The importance of adrenal function to survival in critically ill patients has been established; however, identifying the best method to diagnose adrenal insufficiency has been problematic. Multiple methods of determining adrenal function have been developed, each with its advantages and disadvantages. Serum-free cortisol levels are probably the most accurate, although obtaining this result is technically demanding. Cohen and colleagues investigated the feasibility of measuring tissue cortisol levels in burn patients and whether tissue cortisol levels could be used as a surrogate for plasma-free cortisol levels.