing. By using Receiver Operating Characteristic (ROC) curves, it is shown that calculations with the Reiber formula agree best with IEF. Virtually the same results are obtained with the "ln IgG index + 1" and "IgG index".