A Case of Myelin Oligodendrocyte Glycoprotein Antibody‐Associated Disease With a False‐Positive Aquaporin‐4 Antibody Test

ABSTRACT Antibodies to aquaporin‐4 (AQP4‐Ab) are sensitive and highly specific serum markers of neuromyelitis optica spectrum disorder (NMOSD). A 32‐year‐old man presenting with left optic neuritis was diagnosed with NMOSD on the basis of AQP4‐Ab positivity by enzyme‐linked immunosorbent assay (ELISA). Three years later, ELISA AQP4‐Ab was also positive when he presented with left leg involuntary movement with right frontal medial cortex hyperintensity on magnetic resonance imaging (MRI). Three years thereafter, he had left facial involuntary movement with hyperintense lesions from the right temporal lobe to the insular cortex on MRI. His serum was negative for AQP4‐Ab by ELISA and cell‐based assay (CBA), but positive for myelin oligodendrocyte glycoprotein (MOG) antibodies by CBA. We retested his preserved serum; the CBA test was negative for AQP4 but positive for MOG antibodies throughout the entire period. The ELISA AQP4‐Ab test result was a false positive, and the patient was diagnosed with MOG antibody‐associated disease. The results of ELISA AQP4‐Ab should be interpreted with caution, especially at low titers.