AQP4positive

AQP4-positive refers to the presence of autoantibodies against aquaporin-4 (AQP4) in a person’s serum, and less commonly in cerebrospinal fluid. The antibodies are typically of the IgG1 subtype and target the water channel located on astrocytic endfeet. AQP4-specific antibodies are most strongly associated with neuromyelitis optica spectrum disorder (NMOSD), a demyelinating disease that classically affects the optic nerves and spinal cord but can involve other regions of the central nervous system.

Testing and interpretation: AQP4-IgG is detected most reliably with cell-based assays, which offer high specificity for NMOSD. A positive result supports a diagnosis of AQP4-positive NMOSD in the appropriate clinical setting. A negative result does not completely exclude NMOSD, particularly early in disease or in seronegative NMOSD; testing for other antibodies, such as MOG-IgG, may be considered in such cases. Diagnostic criteria for NMOSD have evolved, and AQP4-positivity remains a central supportive feature for many cases.

Clinical relevance: Identifying AQP4 positivity informs prognosis and treatment, as NMOSD is typically a relapsing disease requiring long-term immunotherapy to reduce relapse risk. Treatments frequently used include B-cell–depleting therapies (e.g., rituximab, inebilizumab), anti–complement therapy (eculizumab), and anti–interleukin-6 therapy (satralizumab). Acute attacks are managed with high-dose corticosteroids, with plasmapheresis reserved for steroid-refractory cases.

Distinction from related conditions: Some NMOSD-like syndromes are MOG-IgG-positive and AQP4-negative; these conditions often differ in pathophysiology, clinical course, and treatment response from AQP4-positive NMOSD.