Validated Criteria for the Interpretation of a Single Measurement of Serum Cortisol in the Investigation of Suspected Adrenal Insufficiency

Scott D. Mackenzie; Robert M. Gifford; Luke D. Boyle; Mike S. Crane; Mark W. J. Strachan; Fraser W. Gibb


Clin Endocrinol. 2019;91(5):608-615. 

In This Article

Abstract and Introduction


Objective: The diagnostic value of a single measurement of serum cortisol as a first step in the investigation of suspected adrenal insufficiency remains unclear. Previously proposed criteria have not been validated, and little is known regarding the performance of the test outwith morning samples in outpatients. We aimed to identify and validate criteria for morning and afternoon serum cortisol which could be used to determine which individuals require dynamic testing, in both outpatient and medical inpatient settings.

Methods: We performed a retrospective analysis of 2768 patients attending endocrinology clinics and patients admitted to general medical units in two hospitals in Edinburgh, UK. In baseline samples from the short synacthen test, thresholds which identified a subnormal-stimulated serum cortisol (<430 nmol/L using the Abbott Architect assay) with 95% sensitivity were identified. Criteria drawn from data in patients attending outpatient clinics in one hospital were tested in additional outpatient and inpatient validation cohorts.

Results: A morning (8 AM-12 PM) serum cortisol of <275 nmol/L identified subnormal-stimulated cortisol with 96.2% sensitivity. For afternoon (12 PM-6 PM) samples, a cut-off of <250 nmol/L achieved 96.1% sensitivity. Sensitivity was maintained when the criteria were applied to outpatients in the validation cohort for both morning and afternoon samples. For inpatients, the test was sufficiently sensitive in morning samples only.

Conclusions: A single measurement of serum cortisol carries the potential to significantly reduce the need for dynamic testing in the investigation of adrenal insufficiency, whether this is taken in morning or afternoon outpatient clinics, or in morning samples from medical inpatients.


The short synacthen test (SST), comprising measurement of serum cortisol before and typically 30 minutes after the intramuscular injection of 250 μg ACTH1–24 (Synacthen®), is widely used in the diagnosis of adrenal insufficiency (AI). In comparison with the gold standard insulin tolerance test, the SST is less dangerous and demanding, and has been well validated.[1,2] However, it still carries a small risk of severe hypersensitivity reactions,[3] resulting in a Society for Endocrinology recommendation that the test is only performed in units where immediate resuscitation facilities are available.[4] Use of the SST as a first-line test in the investigation of suspected AI also carries significant resource implications, brought to light by well-documented periods of shortage of Synacthen in 2014 and again currently, resulting in a recent price rise to £60 in the UK per 250 μg vial.[5] The requirement for supervision by experienced staff adds a further cost to the test.

A single measurement of serum cortisol as an initial step in the investigation of AI therefore carries a number of advantages over dynamic testing. Although extensively studied,[6–18] this remains a contentious approach, and reports of its diagnostic utility, and suggested thresholds, vary considerably. This may reflect challenges in interpreting single measurements of serum cortisol due to its circadian and ultradian variability,[19] assay variation,[17] and the relative rarity of the condition relative to the size of the population tested, meaning a large cohort is required to draw reliable conclusions. Limitations of the existing literature also include the lack of validation of proposed criteria in additional cohorts. Furthermore, existing studies allow conclusions to be drawn in the relatively confined clinical setting of outpatients attending in the morning. The performance of the test in samples drawn in the afternoon has not been tested, and relatively little is known about the value of the test in the inpatient setting.[16,18]

We aimed to identify thresholds for basal serum cortisol that would be of use in determining which patients require further investigation to confirm or refute AI, and to determine the impact such a strategy on the requirement for dynamic testing. Additionally, we aimed to compare the performance of serum cortisol measured in morning and afternoon samples. Finally, our study aimed to validate proposed criteria using a second outpatient cohort, and a third cohort comprising general medical inpatients.