The Corticotrophin Test in Critical Illness: Traps and Tricks

Abstract

The contribution of adrenal insufficiency to the morbidity of critically ill patients is currently under renewed scrutiny. Absolute adrenocortical insufficiency (diagnosed by very low plasma cortisol concentrations) is uncommon in the intensive care population. The diagnosis of relative adrenocortical insufficiency (RAI, elevated basal plasma cortisol with a subnormal increase in plasma concentrations following an adrenocorticotropic hormone [ACTH] stimulus) continues to generate much debate [1–3]. One of the most common dynamic testing procedures for assessment of adrenocortical function is the standard cosyntropin test (also known as the corticotrophin test, synacthen test, and ACTH stimulation test). The test comprises the measurement of plasma cortisol concentrations immediately prior, and at 30 and 60 minute intervals after, the intravenous administration of 250 μg of 1–24 ACTH. The normal response in unstressed volunteers is a rise in serum levels to above 500–550 nmol/l. In the setting of critical illness, the corticotrophin test has become the mainstay of diagnosis for suspected ‘relative’ adrenal insufficiency [4]. Although many diagnostic thresholds have been advocated, the most recent consensus guidelines suggest that an increase in measured cortisol of less than 250 nmol/l in response to the test is diagnostic of RAI. Early studies using these criteria suggested a strong association between the presence of RAI and poor outcome in patients with septic shock [5].