Kidney pain most often comes from a kidney stone, an infection, or an injury, though several other conditions can be responsible. The pain typically shows up in your flank, the area on either side of your spine just below the ribs and above the hips. Unlike muscle-related back pain, kidney pain doesn’t get better or worse when you shift positions, and it generally won’t improve on its own without treatment.
Understanding what’s behind the pain matters because the causes range from a small stone that passes in a few days to infections and blockages that need prompt medical attention.
Why Kidneys Hurt
Pain in and around the kidneys comes down to two basic mechanisms: something is stretching the organ, or something is inflaming it. Each kidney sits inside a thin, tight-fitting capsule. When swelling, fluid backup, or a growing mass pushes against that capsule, the nerve fibers lining it fire pain signals. The pain tends to be constant and dull, felt deep in the flank and sometimes radiating forward into the lower abdomen.
The tubes connecting your kidneys to your bladder (ureters) generate a different kind of pain. When a stone or other obstruction blocks a ureter, the tube spasms against the blockage and stretches behind it. This produces sharp, wave-like pain that can radiate to the lower abdomen, groin, or inner thighs. The colicky, comes-and-goes quality of this pain is one of the hallmarks of a passing kidney stone.
Kidney Stones
Kidney stones are the most recognized cause of severe kidney pain. They form when minerals, salts, and acids in your urine crystallize into hard clumps. A stone sitting quietly inside the kidney may cause no symptoms at all. The pain starts when a stone drops into a ureter and gets stuck, blocking urine flow and causing the kidney to swell and the ureter to spasm.
The classic symptoms include serious, sharp pain in the side and back below the ribs, pain that spreads to the lower abdomen and groin, and pain that comes in waves of varying intensity. As the stone moves through the urinary tract, the location of the pain shifts too, sometimes changing over the course of hours. Many people also notice blood in their urine, nausea, or a frequent urge to urinate.
Stones 10 millimeters or smaller in the lower ureter have a reasonable chance of passing on their own within about 30 days, sometimes with the help of a medication that relaxes the ureter. Larger stones, or stones that don’t pass, typically require a procedure to break them up or remove them. Stones over 2 centimeters in the kidney itself usually need a more involved surgical approach.
Kidney Infections
A kidney infection, called pyelonephritis, happens when bacteria travel up from the bladder into one or both kidneys. The resulting inflammation causes the kidney to swell inside its capsule, producing a steady, deep ache in the flank. Unlike stone pain, which comes in waves, infection pain tends to be constant.
What sets a kidney infection apart from other causes is the combination of flank pain with systemic illness. Fever, chills, nausea, and vomiting are common. Your urine may look cloudy or smell unusually strong. Diagnosis usually involves checking a urine sample for white blood cells and bacteria, and sometimes blood tests to gauge how well the kidneys are functioning and whether the infection has spread. Kidney infections are treated with antibiotics, but severe cases can require hospitalization.
Polycystic Kidney Disease
Polycystic kidney disease (PKD) is a genetic condition present from birth. Fluid-filled cysts gradually grow on and within the kidneys, sometimes enlarging them dramatically. Pain from PKD is common and often felt in the side or back. It can come and go or persist as a chronic ache.
The pain has several possible triggers. A cyst can bleed internally, a cyst can become infected, or the sheer size of the enlarged kidneys can press on surrounding tissues and stretch the renal capsule. People with PKD are also more prone to kidney stones, which adds another potential pain source. Over time, the accumulating cysts can damage kidney tissue and impair function.
Blockages and Structural Problems
Several structural issues in the urinary tract can cause kidney pain by preventing urine from draining properly. When urine backs up, the kidney swells, and the capsule stretches.
- Ureteropelvic junction obstruction: a blockage at the point where the ureter connects to the kidney. This can be present from birth or develop later, causing the kidney to swell and potentially sustaining damage over time.
- Ureteral stricture: a narrowing of one or both ureters that slows urine flow, leading to pressure buildup in the kidney above the narrowed segment.
- Urinary retention: an inability to fully empty the bladder. The backup of urine can stretch the bladder painfully and, in some cases, cause pressure that reaches the kidneys.
- Vesicoureteral reflux: urine flows backward from the bladder up toward the kidneys, creating pressure and raising the risk of infection.
Injury and Trauma
A direct blow to the back or side from a car accident, a fall, or contact sports can bruise or tear a kidney. Because the kidneys sit relatively close to the surface of the back, just beneath the lower ribs, they’re vulnerable to blunt-force impact. Traumatic kidney pain is usually immediate, located on the injured side, and often accompanied by blood in the urine. Severe trauma may cause internal bleeding that requires emergency care.
Renal Infarction
A less common but serious cause of sudden kidney pain is renal infarction, where a blood clot blocks an artery supplying the kidney. Symptoms appear suddenly and can include flank or belly pain, nausea, vomiting, blood in the urine, and foamy urine (a sign of protein leaking into it). People most at risk include those over 40 with heart conditions like atrial fibrillation, high blood pressure, high cholesterol, diabetes, or a history of blood clots. Cocaine use and oral contraceptives also increase risk.
Kidney Cancer
Tumors in the kidney often cause no symptoms in their early stages. When they do produce pain, it’s typically a persistent ache in the flank, sometimes accompanied by blood in the urine. Because kidney cancer is usually silent early on, pain from a tumor generally means it has grown large enough to press against the renal capsule or surrounding structures.
Kidney Pain vs. Back Pain
One of the most common reasons people search for kidney pain is that they’re trying to figure out whether what they feel is actually a kidney problem or just a sore back. The two can feel similar in location, but they behave differently.
Muscle-related back pain is often described as a dull ache, stiffness, or soreness that gets worse with certain movements and improves when you find a comfortable position. It may radiate into the legs if a nerve is involved. Kidney pain, by contrast, stays in one area beneath the ribs and doesn’t change with movement. It may spread to the lower abdomen or inner thighs but won’t shoot down the legs. Accompanying symptoms like fever, changes in urination, nausea, or blood in the urine all point toward a kidney source rather than a muscular one.
How Kidney Pain Is Diagnosed
When kidney pain brings you to a doctor, the first step is usually a urine test checking for blood, white blood cells, and bacteria. Blood tests can reveal signs of infection and show how well your kidneys are filtering.
Imaging is where the real answers often come from. A CT scan without contrast dye is the gold standard for detecting kidney stones, with a sensitivity of 95 to 96 percent and specificity of 98 percent. Ultrasound is a radiation-free alternative that’s excellent at spotting signs of obstruction like a swollen kidney, but it catches only 24 to 57 percent of actual stones and is especially poor at finding small ones. For that reason, CT is usually the first choice when a stone is suspected, while ultrasound works well as an initial screen for other causes like cysts or structural problems.
Signs That Need Urgent Attention
Some combinations of symptoms signal that kidney pain needs same-day or emergency evaluation. Flank pain paired with a high fever and chills suggests a kidney infection that could spread to the bloodstream. Severe pain with an inability to urinate points to a complete blockage. Visible blood in the urine after an injury, pain so intense you can’t sit still or find a comfortable position, and persistent vomiting that prevents you from keeping fluids down are all situations where waiting it out can lead to complications.