Serum Aquaporin-4 Antibody Testing in NMOSD
Serum AQP4 antibody testing is essential for diagnosing NMOSD and should be performed using cell-based assays on serum samples, as it provides high diagnostic specificity (85.8-100%) and sensitivity (51.5-100%), guides critical treatment decisions including the use of B-cell depleting therapies and avoidance of MS drugs that can worsen disease, and may indicate disease severity during acute attacks though it does not reliably track disease activity between relapses. 1, 2, 3
Diagnostic Role and Testing Methodology
When to Test for AQP4 Antibodies
Testing is particularly indicated when patients present with: 1
- Longitudinally extensive transverse myelitis (LETM) affecting ≥3 vertebral segments
- Severe optic neuritis with significant visual deficit or blindness
- Area postrema syndrome with intractable nausea, vomiting, or hiccups
- Brainstem encephalitis
- Acute respiratory insufficiency with radiological demyelination signs
Optimal Testing Approach
Use cell-based assays (CBA) on serum samples, not CSF. Cell-based assays demonstrate superior sensitivity and specificity compared to other methods including ELISA, immunohistochemistry, or flow cytometry. 2, 3 Serum is the specimen of choice because AQP4-IgG is produced extrathecally, resulting in higher serum titers than CSF. 4
When cost is a limiting factor and disease is stable, test AQP4-IgG first since it is more frequent in NMOSD than MOG-IgG; if disease is active or costs are not a concern, test both antibodies in parallel. 1
Testing Pitfalls and Red Flags
AQP4-IgG/MOG-IgG "double-positive" results are extremely rare and should prompt retesting for both antibodies using alternative methodologies. 5 This finding suggests potential false-positive results in one or both assays.
Additional red flags that should prompt retesting include: 5
- MOG-IgG positivity in CSF but not serum (MOG-IgG is typically produced extrathecally)
- Chronic progressive disease course without relapses
- MRI findings typical of MS (Dawson's fingers, ovoid periventricular lesions)
Clinical Utility Beyond Diagnosis
Disease Severity Correlation
Higher AQP4 antibody titers correlate with increased disease severity during acute attacks but do not reliably distinguish between relapse and remission phases. 6, 7
Specifically, serum AQP4-IgG titers show: 6
- Positive correlation with number of involved spinal cord segments (correlation coefficient 0.70)
- Positive correlation with EDSS scores during attacks (correlation coefficient 0.54)
- No significant difference between relapse and remission phases overall
Patients with high AQP4 titers (≥1:320) experience more severe optic neuritis attacks, higher rates of visual disability, and increased annual recurrence rates compared to low-titer patients (≤1:100). 7
Monitoring Treatment Response
The absence of AQP4 antibodies following treatment has been associated with durable disease remission in some cases. 1 In patients undergoing autologous hematopoietic stem cell transplantation (AHSCT), elimination of AQP4 antibodies could serve as a biomarker of treatment response. 1
However, routine serial AQP4 antibody monitoring for disease activity is not recommended since titers do not reliably distinguish active from inactive disease. 6
Critical Treatment Implications
Therapies to Use
Positive AQP4 antibody status mandates specific therapeutic approaches: 1, 8
For acute attacks:
- High-dose intravenous methylprednisolone as first-line
- Plasma exchange or immunoadsorption if steroids fail (AQP4-positive patients show particular responsiveness to antibody-depleting treatments)
- Mandatory slow steroid taper due to high flare-up risk
For long-term prevention:
- FDA-approved options: eculizumab (complement-inhibiting) and satralizumab (anti-IL-6 receptor)
- B-cell depleting therapies (rituximab, ocrelizumab, ofatumumab)
- Start preventive therapy immediately after first attack
Therapies to Avoid
Beta-interferons, natalizumab, and fingolimod—approved for MS—can exacerbate NMOSD due to differences in immunopathogenesis and should be avoided in AQP4-positive patients. 8, 4 This represents a critical distinction from MS management and underscores why accurate antibody testing is essential.
Distinguishing AQP4-Positive from Seronegative NMOSD
Approximately 20% of NMOSD patients are AQP4 antibody negative, and these patients require different management approaches. 3 When evaluating assay performance, both appropriate control samples and defined seronegative NMOSD samples are essential for clinically meaningful evaluation. 2