Aquaporin-4 (AQP4/NMO) IgG
A blood test for an antibody to a water channel in the brain and spinal cord. It helps tell neuromyelitis optica spectrum disorder (NMOSD) from other nerve conditions.
AI-verified against cited sources. Awaiting expert reviewAlso called AQP4-IgG, AQP4-Ab, NMO-IgGLOINC 43638-6
What it is
Aquaporin-4 (AQP4) is a water channel found on star-shaped support cells in the brain, optic nerves and spinal cord. In neuromyelitis optica spectrum disorder (NMOSD), the immune system can make an IgG antibody that targets this channel. The test looks for that antibody in blood serum. It is usually reported as negative or positive, and a positive result may come with a titer. The most sensitive method is a cell-based assay, which uses cells that display the AQP4 protein.
Why it matters
NMOSD is a rare condition that inflames the optic nerves and spinal cord, and sometimes parts of the brain. It can look like multiple sclerosis (MS), but the two are treated differently, so telling them apart matters. This antibody is rarely found in healthy people, so a positive result is a strong clue when it matches your symptoms and scans. A negative result does not rule NMOSD out, because some people with the condition do not have the antibody.
If it’s high
A positive result means AQP4 antibodies were found. With matching symptoms and scan findings, this supports NMOSD. A neurology specialist usually guides the next steps. Low-level positives from older test types (ELISA or tissue-based tests) are sometimes false positives. A second type of test may be used to confirm.
Common causes
- Neuromyelitis optica spectrum disorder (NMOSD), the condition this antibody is linked to
- A false-positive result, which is more likely with ELISA or tissue-based tests at low levels
What can help
- Ask your neurologist whether the result should be confirmed with a cell-based test
- Ask what the result means for your symptoms, MRI scans and next steps
What can affect your result
- Treatment: steroid infusions, plasma exchange and immune-suppressing medicines can lower the antibody below what the test can detect
- Test type: cell-based assays find more true positives than ELISA or tissue-based tests
- Timing: if NMOSD is still suspected after a negative result, some labs suggest testing again in 3 to 6 months
- Not everyone with NMOSD has this antibody
- Steroid infusions, plasma exchange or immune-suppressing medicines can lower the antibody below the level the test can detect.
- ELISA and tissue-based tests find fewer true positives than cell-based assays and can give false positives at low levels.
- Different labs use different methods (flow cytometry, cell-based microscopy, ELISA), so results are not interchangeable.
The result reflects the day the sample was drawn. Treatment can make the antibody fall below the detection limit.
When to talk to a clinician
Get checked promptly for sudden vision loss or eye pain. The same goes for new weakness or numbness in the arms or legs, trouble walking, loss of bladder or bowel control, or hiccups and vomiting that will not stop. This is true whatever this test shows.
Questions you could ask
- Could any treatment I have had affect this result?
- Was my result found with a cell-based test, and does it need to be confirmed?
- What does this result mean for my scans and symptoms?
- If my test is negative, should we repeat it later or look for other antibodies?
Questions people ask
Can I have NMOSD if my AQP4 result is negative?
Yes. Some people with NMOSD do not have this antibody, or have levels the test cannot detect, for example after treatment. Your neurologist then relies more on symptoms and scans, and may test for other antibodies such as MOG-IgG.
Why might my clinician repeat or confirm the test?
Test types differ in how well they work. A low-level positive from an ELISA can be a false positive, so a second type of test may be used. A negative result may be repeated in 3 to 6 months if NMOSD is still suspected.
Is this the same as the MOG antibody test?
No. They are separate antibody tests for different targets. Your neurologist may order one or both.
Show the science and sources
For clinicians and researchers
AQP4 is the water channel on astrocyte end-feet, and AQP4-IgG is the serum biomarker for NMOSD. The 2015 International Panel for NMO Diagnosis recommends cell-based serum assays (microscopy or flow cytometry) whenever possible. In a pooled analysis they had a mean sensitivity of 76.7% and a 0.1% false-positive rate in a multiple sclerosis clinic cohort. Indirect immunofluorescence and ELISA had mean sensitivities of about 63% to 64%, and ELISA gave false positives in 0.5% to 1.3%, often at low titer. The panel urges caution with low-titer ELISA results in people whose symptoms are less typical, and recommends confirming with a different assay technique. In a blinded Mayo, Oxford and McGill comparison cited by Mayo, sensitivity was 77% for flow cytometry (M23 isoform), 73% for cell-based assay (M1), 60% for M1-ELISA, 53% for immunoprecipitation and 48% for tissue immunofluorescence, with specificity of 100% for all except the Mayo immunoprecipitation assay (97%). Mayo's serum flow cytometry assay reports negative or positive and can reflex to a titer. Samples taken after high-dose steroids, plasma exchange or immunosuppression can be falsely negative. NMOSD without detectable AQP4-IgG is diagnosed under stricter clinical and MRI criteria, and MOG-IgG is tested separately.
LOINC 43638-6 (primary), 95272-1, 95273-9, 68548-7, 43752-5. Specimen: serum.
Sources
- Neuromyelitis Optica (NMO)/Aquaporin-4-IgG Fluorescence-Activated Cell Sorting (FACS) Assay, Serum (test NMOFS), Mayo Clinic Laboratories
- International consensus diagnostic criteria for neuromyelitis optica spectrum disorders, Neurology (International Panel for NMO Diagnosis) (2015)
- LOINC codes and long common names verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.