Introduction
The Adrenal Washout Calculator helps doctors tell if an adrenal gland mass is a benign adenoma or something that needs more testing. It uses CT scan numbers called Hounsfield Units (HU) from three phases: pre-contrast, post-contrast, and a 15-minute delayed scan. From these values, it computes two key metrics: Absolute Percentage Washout (APW) and Relative Percentage Washout (RPW).
Adrenal adenomas wash out contrast dye faster than malignant tumors. An APW of 60% or higher or an RPW of 40% or higher strongly suggests the mass is a benign adenoma.2 A recent study found those cut-offs misclassified about a third of masses whose unenhanced attenuation was above 10 HU, and suggested 58% for RPW instead — a figure that still needs confirmation.2
Enter your CT attenuation values below. The calculator will show your washout percentages, compare them against adenoma thresholds, provide a step-by-step solution, and generate a structured report you can copy directly into your notes. Pre-contrast HU is optional. If left blank, only the Relative Washout will be calculated.
How to Use Our Adrenal Washout Calculator
Enter the Hounsfield Unit (HU) values from your adrenal CT scan phases below. The calculator will return the Absolute Percentage Washout (APW) and Relative Percentage Washout (RPW), a step-by-step solution, a visual chart, and a ready-to-use clinical report.
Pre-Contrast Attenuation (HU): Enter the HU value from the non-contrast CT phase. This field is optional. If you leave it blank, only the Relative Percentage Washout will be calculated. You need this value to get the Absolute Percentage Washout result.
Post-Contrast Attenuation (HU): Enter the HU value measured during the portal venous phase, taken 60 to 75 seconds after contrast injection. This field is required.
Delayed Attenuation (HU): Enter the HU value from the 15-minute delayed phase scan. This field is required.
Click Calculate to see your results. The tool will show whether the washout meets the standard adenoma thresholds: APW ≥ 60% or RPW ≥ 40%. Click Start Over to clear all fields and begin a new calculation.
What Is an Adrenal Washout Study?
An adrenal washout study is a CT scan test that helps doctors tell if a lump on the adrenal gland is harmless or possibly dangerous. The adrenal glands sit on top of your kidneys and make important hormones. Sometimes, a CT scan done for another reason finds a small mass on one of these glands. This is called an adrenal incidentaloma. Most of these lumps are benign (not cancer) and are called adenomas. But doctors need a way to tell them apart from other growths.
How Adrenal Washout Works
The test measures how fast contrast dye leaves the adrenal mass over time. Adenomas lose contrast dye quickly, while other types of masses hold onto it longer. A CT scanner takes images at up to three different times: before contrast is given (pre-contrast), shortly after contrast is injected (post-contrast at 60–75 seconds), and again after a 15-minute delay. The Hounsfield Unit (HU) number at each phase shows how dense the tissue is.
Absolute Washout vs. Relative Washout
Absolute Percentage Washout (APW) uses all three measurements: pre-contrast, post-contrast, and delayed. An APW of 60% or higher strongly suggests the mass is a benign adenoma.
Relative Percentage Washout (RPW) uses only the post-contrast and delayed values. This is helpful when a pre-contrast scan was not done. An RPW of 40% or higher suggests an adenoma.
Key Thresholds to Know
A pre-contrast value of 10 HU or less already points to a lipid-rich adenoma, even without a washout calculation.1 A high unenhanced attenuation raises concern for a malignant mass and needs further workup whatever the washout numbers show.
How to Use This Calculator
Enter the HU values from your CT report into the three fields above. The post-contrast and delayed values are required. The pre-contrast value is optional but lets the calculator find both APW and RPW. After you click Calculate, you will see the washout percentages, a step-by-step breakdown of the math, a comparison chart, and a ready-to-copy radiology report.
When evaluating adrenal masses, clinicians often assess kidney function in parallel. Our GFR Calculator and Creatinine Clearance Calculator can help estimate renal function, which is especially relevant before contrast administration.
Important: This calculator is a clinical decision-support tool. It does not replace the judgment of a qualified physician. All results should be interpreted by a radiologist or endocrinologist in the context of the full clinical picture.