An internal standard for LC-MS/MS testing of the immunosuppressants tacrolimus, sirolimus, and everolimus. This Certified Spiking Solution® is suitable for critical quantitative applications in clinical and diagnostic testing such as therapeutic drug monitoring assays to ensure patients remain within the narrow therapeutic range of these immunosuppressants. The Snap-N-Spike® format provides laboratories the ability to spike into their matrix of choice just before use and allows them to eliminate weighing operations of hazardous substances which significantly reduces occupational exposure hazards related to handling acutely toxic powders such as immunosuppressants.