Aquaporin-4 antibody (AQP4-IgG)
An antibody that attacks a water channel on the support cells of the brain and spinal cord. It is the defining blood test for neuromyelitis optica spectrum disorder (NMOSD), a serious condition that can look like multiple sclerosis but is treated differently.
What it measures
Aquaporin-4 is a water-channel protein sitting on astrocytes — the support cells of the central nervous system — concentrated around the optic nerves and spinal cord. In neuromyelitis optica spectrum disorder (NMOSD), the immune system makes an antibody against it, triggering inflammation that damages the optic nerve (causing painful vision loss) and the spinal cord (causing weakness, numbness and bladder problems), often in severe attacks. Detecting this antibody in the blood is the key test: a positive AQP4-IgG in someone with typical symptoms confirms NMOSD and, crucially, distinguishes it from multiple sclerosis, which it can closely resemble on scans. This distinction matters enormously because several drugs used for multiple sclerosis can actually worsen NMOSD, and NMOSD has its own specific treatments. The test uses a sensitive cell-based method. A negative result does not fully exclude the condition — some people are instead positive for a different antibody (MOG) or are antibody-negative — so it is interpreted with the clinical picture and MRI. AQP4-IgG is a specialist test, ordered by neurologists working up inflammatory disease of the optic nerves and spinal cord.
What a high value can mean
- Neuromyelitis optica spectrum disorder (NMOSD) — the defining finding; a positive AQP4-IgG with typical symptoms confirms the diagnosis.
- Distinguishes NMOSD from multiple sclerosis — critical, because they need different, sometimes opposite, treatments.
- Supports a diagnosis after optic neuritis or a spinal cord attack — especially if severe or recurrent.
What a low value can mean
- Negative — the normal result; makes NMOSD less likely but does not fully exclude it.
- MOG-antibody disease or seronegative NMOSD — a negative AQP4 result in someone with typical symptoms may reflect these, needing further testing.
When to discuss with a doctor
AQP4 antibody testing is a specialist request, ordered by neurologists when someone has inflammation of the optic nerve (optic neuritis, with painful loss of vision) or the spinal cord (transverse myelitis, with weakness, numbness or bladder problems), especially when attacks are severe or recurrent, or when telling NMOSD apart from multiple sclerosis will change treatment. A positive result confirms NMOSD and directs specific therapy while steering away from certain multiple-sclerosis drugs that can worsen it. Acute attacks — sudden vision loss or new limb weakness — are urgent and need immediate neurological care. Mediora.AI shows the AQP4 antibody in context, but this is a specialist result: its interpretation, and the diagnosis and treatment of NMOSD, belong with a neurologist.