Kidney pain most commonly comes from kidney stones, infections, or swelling caused by a urine blockage. It shows up as a deep ache or sharp pain in your flank, the area on either side of your spine just below your rib cage and above your hips. Roughly 1 in 10 adults will deal with a kidney stone at some point, making it the single most frequent cause of sudden kidney pain.
How Kidney Pain Feels Different From Back Pain
Your kidneys sit against the back muscles just below the rib cage, so it’s easy to confuse kidney pain with a pulled muscle or spinal issue. The key difference is movement. Musculoskeletal back pain tends to feel like a dull ache, stiffness, or soreness that gets worse when you bend or twist and improves when you find a comfortable position. Kidney pain does not change with movement. It stays constant or comes in waves regardless of how you sit, stand, or stretch, and it typically won’t improve without treatment.
Location matters too. Back pain from muscles or nerves often runs along the center of the spine or radiates down into the legs. Kidney pain stays in the flank and may spread forward into the lower abdomen or inner thighs. If you press on the area just below your lowest rib near the spine (doctors call this the costovertebral angle) and it’s tender, that points toward a kidney source rather than a muscular one.
Kidney Stones
Kidney stones are hard deposits, mostly made of calcium and oxalate, that form inside the kidney and can get stuck as they travel down the ureter toward your bladder. The pain isn’t caused by the stone itself scraping tissue. It’s caused by the sudden backup of urine behind the stone, which stretches the kidney’s outer lining and the walls of the ureter. That acute stretching and dilation is what produces the intense, wave-like pain known as renal colic.
The pain often starts suddenly, sometimes in the middle of the night, and can shift location as the stone moves. A stone high in the ureter typically causes flank pain, while one closer to the bladder may cause pain in the lower abdomen or groin, along with a strong, frequent urge to urinate. Blood in the urine is common and can be visible to the naked eye or only detectable on a lab test. Nausea and vomiting frequently come along for the ride.
Stones smaller than about 5 millimeters often pass on their own with fluids and pain management. Stones larger than 5 millimeters are more likely to need medical intervention, which can range from sound-wave therapy that breaks the stone into smaller pieces to a minimally invasive procedure using a small scope threaded through the urinary tract.
Kidney Infections
A kidney infection, or pyelonephritis, usually starts as a lower urinary tract infection that climbs upward from the bladder into one or both kidneys. What sets it apart from a regular bladder infection is the combination of systemic illness and flank pain. You’ll likely develop a fever, chills, nausea, and pain in your lower back or side that feels deeper and more constant than the burning discomfort of a simple UTI. Urine may appear cloudy or bloody and smell unusually strong.
Kidney infections can become dangerous quickly. If you develop a fever above 101°F along with flank pain and chills, that combination suggests possible sepsis and warrants urgent medical attention. Heart rate above 120 beats per minute or a significant drop in blood pressure alongside these symptoms are red flags that the infection may be spreading into the bloodstream.
Hydronephrosis (Kidney Swelling)
Hydronephrosis is the medical term for a kidney that has swollen because urine can’t drain properly. A kidney stone is the most common culprit, but other causes include blood clots, scar tissue from past infections, an enlarged prostate, or external pressure on the ureter. In people with a uterus, pregnancy is a notable cause: as the uterus expands, it can press against the ureters and partially block urine flow, especially in the second and third trimesters.
The swelling stretches the kidney’s outer capsule, producing a deep, steady ache in the flank. If the blockage comes on slowly, you may not notice pain at first. Sudden blockages tend to produce sharp, intense pain similar to kidney stone colic.
Polycystic Kidney Disease
Polycystic kidney disease (PKD) is a genetic condition in which fluid-filled cysts grow throughout both kidneys, gradually enlarging them and impairing their function. Pain in the side or back is one of the most common symptoms and can come from several triggers: a cyst bleeding internally, a cyst rupturing, a kidney stone forming within the affected kidney, or simply the mechanical bulk of enlarged kidneys pressing against surrounding tissues. Some people notice their abdomen growing larger as the kidneys expand over time.
PKD pain can come and go or become a chronic issue. Because the cysts also raise the risk of urinary tract infections, new or worsening pain in someone with PKD sometimes signals an infection on top of the underlying disease.
Renal Infarction
Less commonly, kidney pain can come from a blood clot that blocks the artery supplying the kidney. This is called a renal infarction, and it cuts off blood flow to part of the kidney tissue. Symptoms appear suddenly: flank or abdominal pain, nausea, vomiting, and sometimes blood or foam in the urine. Some people have no symptoms at all, and the event is only discovered later.
People at highest risk include those with atrial fibrillation (an irregular heart rhythm that allows clots to form in the heart and travel to the kidneys), high blood pressure, high cholesterol, diabetes, or a history of blood clotting disorders. Cocaine use, oral contraceptive use, and obesity also raise the risk. Trauma to the kidney area or a tear in the renal artery or aorta can trigger the same problem.
When Kidney Pain Needs Urgent Attention
Certain combinations of symptoms suggest something more serious than a stone working its way through. Fever above 100.4°F with flank pain and chills points toward infection that may need immediate antibiotics. A rapid heart rate, low blood pressure, or shaking rigors alongside kidney pain are signs of sepsis. Complete inability to urinate despite having two functioning kidneys is another emergency scenario.
Severe flank pain that doesn’t improve after several hours of over-the-counter pain relief also warrants evaluation, especially if it’s accompanied by vomiting that prevents you from keeping fluids down. In some cases, what seems like kidney pain turns out to be something else entirely. Conditions that can mimic kidney pain include an abdominal aortic aneurysm, appendicitis, an ectopic pregnancy, ovarian cysts, or pelvic inflammatory disease.
How Kidney Pain Is Diagnosed
A non-contrast CT scan is the gold standard for evaluating sudden flank pain, particularly when a kidney stone is suspected. It picks up stones with 95 to 96 percent sensitivity and 98 percent specificity, takes only a few minutes, and doesn’t require any injected dye. It can also reveal other causes like hydronephrosis, kidney enlargement, or signs of infection.
Ultrasound is less accurate for finding stones, catching only 24 to 57 percent of them, and it can miss over 30 percent of acute obstructions in the early hours before the kidney has had time to visibly swell. However, ultrasound uses no radiation, which makes it the preferred first test for pregnant patients or situations where repeated imaging is needed. A urinalysis will typically accompany imaging to check for blood, bacteria, or white blood cells, helping narrow the diagnosis toward stones, infection, or another cause.