This test measures the level of adrenocorticotropic hormone (ACTH) in blood plasma at any time of day, without being restricted to a specific morning or evening schedule. It is less accurate than timed (morning/evening) tests for diagnosing circadian rhythm disorders (such as Cushing's syndrome), but it is useful in the initial or emergency assessment of suspected adrenal insufficiency or hyperactivity, especially when combined with a simultaneous measurement of cortisol levels.