CPT® Code 86053

Aquaporin-4 (AQP4) antibodies serve as a highly specific and sensitive serological marker for neuromyelitis optica (NMO), which is an autoimmune, inflammatory, demyelinating disease affecting the central nervous system (CNS). NMO is recognized as a spectrum disorder that primarily impacts the optic nerves and spinal cord, but it can also involve the brain. The presence of these antibodies is critical as they can lead to severe clinical manifestations, including blindness, weakness or paralysis in the limbs, uncontrollable vomiting or hiccups, painful spasms, and dysfunction of the bladder or bowel. In pediatric patients, NMO may present with seizures or even coma. Flare-ups of the disease can result in irreversible vision loss and mobility issues. The antibodies, often referred to as NMO-IgG or AQP4, represent the first biomarker identified for any form of CNS inflammatory demyelinating disease. Their detection is essential for distinguishing NMO from multiple sclerosis (MS), as AQP4 antibodies are typically present in NMO but absent in classical MS. The testing process involves obtaining a venous blood sample from the patient, which is then analyzed using flow cytometry, specifically fluorescence-activated cell sorting (FACS). This advanced technique allows for the sorting of individual cells based on their characteristics. In the laboratory, human embryonic kidney (HEK) cells are modified to express green fluorescent protein (GFP)-tagged AQP4 and are mixed with untransfected HEK cells. The patient's serum sample is introduced, followed by the addition of a secondary anti-human antibody that is conjugated with fluorescein isothiocyanate, which binds to the human antibodies present in the serum. As the cells pass through a laser beam, their fluorescence is measured, and they are sorted based on whether they carry the fluorescently tagged human antibody. Additional titration studies may be conducted to determine the dilution factor at which specific fluorescence remains detectable, providing a semi-quantitative result for the presence of AQP4 antibodies.