Intrathecal Ig synthesis. Its detection by isoelectric focusing and IgG index

In a retrospective study on 999 patients, the likelihood ratios of the IgG index, Tourtellotte formula and IgG concentration in CSF and in serum, the albumin concentration in CSF and in serum, and the total protein in CSF, were compared in predicting intrathecal Ig synthesis. This synthesis was detected with isoelectric focusing (IEF). No patient was included more than once in the data collection. All patients with high IgG and other abnormalities in serum, as well as all xanthochromic and blood-tinged CSF specimens, were excluded from the study. Construction of ROC curves established that the IgG index, Tourtellotte formula and CSF IgG yield the same information: these parameters indicate the presence of intrathecal IgG synthesis. The likelihood ratio for the IgG index at a cut-off point of 0.80 for a positive test is 20; at a cut-off point of 0.5 for a negative test it is 5. For the Tourtellotte formula, it was found that the likelihood ratio reached a maximal value of 7 at a cut-off value of synthesis of 10 mg/day for a positive test. For a negative result, the formula had a likelihood ratio of 6 at a cut-off value of -5. For CSF IgG, it was found that the likelihood ratio was 4 for a positive test with a cut-off value of 0.1 g/l. For a negative outcome, the determination of CSF IgG is only meaningful at a very low cut-off value (0.03 g/l). The other parameters studied (serum IgG concentration, albumin concentration in CSF and in serum, and total protein in CSF) showed a likelihood ratio equal to 1. It is concluded that only the IgG index, the Tourtellotte formula and the CSF IgG concentration have predictive value for intrathecal Ig synthesis as recorded with IEF.