The invention relates to of analyzing vector supernatants useful for transducing T cells destined for administration to a human subject. The invention also related to methods of analyzing transduced T cells destined for administration to a human subject. The invention provides a method of analyzing a genetically modified T cell to detect a contaminant, wherein the genetically modified T cell comprises a nucleic acid encoding a chimeric antigen receptor (CAR), wherein the CAR comprises an antigen binding domain, a transmembrane domain, a costimulatory signaling region, and a signaling domain. In one embodiment, the contaminant is at least one selected from the group consisting of endotoxin, mycoplasma, replication competent lentivirus (RCL), p24, VSV-G nucleic acid, HN gag, residual anti-CD3/anti-CD28 coated beads, mouse antibodies, pooled human serum, bovine serum albumin, bovine serum, culture media components, vector packaging cell or plasmid components, a bacterium and a fungus.
