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The following is a summary of “Predicting intrathecal immunoglobulin synthesis in the ICU: a comparative study of IgG-based indexes,” published in the April 2025 issue of Annals of Intensive Care by Balcerac et al. 

Central nervous system autoimmune diseases (CNS-AD) often required intensive care unit (ICU) admission and were difficult to diagnose early due to limited acute-phase markers and reliance on delayed confirmatory tests.  

Researchers conducted a retrospective study to evaluate surrogate markers, such as the IgG quotient (QIgG), IgG index, and Reiber’s formula, for predicting intrathecal immunoglobulin synthesis (ISI) in the ICU setting to expedite treatment initiation.  

They screened all neuro-ICU admissions from 2008 to 2022 at their center, including patients who underwent lumbar puncture (LP) and were tested for ISI via iso-electric focalization (IEF). Patients lacking concomitant cerebrospinal fluid (CSF)/serum albumin and IgG data were excluded. Patients were categorized by final diagnosis into “CNS-AD” or “other” groups based on whether ISI was present. The QIgG, IgG index, and Reiber’s formula were calculated, and their performance was compared to IEF in terms of sensitivity (Se) and specificity (Sp).  

The results showed that 35% of patients (93/266) had intrathecal ISI. In the CNS-AD group, 54% were ISI-positive, while 21% of patients in the “Other” group also showed ISI. Among the 3 indexes, only the IgG index exhibited strong Sp (95%) but moderate Se (56%). Both the QIgG and Reiber’s formula demonstrated similar Se (67% and 66%, respectively) but lower Sp (41% for both). Multivariable analysis identified age < 50 years (odds ratio [OR] 2.5 [95% CI 1.3–4.7]) and an IgG index > 0.7 (OR 14.2 [95% CI 6.6–32.0]) as factors independently associated with ISI positivity. Thresholds were recalibrated using the Youden index and likelihood ratio to improve performance. A “grey zone” for the IgG index was defined as 0.67–0.80, below which ISI was unlikely and above which ISI was considered probable.  

Investigators concluded that although the IgG index had low Se and limited its independent diagnostic application, its high Sp rendered it a valuable positive indicator for informing treatment decisions in suspected Central nervous system autoimmune diseases (CNS-AD) while awaiting potentially delayed IEF results. 

Source:annalsofintensivecare.springeropen.com/articles/10.1186/s13613-025-01475-7