Predicting intrathecal immunoglobulin synthesis in the ICU: a comparative study of IgG-based indexes
Authors' conclusion
While the IgG index's low sensitivity limits its standalone diagnostic use, its high specificity makes this index a good one when positive, to weigh in the decision-making process to treat or not a patient with suspected CNS-AD, while awaiting IEF results, which can take days or even weeks in some centers.
Study context
- Population
- Neuro-ICU admissions 2008–2022 who underwent lumbar puncture (LP) and immunoglobulin synthesis (ISI) testing via iso-electric focalization (IEF), with concomitant CSF/serum albumin and IgG data
- Participants
- 266 admissions
- Comparison
- QIgG, IgG index and Reiber's formula vs IEF for detection of ISI
- Outcome
- Sensitivity and specificity for ISI vs IEF
Methodological detailsretrospective, single-centre
- Retrospective
- Single-centre
Performance
As reported in the abstract
Diagnostic performance for ISI
| Test or model | Sensitivity | Specificity |
|---|---|---|
| IgG index | 56% | 95% |
| QIgG | 67% | 41% |
| Reiber's formula | 66% | 41% |
Sensitivity
IgG index56%
QIgG67%
Reiber's formula66%
Specificity
IgG index95%
QIgG41%
Reiber's formula41%
Further findings
Factors independently associated with ISI positivity (multivariable analysis)
| OR (95% CI) | ||
|---|---|---|
| Age < 50 years | 2.5 (1.3–4.7) | |
| IgG index > 0.7 | 14.2 (6.6–32.0) | |
| 0.050.21520Odds ratio (log scale) |
ISI prevalence
- ISI detected, all patients
- 35% (93/266)
- ISI-positive, central nervous system autoimmune diseases (CNS-AD)
- 54%
- ISI-positive, Other
- 21%
Recalibrated IgG index thresholds (Youden index, likelihood ratio)
- Grey zone for IgG index; below, ISI unlikely; above, probable
- 0.67-0.80
Original publication
Predicting intrathecal immunoglobulin synthesis in the ICU: a comparative study of IgG-based indexes. Ann Intensive Care. Published 30 April 2025. doi:10.
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