Inferior petrosal sinus sampling is a relatively new approach to the diagnosis of Cushing's disease.
In essence, it tests to see the source of the raised ACTH levels in a patient with diagnosed Cushing's syndrome and high or normal serum ACTH levels. The inferior petrosal sinus is where the pituitary gland drains. Therefore, a sample from here showing raised ACTH compared to the periphery suggests that it is a pituitary cause of Cushing's, i.e. Cushing's disease. Equivocal levels of ACTH indicate ectopic or Paraneoplastic Cushing's Syndrome. The sample is usually taken after administration of Corticotropin-releasing hormone or, more recently, DDAVP, which have been shown to increase ACTH production in active ACTH-producing pituitary tumors. Increasingly, it is known as a gold-standard method for diagnosing Cushing's disease.