The time course of complement blockage by ravulizumab in neuromyelitis optica spectrum disorder

The classical 50% hemolytic complement test is commonly used to monitor complement function. However, its use in monitoring the effect of ravulizumab on complement assays is unknown. A 47‐year‐old man with anti‐aquaporin 4 antibody‐positive neuromyelitis optica spectrum disorder who was treated with oral prednisolone and azathioprine experienced several episodes of optic neuritis or myelitis. After his latest relapse, ravulizumab was administered to prevent relapse. Serum classical 50% hemolytic complement activity decreased below detectable levels (<10 U/mL) within 1 h of ravulizumab injection. Eighteen weeks later, the patient showed no adverse events or relapse, suggesting that serum classical 50% hemolytic complement activity can monitor ravulizumab effect. In Japan, where 3.5% of the population shows a reduced response to anti‐C5 therapy, serum classical 50% hemolytic complement activity proves useful in confirming ravulizumab efficacy.