An improved formula for the judgement of intrathecally produced IgG in the presence of blood brain barrier damage

In order to develop a better formula for discrimination between transsudated and intrathecally produced IgG, we studied the relation between cerebrospinal fluid (CSF) IgG, serum IgG, CSF albumin, and serum albumin for patients with blood brain barrier (BBB) damage. From this study we derived a new formula, IgG extended index = (CSF IgG/serum IgG)/(CSF albumin/serum albumin) 1.12 Applying this index on 152 patients with BBB damage, but no expected intrathecal IgG synthesis, yielded 14 patients (9.2%) with increased (> 1.24) IgG extended index. In comparison, ‘conventional’ IgG index was increased in 34 of the patients (22%). The diagnostic sensitivity for 68 patients with multiple sclerosis (MS), did not differ significantly between the two formulas (81 and 87%, respectively). A formula for the estimation of 24 hours IgG production was also developed, but this formula was not better than IgG index. Children (0.5–16 yr) had similar IgG patterns as adults.