# What is the adrenal washout percent?

Works out the absolute and relative contrast washout of an adrenal mass from unenhanced, enhanced, and delayed CT attenuation, and compares them with the published adenoma thresholds.

- Page: https://www.acalculator.org/health/adrenal-washout-calculator
- JSON spec: https://www.acalculator.org/health/adrenal-washout-calculator.json
- Version: 9fff7e39d7e8

## Default answer

Example with the default inputs (Unenhanced (HU) 20, Enhanced (HU) 80, Delayed (HU) 30): The absolute washout is 83.3% (60% or more: in the adenoma range) and the relative washout is 62.5% (40% or more: in the adenoma range).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| pre | Unenhanced (HU) | Attenuation of the mass before contrast. In Hounsfield units (HU), -100 to 100. |
| enh | Enhanced (HU) | Attenuation in the portal venous phase, about 60 s after contrast. In Hounsfield units (HU), -100 to 400. |
| del | Delayed (HU) | Attenuation on the delayed scan, 15 minutes after contrast. In Hounsfield units (HU), -100 to 400. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| apw | Absolute washout (%) | (Enhanced − delayed) ÷ (enhanced − unenhanced) × 100, to 1 decimal. |
| rpw | Relative washout (%) | (Enhanced − delayed) ÷ enhanced × 100, to 1 decimal. |
| apwReading | Absolute washout | Against the 60% threshold. |
| rpwReading | Relative washout | Against the 40% threshold. |
| preReading | Unenhanced | Against 10 HU. |

## Method

Absolute washout = (enhanced − delayed) ÷ (enhanced − unenhanced) × 100. Relative washout = (enhanced − delayed) ÷ enhanced × 100. Adenoma range: absolute 60% or more, relative 40% or more, or unenhanced 10 HU or less.

## Assumptions

- The three values are region-of-interest means of the same mass, in Hounsfield units, from an adrenal protocol CT with a 15-minute delay.
- Each washout is rounded once to 1 decimal, halves away from zero, and the shown value is compared with its threshold.

## Worked examples

1. pre = 20, enh = 80, del = 30 gives apw = 83.3, rpw = 62.5, apwReading = 60% or more: in the adenoma range, rpwReading = 40% or more: in the adenoma range, preReading = Over 10 HU: washout decides. Source: Caoili 2002 formulas: 50 ÷ 60 × 100 = 83.33.
2. pre = 30, enh = 100, del = 75 gives apw = 35.7, rpw = 25, apwReading = Under 60%: not in the adenoma range, rpwReading = Under 40%: not in the adenoma range.
3. pre = -5, enh = 60, del = 20 gives apw = 61.5, rpw = 66.7, preReading = 10 HU or less: in the lipid-rich adenoma range.
4. pre = 35, enh = 90, del = 60 gives apw = 54.5, rpw = 33.3.
5. pre = 30, enh = 80, del = 90 gives apw = -20, rpw = -12.5, apwReading = Under 60%: not in the adenoma range.

## FAQ

### How is adrenal washout calculated?

Absolute washout = (enhanced − delayed) ÷ (enhanced − unenhanced) × 100. Relative washout = (enhanced − delayed) ÷ enhanced × 100. All three values are CT attenuations of the mass in Hounsfield units (HU).

### What washout suggests an adrenal adenoma?

An absolute washout of 60% or more and a relative washout of 40% or more are the usual adenoma thresholds. With the 60% rule and an unenhanced cut-off of 10 HU, Caoili et al. (2002) characterized 98% of adenomas and 92% of other masses correctly.

### Why does an unenhanced value of 10 HU or less matter?

Fat lowers the CT value. A mass of 10 HU or less before contrast is in the lipid-rich adenoma range, so Caoili et al. did not need a washout scan for it.

### When do I use the relative washout?

When there is no unenhanced scan. Relative washout needs only the enhanced and delayed values.

### Can the washout be negative?

Yes. If the delayed value is higher than the enhanced value, the mass keeps gaining contrast and the washout is below 0, which is not in the adenoma range.

### Are the thresholds certain?

No. The 2023 European Society of Endocrinology guideline says the evidence for the 60% and 40% cut-offs in incidentally found masses is weak, and some tumors that are not adenomas also wash out fast (Tay 2026). A radiologist reads the washout with the size and look of the mass.

## Sources

- Caoili EM, Korobkin M, Francis IR, et al. Adrenal masses: characterization with combined unenhanced and delayed enhanced CT. Radiology. 2002;222(3):629-633. https://doi.org/10.1148/radiol.2223010766 (retrieved 2026-10-05)
- Tay BHB, Sim JZT, Low G. Pitfalls of adrenal washout on CT in the adult population: a pictorial review. Diagnostics (Basel). 2026;16(6):920. https://doi.org/10.3390/diagnostics16060920 (open at https://pmc.ncbi.nlm.nih.gov/articles/PMC13025807/, retrieved 2026-10-05)
- Peña CS, Boland GW, Hahn PF, et al. Characterization of indeterminate (lipid-poor) adrenal masses: use of washout characteristics at contrast-enhanced CT. Radiology. 2000;217(3):798-802. https://doi.org/10.1148/radiology.217.3.r00dc29798 (retrieved 2026-10-05)