A composition that binds to an anti-CD38 antibody includes a specific sequence of a recombinant soluble form of an extracellular domain of CD38 and/or a fragment thereof that interferes with binding activity of the anti-CD38 antibody. The composition can be included in a kit for bio-monitoring research and diagnostic assays. The composition can be used to neutralize an anti-CD38 antibody in a sample and/or to select a suitable red blood cell unit for a patient treated with anti-CD38 antibodies.