The ACTH 90-minute test is performed by administering synthetic ACTH and measuring adrenocorticotrophic hormone levels after 90 minutes. The test helps healthcare providers evaluate baseline adrenal gland function and adrenal response to synthetic ACTH stimulation by assessing levels at 90 minutes. It is an important adrenal function test, providing insights about the hypothalamic-pituitary-adrenal axis. A suboptimal increase in cortisol level indicates adrenal dysfunction, guiding treatment decisions.