Adrenal Washout Calculator

What Is Adrenal Washout?

Adrenal washout analysis is a CT-based technique used to characterize adrenal lesions detected incidentally on cross-sectional imaging — commonly called adrenal incidentalomas. These lesions appear in approximately 4–7% of abdominal CT scans, and the vast majority are benign adrenal adenomas. However, distinguishing adenomas from malignant processes such as adrenocortical carcinoma or metastatic disease is clinically essential, and CT washout analysis provides a validated, noninvasive method to do so.

The underlying principle is straightforward: benign adenomas release iodinated contrast medium rapidly after intravenous injection, whereas malignant lesions retain contrast longer. By measuring attenuation values — expressed in Hounsfield Units (HU) — at three time points on CT, radiologists can compute a percentage washout that reliably separates benign from malignant tissue in most clinical scenarios.

The Adrenal Washout Formulas

Absolute Percentage Washout (APW)

APW is the preferred and more accurate formula when an unenhanced (pre-contrast) CT image is available:

APW = [ (HU-enhanced − HU-delayed) ÷ (HU-enhanced − HU-unenhanced) ] × 100

An APW value of 60% or greater is consistent with a benign lipid-poor adrenal adenoma, even when unenhanced attenuation exceeds the 10 HU lipid-rich threshold.

Relative Percentage Washout (RPW)

RPW is used when no unenhanced phase is available — a common scenario when patients undergo contrast-enhanced CT without a dedicated pre-contrast series:

RPW = [ (HU-enhanced − HU-delayed) ÷ HU-enhanced ] × 100

An RPW value of 40% or greater suggests a benign adenoma. RPW carries slightly lower sensitivity than APW but remains a clinically reliable alternative.

Understanding the Input Variables

• Unenhanced (Pre-contrast) HU: Attenuation of the adrenal lesion before IV contrast injection. Lesions measuring 10 HU or less on unenhanced imaging are already consistent with lipid-rich adenomas and may not require washout calculation. This value is required only for APW.
• Enhanced (Portal Venous Phase) HU: Attenuation measured 60–90 seconds after intravenous contrast injection, capturing peak lesion enhancement. This value appears in both the APW and RPW formulas.
• Delayed Phase HU (10–15 min): Attenuation measured 10 to 15 minutes after contrast injection. Adenomas drop substantially in attenuation at this phase due to rapid washout, whereas malignant lesions retain enhancement.

Diagnostic Thresholds and Sensitivity Data

The validated cutoff values and their performance characteristics are as follows:

• APW ≥ 60%: Sensitivity approximately 88%, specificity approximately 96% for adrenal adenoma
• RPW ≥ 40%: Sensitivity approximately 82%, specificity approximately 92% for adrenal adenoma

Lesions that fall below these thresholds warrant further workup, which may include adrenal-protocol MRI, FDG-PET/CT, or tissue sampling depending on clinical context and the presence of a known primary malignancy.

Worked Clinical Example

A 55-year-old patient has an incidentally discovered right adrenal mass. CT attenuation values are: unenhanced HU = 28, enhanced HU = 115, delayed HU = 42.

APW = [(115 − 42) ÷ (115 − 28)] × 100 = [73 ÷ 87] × 100 = 83.9%

Because 83.9% exceeds the 60% APW threshold, the lesion is characterized as a benign lipid-poor adrenal adenoma. In the absence of a known primary malignancy, no further oncologic workup is required.

Factors Affecting Washout Analysis and Clinical Interpretation

Several patient and imaging variables can influence the interpretation of washout values. Renal function, particularly in patients with reduced glomerular filtration rate, may alter the kinetics of contrast washout; in such cases, delayed imaging may be performed at intervals longer than the standard 10–15 minutes to allow adequate differentiation. Lesion size and composition also play important roles: smaller lesions may exhibit noise-related variation in HU measurements, whereas lesions with areas of necrosis, calcification, or hemorrhage can yield unreliable washout percentages. Careful region-of-interest placement, standardized imaging protocols, and awareness of these technical factors are essential for accurate calculation and interpretation.

The complementary use of unenhanced imaging — particularly HU ≤ 10, which is pathognomonic for lipid-rich adenoma — provides additional confidence in benign characterization. When lesions are lipid-rich (HU ≤ 10 on unenhanced imaging), a diagnosis of adenoma can be rendered confidently even without washout analysis. Conversely, lesions that are lipid-poor (unenhanced HU > 10) but exhibit favorable washout (APW ≥ 60% or RPW ≥ 40%) are also reliably benign in non-oncologic patients. This integrated approach to imaging interpretation, combining attenuation thresholds with washout kinetics, optimizes the specificity and sensitivity of CT characterization.

Methodology and Authoritative Sources

The formulas in this calculator derive from landmark peer-reviewed studies in adrenal CT characterization. Korobkin et al. (Radiology, 1998) established the delayed enhanced CT protocol and first validated the APW threshold of 60% for differentiating benign from malignant adrenal masses in a cohort of surgically confirmed lesions. Caoili et al. (AJR, 2002) subsequently confirmed both APW and RPW thresholds in a prospective study, providing the sensitivity and specificity benchmarks now cited in radiology society guidelines. Additional clinical reference material is available at Radiopaedia's adrenal washout article and the Harvard MEEI Adrenal Washout Calculator.