Haemorrhagic (bloody) kidney cyst — what it means, and when it needs watching

It is an ordinary kidney cyst that has bled into itself. A simple cyst is a thin-walled sac of clear fluid. Tiny vessels in its wall can leak, and the fluid then contains fresh or old blood, sometimes mixed with thick protein. Nothing has grown inside the cyst and nothing has invaded it. What has changed is only what the sac is filled with. Because blood and protein look different from water on a scan, the radiologist may describe the cyst as haemorrhagic, complicated or hyperdense. That wording describes the contents, not a diagnosis of cancer.

Usually it is not. Kidney cysts are common, they become more common with age, and a bleed into one is a recognised, benign event. Up to a third of small kidney masses turn out to be benign, and most kidney cancers that are picked up by chance on a scan are early and highly curable. What a specialist looks for is not the blood but whether any part of the cyst takes up contrast, and whether the walls or internal divisions are thickened or irregular. A cyst that is dense with blood but has thin, smooth walls and takes up no contrast is treated as benign.

Blood and thick protein are denser than the clear fluid inside a simple cyst, so they show up brighter on CT even before any contrast is injected. That is what hyperdense means in a report. It is easy to confuse with enhancement, but the two are different. Density is how bright something looks on its own. Enhancement is a rise in brightness after contrast is injected, and it means the tissue has its own blood supply. A cyst can be dense with old blood and still show no enhancement at all, which is reassuring rather than worrying.

Most cause nothing at all and are found by chance on a scan arranged for something else. When a cyst bleeds it can cause a sudden, dull ache low on one side that settles over days. Rarely, a cyst that leaks towards the collecting system can put visible blood in the urine. Any blood in the urine should be checked promptly, even a single painless episode that clears on its own, because the tests that sort out the cause are simple. Seek care the same day if the pain is severe, or comes with a high fever and shaking chills.

Most need neither. A cyst with thin smooth walls, at most a few hairline internal divisions and no enhancement is considered benign, and no repeat scan is needed for it. Cysts in the middle ground, where the divisions are slightly thicker or more numerous, are watched with repeat imaging at set intervals instead of being operated on, because stability over time is itself the answer. A cyst is only removed when the scan features suggest it could be a cancer. Draining a benign cyst is not usually helpful, because the fluid tends to come back.

When the scan shows enhancement. A thickened wall, a thick or irregular internal division, or a solid nodule that brightens after contrast all suggest living tissue with a blood supply rather than trapped blood, and those cysts are assessed as possible kidney cancers. A cyst that has clearly grown or changed shape since an earlier scan also needs review. At CION these cases go to the uro-oncology tumour board, and where an operation or an ablation is the right answer it is coordinated with specialist urology, uro-oncology and interventional radiology partners who deliver it.