Fig. 2
From: S1 guidelines “lumbar puncture and cerebrospinal fluid analysis” (abridged and translated version)
Quotient diagram. Logarithmically the albumin quotient is plotted against the IgG quotient. The thick diagonal line represents the QLim. This corresponds to the mean value of the expected IgG concentration plus 3 times the standard deviation. For IgG quotients above this line, an intrathecal IgG synthesis can therefore be assumed with a probability of a false-positive result of < 0.5%. The red vertical line represents the age-related limit value for barrier function (formula: age/15 + 4, the upper normal range of a 60-year-old person would be 8 × 10− 3). This results in different areas with differently interpreted findings (and disease examples). An advantage of the quotient diagrams over a numerical calculation is that typical finding constellations can be assigned to a disease at a glance: Possible constellations resulting from QIgG and QAlb are: (1) Normal findings, e.g., no indication of inflammatory CNS process. (2) Isolated barrier dysfunction, e.g., Guillain-Barré syndrome or spinal canal stenosis. (3) Isolated inflammation in the CNS, e.g., multiple sclerosis or past infectious encephalitis. (4) The combination of (2) and (3), e.g., acute neuroborreliosis, neurotuberculosis. (5) Implausible findings (e.g., high-dose hook effect, puncture soon after immunoglobulin infusion)