Persistent flank or side pain — and when kidney cancer needs ruling out

It can, but pain is not the usual way kidney cancer shows itself. Most kidney cancers grow quietly and are found by chance on a scan arranged for something else. When a kidney tumour does cause pain, it is typically a dull, steady ache low on one side, between the lowest rib and the hip, that does not shift when you change position and does not settle over weeks. Sharp pain that comes in waves and travels towards the groin is far more often a kidney stone. A persistent one-sided ache still deserves a check, because the first test that looks at it is quick and painless.

The kidneys sit high at the back of the abdomen, tucked under the lowest ribs on either side of the spine. Pain coming from a kidney is therefore felt in the flank, the band between the bottom rib and the top of the hip, and it can wrap round towards the front or up under the ribs. It is usually deeper and duller than muscle pain, does not ease when you shift position, and is not clearly worsened by bending or lifting. Pain in the small of the back, at belt level, is much more often muscular or spinal than kidney related.

Three things help. Position: muscle and spine pain usually changes with movement, bending or lifting and eases with rest, while kidney pain tends to stay much the same whatever you do. Site: muscle pain is often central and low, while kidney pain sits higher and to one side, under the ribs. Company: kidney pain is more likely to arrive with fever, burning on passing urine, blood in the urine, nausea, or weight loss you cannot explain. None of this is a diagnosis. If a one-sided ache has lasted more than two or three weeks, or comes with any of those extras, have it examined.

It is the combination that matters. Blood in the urine, whether you can see it or it is picked up on a dipstick test, should always be checked, even if it happens only once, clears on its own and causes no pain at all. Alongside a persistent one-sided ache it is a clear reason to arrange imaging promptly rather than wait and watch. Most people who have both turn out to have a stone or an infection rather than a cancer. The tests that separate those causes are simple, and doing them early is what makes an early diagnosis possible.

An ultrasound of the kidneys is almost always the first scan. It uses no radiation, needs no injection, takes minutes, and is good at showing a stone, a swollen kidney or a solid mass. If the ultrasound is clear and the examination points to a muscular or spinal cause, that is often the end of it. If it shows something, or the picture is not conclusive, a contrast CT scan follows, because NCCN guidance treats contrast CT or MRI as the standard way to characterise a kidney mass. At CION the ultrasound, CT, MRI and the blood and urine tests are all arranged in-house.

If the pain is severe, comes with a high fever and shaking chills, or you cannot pass urine, seek care the same day. Otherwise a one-sided ache that has not settled after two or three weeks is worth an appointment, and one that arrives with blood in the urine, a fever that keeps returning, a lump you can feel in the side, or unexplained weight loss should be seen sooner. Waiting a few days after an obvious strain is sensible. Waiting months for an ache that has quietly persisted is not, because the check itself is easy.