Normally, the sample used to test the presence of AQP4-Ab is blood serum, but the presence of AQP4-Ab in the CSF and not in the serum has been reported in some cases of NMO [25]; therefore, it could be clinically useful to test for the presence of CSF AQP4-Ab to consider a diagnosis of seronegative NMOSD. Here, AQP4 is linked to neuromyelitis optica.