An IgG index of 0.8 is most indicative of which phenomenon?

• A. Intrathecal IgG synthesis
• B. CSF leakage
• C. Fungal infection
• D. Malignancy

Answer: (A)

The IgG index measures intrathecal (within the CNS) IgG production by adjusting CSF IgG for what leaks in from serum, using the CSF/serum albumin ratio as a backbone. Normal values are around up to 0.7–0.8, and values at or above this threshold point to IgG being produced inside the CNS rather than just reflecting serum IgG crossing the barrier. An index of 0.8 sits at the upper limit of normal, indicating intrathecal IgG synthesis. This is the hallmark of CNS-restricted immune activity, as seen in inflammatory or demyelinating processes. CSF leakage tends to disturb the barrier differently and does not inherently raise the IgG index. While infections or malignancies can involve intrathecal IgG production, the index by itself specifically signals intrathecal synthesis rather than pinpointing a particular diagnosis.