What does a milk-of-calcium cyst on renal ultrasound indicate, and does it cause a reduction in estimated glomerular filtration rate (eGFR)?

Milk-of-Calcium Cysts: Benign Finding with No Impact on eGFR

Milk-of-calcium cysts in the kidney are benign, asymptomatic findings that do not cause reduction in eGFR and require no treatment.

What Are Milk-of-Calcium Cysts?

Milk-of-calcium (MOC) cysts are small renal cysts or calyceal diverticula containing a colloidal suspension of calcium crystals. These are quite common—one study identified 91 such cysts in 61 patients over approximately 4.5 years 1.

Characteristic Ultrasound Features:

• Echogenic material layering in the dependent (bottom) portion of the cyst
• Reverberation echoes without shadowing (when MOC is in small amounts)
• Posterior shadowing when larger amounts of calcium are present 1
• Ultrasound detects MOC more readily than radiography or CT 1

Important caveat: In rare cases where a cyst is completely filled with MOC, it may paradoxically appear as a simple cyst on ultrasound (sonolucent with enhanced through transmission), which can be misleading 2.

Clinical Significance and Management

MOC cysts are typically asymptomatic and require no treatment 1. The key clinical importance is distinguishing them from other lesions that would require intervention:

• Not renal calculi (stones)—though they can be mistaken for stones on routine imaging 3
• Not angiomyolipomas or other solid masses
• Not malignant lesions

Diagnostic Pitfall to Avoid:

MOC cysts can masquerade as renal calculi on routine radiography and supine ultrasound, potentially leading to unnecessary surgical intervention 3. If there's diagnostic uncertainty:

• Obtain upright plain films showing characteristic half-moon contour
• Consider CT imaging to demonstrate typical calcific suspension layering 3

Does MOC Cause eGFR Reduction?

No, milk-of-calcium cysts do not cause eGFR reduction. The provided evidence addresses simple renal cysts (SRC) and their relationship to eGFR, but MOC cysts are fundamentally different entities:

Simple Renal Cysts vs. MOC Cysts:

While recent research shows that multiple and large simple renal cysts (≥2 cysts, ≥2 cm) are associated with eGFR decline 4, 5, this relationship does not apply to MOC cysts because:

1. MOC cysts are typically small and represent a specific benign variant
2. MOC cysts are asymptomatic and don't indicate underlying renal dysfunction 1
3. The association between simple cysts and eGFR decline likely reflects age-related kidney changes or underlying chronic kidney disease, not a causal relationship from the cysts themselves

Key Distinction:

The evidence showing eGFR decline with simple renal cysts 4, 5 studied large cohorts where cyst burden correlated with declining kidney function—but MOC cysts are a distinct radiologic finding that represents calcium precipitation within an existing benign cyst, not a marker of progressive kidney disease.

Bottom line: Finding MOC on ultrasound should reassure you that this is a benign incidental finding requiring no intervention and no concern about kidney function impact.