Specimen requirement

Adult patient:

  Clotted blood tube (Serum gel separator clot activator tube)

Minimum blood volume: 5 mL

Paediatric patient:

Paediatric clotted blood tube

Number of vials: 2

Minimum blood volume: 1.6 mL

Authorisation code required

No

24 Hr available service

No

Precaution

Sample collection time must be noted at the time of collection.

Method

Immunoassay

Reference interval

Morning cortisol: 166 - 507 nmol/L (5th - 95th percentile)
Afternoon cortisol: 74 - 291 nmol/L   (5th - 95th percentile)

(Source: from manufacturer's kit insert 2015-11, V2.0)

Clinical indication

Assessment of adrenocortical function as part of a suppression or stimulation test

See overnight dexamethasone suppression test, low dose dexamethasone suppression test, short synacthen test, glucagon stimulation test and insulin stimulation test.

Random cortisol assay is only indicated as an initial test to exclude pituitary insufficiency or in the confirmation of Addisonian crisis.

Result interpretation

Effective from 21 Apr 2016, a new cortisol assay traceable to LC-MS method is used. New assay results are ~ 20% lower than that of the old assay.

1. Low dose short synacthen test could be considered to confirm or exclude adrenal insufficiency if the new assay 'Morning Cortisol' result is between 80 and 380 nmol/L (for in patients), 100 and 330 nmol/L (for out-patients), respectively.
   
   Values of 80 and 380 (new assay) correspond to 92 and 494 (old assay); 100 and 330 (new assay) correspond to 124 and 428 (old assay) nmol/L.  (Local population derived decision cutoff, JAFES 2015; 30: 147 - 154)
   
   
2. Effective from 21 Apr 2016, a new cortisol assay traceable to LC-MS method is used. New assay results are ~20% lower than that of the old assay. Values of 427 and 384 nmol/L correspond to the conventional 550 and 500 nmol/L decision cutoff, respectively.
   
   
3. In overnight dexamethasone suppression test, cortisol level < 50 nmol/L indicates a negative screening test and endogenous Cushing's syndrome is very unlikely.
   
   
4. A random serum cortisol is rarely informative. The presence of a normal diurnal rhythm (9 - 10 am and 11 pm - 12 mid-night) is strong evidence against Cushing's syndrome. Loss of diurnal rhythm occurs very early in the development of Cushing's syndrome but can also occur in acute stress, endogenous depression, gross obesity, alcoholism and hyponatremia.

Measurement of uncertainty

See table.

Frequency of measurement

Daily