Kidney cyst

3 things to know first

1. Simple kidney cysts are usually harmless. They are one of the most common findings on abdominal imaging.
2. They become more common with age. Many people over 50 have at least one kidney cyst without knowing it.
3. Only complex cysts usually need follow-up. A cyst with internal walls, solid parts, or irregular features may need closer evaluation.

What it is

A kidney cyst is a round, fluid-filled sac that forms in the kidney. Simple kidney cysts have thin walls, contain clear fluid, and do not communicate with the urine-collecting system. They are different from the cysts seen in polycystic kidney disease, which are numerous and often affect kidney function.

Most simple kidney cysts cause no symptoms and do not affect how the kidneys work.

Why it appears on your report

Kidney cysts are often found incidentally on ultrasound, CT, or MRI done for other reasons. The radiologist describes the cyst's size, location, and features. If it looks simple, the report may simply state that a benign-appearing cyst is present.

How serious is it

The vast majority of kidney cysts are not serious:

• Simple cysts (thin wall, clear fluid, no internal structures) are almost always benign and need no treatment.
• Minimally complex cysts may be followed with imaging to confirm stability.
• More complex cysts are often graded using the Bosniak classification. Higher Bosniak categories have a greater chance of being cancerous and may need specialist review.

Your doctor will consider the cyst's appearance, size, and whether it is growing.

When to seek care urgently

Simple kidney cysts rarely cause emergencies. Seek urgent care if you have:

• Severe flank or back pain with fever
• Blood in the urine with clots or severe pain
• Signs of infection such as shaking chills and high fever
• A sudden, severe headache with very high blood pressure (rare, in polycystic kidney disease)

These may indicate a cyst infection, bleeding, rupture, or another kidney problem.

What usually happens next

1. Confirm it is simple. The radiologist's description is usually enough. If the cyst has complex features, dedicated renal imaging may be recommended.
2. Assess kidney function. Blood or urine tests may be done if there are many cysts or concerns about kidney function.
3. Plan follow-up if needed. Simple cysts often need no follow-up. Complex cysts may be monitored with periodic imaging.
4. Consider treatment if symptomatic. Large cysts that cause pain, blockage, or infection can sometimes be drained or surgically treated.

A simple analogy

Think of a kidney as a grape cluster. A simple cyst is like a single grape that has filled with clear water instead of tissue. It sits quietly among the other grapes and usually does not affect how the whole cluster works.

Further reading

• MedlinePlus — Kidney cysts
• NHS — Kidney disease