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Cortisol

Cortisol is the primary glucocorticoid of the adrenal cortex. In the circulation, 80 to 90% of cortisol is bound to corticosteroid binding globulin (CBG) and 7% to albumin. The remaining 3% is unbound. Clinical laboratory measurement of cortisol includes all three forms.

Cortisol secretion has a pronounced diurnal variation. Concentrations are highest in the morning, around 08:00, and lowest at midnight. 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. 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.

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 performed. The following table summarizes the degree of crossreactivity with cortisol measured on a Siemens 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 ranges vary by the time of day.

 

Time

Concentration

Morning

7-25 ug/dL

Evening

2-9 ug/dL

Midnight

0-5 ug/dL

 

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

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