Timing of Serum Cortisol Measurement After IV Hydrocortisone Administration
Serum cortisol levels should be measured at least 24 hours after the last dose of intravenous (IV) hydrocortisone to obtain accurate results reflecting endogenous adrenal function. 1
Rationale for 24-Hour Washout Period
- The American College of Clinical Oncology notes that therapeutic steroids interfere with cortisol assays, making cortisol measurements in patients on corticosteroids non-diagnostic 1
- For accurate assessment of endogenous adrenal function, hydrocortisone should be held for at least 24 hours before measuring serum cortisol 1
- This washout period is necessary to prevent exogenous hydrocortisone from being detected in the cortisol assay, which would lead to falsely elevated results 1, 2
Alternative Testing Approaches
- If a patient cannot safely discontinue hydrocortisone for 24 hours due to risk of adrenal crisis, a standard-dose ACTH stimulation test may be more appropriate than a single cortisol measurement 1, 3
- The standard ACTH stimulation test involves administration of 0.25 mg cosyntropin (synthetic ACTH) intramuscularly or intravenously, followed by measurement of serum cortisol after 30 and/or 60 minutes 4
- A normal response is defined as a cortisol value exceeding 550 nmol/L at either 30 or 60 minutes after ACTH administration 4, 5
Important Considerations for Cortisol Testing
- Morning cortisol measurements (7:30-8:00 AM) are preferred as cortisol levels are naturally highest at this time 1
- For patients with known adrenal insufficiency on hydrocortisone replacement, discontinuing medication for testing should only be done under medical supervision 1, 6
- If clinical suspicion of impending adrenal crisis exists, treatment with IV hydrocortisone should never be delayed for diagnostic procedures 4, 6
- Blood samples for cortisol and ACTH measurement should ideally be secured prior to treatment if possible, but diagnosis can be established later even if treatment has commenced 4
Special Circumstances
- In patients receiving long-term glucocorticoid therapy, pituitary-adrenal function cannot be reliably estimated from the dose of glucocorticoid, duration of therapy, or basal plasma cortisol concentration 2
- For patients requiring stress-dose hydrocortisone (such as during major surgery or severe illness), continuous IV infusion of hydrocortisone is the preferred method of administration 7
- When monitoring hydrocortisone replacement therapy, a single serum cortisol measurement taken 4 hours after oral hydrocortisone administration can predict cortisol area under the curve 8
By ensuring proper timing of cortisol measurement after discontinuation of IV hydrocortisone, clinicians can obtain accurate assessments of adrenal function, which is crucial for appropriate diagnosis and management of adrenal insufficiency.