AQP4 and myeloid sarcoma: Even though the presence of AQP4-Ab is essential for the diagnosis of AQP4-Ab-seropositive NMOSD, clinical and radiological differentiation between AQP4-Ab-seropositive NMOSD and MS remains crucial for the following reasons: (i) clinicians need to identify patients on whom the AQP4-Ab test should be performed; (ii) the result of an AQP4-Ab assay can be influenced by assay methodology and the patient's clinical status (3, 7, 8).