Petrosal Sinus Sampling
This test can be used to accurately distinguish pituitary and ectopic sources of ACTH causing Cushing's syndrome. The principle of the test is to sample the blood from the petrosal sinuses, draining the pituitary gland, to compare the levels of ACTH with those found in the peripheral blood. A petrosal: peripheral ratio of > 2, indicating excess ACTH from the pituitary, is necessary to diagnose Cushing's disease with confidence. Accuracy can be improved using CRH stimulation to exaggerate the difference.