A stable-labeled internal standard for the immunosuppressant Everolimus, marketed as Zortress® and Certican® for use in organ transplantation and Afinitor® for treatment of kidney and pancreatic cancers. Clinical laboratories quantitate everolimus levels in patient whole blood samples using LC-MS/MS. While many everolimus LC-MS/MS methods use 32-desmethoxyrapamycin or ascomycin as an internal standard, higher levels of variability / lower levels of reproducibility may occur in the analysis along with the potential for interferences. Use of a stable-labeled analog of everolimus is the best choice to minimize variability or mitigate any potential interference in LC-MS/MS analysis.