Laboratory Test Reference Guide

Most commonly, an early morning cortisol is requested, typically collected between 0800 and 0900.

The reference interval for early morning cortisol applies to samples collected between 0600 and1000.

Samples collected outside of these hours will be reported without a reference interval.

For community-based patients, the earliest specimen collection time is governed by the operating hours of the outpatient phlebotomy service

0600 – 1000 hrs : 170 - 500 nmol/L

Samples collected outside this period will be reported with the following comment:

“The cortisol reference interval is not well defined for the time of collection.”

• Additional comments are added:

When cortisol is < 100 on a sample collected between 0600 – 1000:

“A cortisol result < 100 nmol/L is suspicious of adrenal insufficiency. Consider an endocrine opinion.”

When cortisol is > 300 on a sample collected between 0600 – 1000:

“In the absence of acute illness, this cortisol result suggests adrenal insufficiency is unlikely.”

Following Synacthen Stimulation (0.25 mg IM)

30 minutes:” A result ≥ 400 nmol/L suggests adequate adrenal reserve”

60 minutes:” A result ≥ 460 nmol/L suggests adequate adrenal reserve”

Following Dexamethasone Suppression (1mg oral given at 2300 hr) cortisol collected between 
0800 to 0900 hrs the cut-off for “normal suppression” is 50 nmol/L or below

(Implemented by Dr S du Toit on 21 September 2015 following discussion with Waikato Endocrinologists)

Please note that the lab neither supplies the synacthen nor performs the test. Please contact the Medical Day Care Unit or Pharmacy for further assistance.

0.2 mL plasma or serum.