Blood Tests Used for Diagnosis

Reviewed by: HU Medical Review Board | Last updated: June 2023

Diagnosing neuromyelitis optica spectrum disorder (NMOSD or NMO) can be challenging. In addition to physical exams, your doctor will order different blood tests. Because NMO can present in a similar way as other diseases, these lab tests are important to make sure you get the correct diagnosis.1

What blood tests are done for NMO?

Your doctor will order various blood tests to diagnose NMO. The 2 most common blood tests include:1,2

  • AQP4 antibody (AQP4-IgG)
  • MOG antibody (MOG-IgG)

Along with these tests, your doctor may order other blood tests to rule out:1

AQP4 and MOG antibodies

Antibodies are proteins the immune system makes to kill germs. In some diseases, antibodies can also be harmful. One type of antibody that causes inflammation in the brain targets a protein called aquaporin-4 (AQP4).3

AQP4 is found on brain cells called astrocytes and helps control water balance in the brain. The brain, spinal cord, and eye nerve (optic nerve) have AQP4. AQP4 antibodies (AQP4-IgG) attack these areas and damage the protective coating (myelin) of the brain cells, producing more proteins that lead to inflammation.3

While many with NMO test positive for AQP4 antibodies (AQP4-IgG), about 1 in 4 do not. This adds to the mystery behind the cause of the disease.1

Antibodies targeting another protein called myelin oligodendrocyte glycoprotein (MOG) can also be present in NMO. They are believed to cause similar optic nerve or spinal cord attacks in MOG antibody (MOG-IgG) NMO. However, symptoms of this condition develop differently and need to be treated differently from those of AQP4 antibody (AQP4-IgG) NMO. Men and women are equally likely to get MOG antibody (MOG-IgG) NMO. Children have this disease more often than they have AQP4 antibody (AQP4-IgG) NMO.1,3-5

If you test negative for AQP4 antibodies

Testing negative for AQP4 antibodies (AQP4-IgG) does not mean you do not have NMO. This can be frustrating for both you and your doctor. A negative result is called seronegative. Reasons for a negative AQP4 antibody (AQP4-IgG) test include:1

  • No AQP4 antibody (AQP4-IgG) exists in your body.This means that, although your symptoms may suggest NMO, another condition that mimics NMO is likely the cause.
  • The type of specimen tested did not contain the antibodies. For example, the fluid surrounding the spine (CSF) may be negative, but the blood serum is positive.
  • Your AQP4 test gave a false negative. Different people with NMO can have much different levels of AQP4 in their blood. Depending on the lab the test was sent to and the method used to test the blood, the test may come back negative even though you have NMO with a low level of antibodies. For this reason, your doctor may sometimes order blood tests more than once.

If you test positive for MOG antibodies

Despite advances in testing, doctors are still not sure the exact role that MOG antibodies (MOG-IgG) play in disease and inflammation. If you test positive for MOG antibodies (MOG-IgG), you will likely be diagnosed with MOG-IgG NMO.1,4

It is possible that MOG antibody (MOG-IgG) NMO and AQP4 antibody (AQP4-IgG) NMO will eventually be considered separate diseases, though this is debated. For now, they are considered subtypes of NMO.4,5

Your doctor will best guide you on your lab results.

What if both are negative?

Testing negative for both AQP4 (AQP4-IgG) and MOG (MOG-IgG) antibodies is not common if you have symptoms similar to those in NMO. However, this may happen. Again, your doctor will be able to guide you on your results and what your results might mean.4,5

Blood tests for NMO are important tools your doctor will use to help you get the diagnosis you need. Because other conditions can mimic NMO, these tests are helpful to rule out other diagnoses. This helps get you the treatment you need as soon as possible.

Treating NMO is key to preventing harmful attacks, so having the right tests done is the first step to a treatment plan that will be best for you.

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