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Cortisol

Cortisol is the primary glucocorticoid of the adrenal cortex. Ten to 20% of circulating cortisol is loosely attached to albumin and 70 or 80% is bound to transcortin. The remaining 10% is unbound. Total serum cortisol is measured in the laboratory. Serum cortisol levels are lowest about 04:00, rise to their highest levels about 08:00, and then fall throughout the day to a nadir of 50% of the peak level by 20:00 hours. Obese patients tend to have increased cortisol production.

In patients with Cushing's syndrome, cortisol secretion is increased and the normal circadian rhythm is absent. If the 16:00 cortisol level is greater than 75% of the 08:00 level, cortisol secretion is considered to be abnormal. This criterion has a sensitivity and specificity for Cushing's syndrome of 70% and 82% respectively. If a cut off point for the 16:00 cortisol level of greater than 50% of the 08:00 level is used, then all patients with Cushing's syndrome will lack diurnal variation. However, approximately one third of obese patients will also not exhibit diurnal variation.

Random measurements of plasma cortisol are of little value in evaluating the patient with suspected adrenal cortical insufficiency because of the diurnal variation and the effects of stress.

With the increasing use of steroid replacement therapy for critically ill patients, an important issue is the crossreactivity of steroid medications with the cortisol assay. The following table summarizes the degree of crossreactivity with cortisol on the Bayer Centaur analyzer. This type of study is accomplished by comparing cortisol levels in a serum that has been spiked with a particular steroid with the same sample that has not been spiked.

Steriod	% Crossreactivity
Corticosterone	2.8
Cortisone	7.4
Dexamethasone	0.2
Fludrocortisone	1.2
Hydrocortisone	0.5
Methylprednisolone	20.9
Prednisolone	27.0
Prednisone	6.6

As demonstrated in the table, treatment of patients with dexamethasone, fludrocortisone or hydrocortisone does not significantly affect cortisol measurements.

Reference range for a cortisol drawn at 08:00 is 5-25 ug/dL.
Reference range for a cortisol drawn at 16:00 is 2-17 ug/dL.

Specimen requirement is one SST tube containing a minimum of 2 mL of blood.