Optimal Timing for AM Cortisol Draw
Draw AM cortisol at 8:00-9:00 AM (08:00-09:00 hours) for the most reliable assessment of adrenocortical function. 1
Rationale for 8:00-9:00 AM Timing
The 8:00-9:00 AM window captures the physiologic peak of cortisol secretion that occurs in the early morning hours, which is critical for accurate interpretation of adrenal function. 1
For diagnosing Cushing's disease, morning ACTH and cortisol should be drawn at 08:00-09:00 hours, as this timing provides optimal sensitivity (68%) and specificity (100%) for detecting ACTH-dependent hypercortisolism when using a cutoff of >1.1 pmol/L (>5 ng/L). 1
For evaluating adrenal insufficiency, the 8:00 AM cortisol measurement is the standard reference point used in diagnostic algorithms, including the overnight dexamethasone suppression test where cortisol is measured at 8:00 AM following 11:00 PM dexamethasone administration. 1, 2
Critical Pre-Collection Requirements
Patients must be awake and upright for at least 30-60 minutes before the blood draw to ensure accurate cortisol measurement, as cortisol levels surge immediately upon awakening (the "cortisol awakening response"). 3
The cortisol awakening response causes a 50-75% increase in free cortisol levels within the first 30 minutes after waking, which can confound interpretation if sampling occurs too early. 3
Having the patient arrive at the laboratory by 7:30 AM and drawing blood at 8:00 AM allows adequate time for this awakening surge to stabilize. 3
Important Clinical Caveats
Shift workers and patients with disrupted circadian rhythms should not have AM cortisol testing, as their cortisol peaks occur at different times and will yield unreliable results. 1, 2
- In these populations, alternative testing such as the overnight dexamethasone suppression test is preferred over single AM cortisol measurements. 2
Medications affecting cortisol binding globulin (CBG) must be documented, including oral estrogens, pregnancy status, and chronic active hepatitis, as these increase total cortisol levels and can lead to false interpretation. 1
- Patients on oral contraceptives may show lower early morning cortisol levels. 3
Recent physical or mental stress significantly elevates cortisol, so patients should avoid strenuous exercise for 24-48 hours before testing and should not undergo cognitive assessments immediately before blood draw. 1
Afternoon Cortisol: When AM Timing Is Impossible
If an 8:00-9:00 AM draw is logistically impossible, an afternoon cortisol >234 nmol/L (on Abbott analyzer platforms) effectively excludes adrenal insufficiency with 100% sensitivity. 4
However, afternoon cortisol levels <234 nmol/L require follow-up with a short Synacthen test, as they cannot reliably diagnose or exclude adrenal insufficiency due to the normal circadian decline in cortisol. 4
The acrophase (peak time) of cortisol rhythm varies by chronotype, occurring approximately 55-68 minutes earlier in morning chronotypes ("larks") compared to evening chronotypes ("owls"). 5
Integration with ACTH Measurement
When measuring both ACTH and cortisol together (essential for distinguishing primary from secondary adrenal insufficiency), both must be drawn simultaneously at 8:00-9:00 AM from the same blood draw. 1
ACTH is extremely labile and requires immediate processing on ice, so coordinate with the laboratory before drawing to ensure proper handling. 1
In primary adrenal insufficiency, ACTH will be elevated (>1.1 pmol/L) with low cortisol, while in secondary (central) adrenal insufficiency, both ACTH and cortisol will be low. 1