# Adrenal Washout Calculator

Calculate absolute and relative adrenal washout percentages from CT attenuation values. Differentiate adrenal adenomas from non-adenomas using standard washout criteria.

## What this calculates

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.

## Inputs

- **Pre-contrast HU** (HU) — min -200, max 200 — Attenuation on unenhanced CT (Hounsfield units)
- **Portal Venous Phase HU** (HU) — min -100, max 400 — Attenuation at 60-70 seconds post-contrast
- **15-min Delayed Phase HU** (HU) — min -100, max 400 — Attenuation at 15 minutes post-contrast

## Outputs

- **Absolute Washout** — Absolute percentage washout (requires pre-contrast phase)
- **Relative Washout** — Relative percentage washout (no pre-contrast needed)
- **Absolute Washout Interpretation** — formatted as text — Whether the absolute washout suggests adenoma
- **Relative Washout Interpretation** — formatted as text — Whether the relative washout suggests adenoma
- **Unenhanced CT Interpretation** — formatted as text — Assessment based on pre-contrast attenuation alone

## Details

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 = 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.

## Frequently Asked Questions

**Q: What is the difference between absolute and relative washout?**

A: Absolute washout uses all three CT phases (pre-contrast, portal venous, and delayed) and is more accurate. Relative washout only needs the portal venous and delayed phases, making it useful when a pre-contrast scan was not obtained. Absolute washout >= 60% or relative washout >= 40% suggests an adenoma.

**Q: When is a washout study not needed?**

A: If the pre-contrast attenuation of the adrenal nodule is 10 HU or less, the mass is almost certainly a lipid-rich adenoma, and no washout analysis is necessary. This threshold has a specificity of about 98%. Nodules smaller than 1 cm are also generally considered too small to characterize and are typically followed with imaging.

**Q: Can all adenomas be detected by washout?**

A: No. About 10-40% of adenomas are lipid-poor and may have pre-contrast attenuation above 10 HU. Most of these can still be identified by washout analysis, but a small percentage of lipid-poor adenomas will have atypical washout. In these cases, chemical-shift MRI or PET-CT may help with diagnosis.

**Q: What timing is used for the delayed phase?**

A: The standard protocol uses a 15-minute delay after contrast injection. The portal venous phase is typically obtained at 60-70 seconds. Using different timing (for example, a 10-minute or 20-minute delay) may affect the accuracy of the washout thresholds, since the published cutoffs were validated at 15 minutes.

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Source: https://vastcalc.com/calculators/health/adrenal-washout
Category: Health & Fitness
Last updated: 2026-04-08