CPT code 81560 describes a laboratory-developed molecular assay and associated algorithm that measures CD154+ T–cytotoxic memory cell responses after incubating a pediatric liver or small–bowel transplant patient’s blood with donor and third-party leukocytes to derive a probability or risk score for allograft rejection; service type is a laboratory immunologic diagnostic test with algorithmic result interpretation, typically performed in a clinical immunology or transplant laboratory (hospital or reference lab) as the site of service.