Adrenal Washout (CT) Calculator

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The Adrenal Washout Calculator works out the absolute and relative contrast washout of an adrenal nodule from CT attenuation values (in Hounsfield units). High washout is characteristic of a benign adrenal adenoma. Enter the unenhanced, enhanced (portal venous), and delayed HU values.

Rates and fees change frequently. Always confirm the current figures with the official authority before relying on this result.

Formula

APW = (Enhanced − Delayed) / (Enhanced − Unenhanced) × 100; RPW = (Enhanced − Delayed) / Enhanced × 100
  • Absolute percentage washout (APW) needs an unenhanced scan: APW = (enhanced − delayed) / (enhanced − unenhanced) × 100.
  • Relative percentage washout (RPW) is used when no unenhanced scan is available: RPW = (enhanced − delayed) / enhanced × 100.
  • APW ≥ 60% or RPW ≥ 40% is the usual threshold suggesting a benign lipid-poor adenoma.
  • Attenuation values are measured in Hounsfield units (HU); enhanced is typically the 60-second portal venous phase and delayed is at about 15 minutes.

Example Calculation

Inputs
  • Unenhanced attenuation (HU): 10
  • Enhanced / portal venous (HU): 80
  • Delayed (15-min) attenuation (HU): 30

With unenhanced 10 HU, enhanced 80 HU, delayed 30 HU: APW = (80−30)/(80−10) ≈ 71.4% and RPW = (80−30)/80 = 62.5% — both meet the benign adenoma thresholds.

Frequently asked questions

What does adrenal washout tell you?
It measures how quickly an adrenal nodule loses contrast over time on CT. Benign adenomas wash out contrast rapidly, while metastases and other lesions retain it.
What are the thresholds?
An absolute washout (APW) of 60% or more, or a relative washout (RPW) of 40% or more, is generally consistent with a benign adenoma.
When do I use relative instead of absolute washout?
Use relative washout (RPW) when no unenhanced (pre-contrast) scan is available, since APW requires the unenhanced HU value.
What scan phases are needed?
Typically an unenhanced scan, an enhanced scan around 60 seconds (portal venous), and a delayed scan at about 15 minutes.
Is this a diagnosis?
No. Washout supports characterisation but must be interpreted by a radiologist alongside the full clinical picture.