Adrenal Washout Calculator
Adrenal washout analysis helps radiologists tell the difference between benign adenomas and other adrenal masses on CT. Enter the Hounsfield unit (HU) values from the pre-contrast, portal venous, and 15-minute delayed phases to calculate both absolute and relative washout percentages.
Adrenal incidentalomas show up on about 4-5% of abdominal CT scans. Most are benign adenomas, but the challenge is distinguishing them from metastases, pheochromocytomas, and adrenocortical carcinomas without invasive biopsy.
The washout technique relies on two formulas:
- Absolute washout = (Portal Venous HU - Delayed HU) / (Portal Venous HU - Pre-contrast HU) x 100
- Relative washout = (Portal Venous HU - Delayed HU) / Portal Venous HU x 100
Adenomas enhance with contrast but wash out quickly because of their rich intracellular lipid content. Non-adenomas tend to hold onto contrast longer.
Key thresholds from the radiology literature:
- Pre-contrast <= 10 HU: Diagnostic for lipid-rich adenoma (sensitivity ~71%, specificity ~98%). No washout study needed.
- Absolute washout >= 60%: Consistent with adenoma (sensitivity ~88%, specificity ~96%)
- Relative washout >= 40%: Consistent with adenoma (sensitivity ~83%, specificity ~93%)
Relative washout is useful when a pre-contrast scan was not performed. However, absolute washout is considered more accurate because it accounts for the baseline attenuation. Both thresholds together provide high diagnostic confidence.
Keep in mind that this tool is for educational reference. Clinical decisions should integrate the full imaging context, patient history, and biochemical workup.