Kidney pain is a deep ache felt under your ribs on one or both sides of your spine, typically in the area known as the flank. Unlike ordinary back pain, which centers over the spine or lower back, kidney pain sits higher and off to the side, and it often radiates toward the groin. The most common causes are kidney stones, infections, and swelling from a blockage, though several other conditions can be responsible.
How to Tell It Apart From Back Pain
The location is the biggest clue. Back pain from muscle strain or spinal issues runs along the middle of your back and sometimes shoots down into your legs. Kidney pain is felt in your flank, the area between your lower ribs and your hip on either side of the spine, and it tends to feel deeper, like it’s coming from inside rather than from the surface. It can also travel forward toward your lower abdomen or groin, which back pain rarely does.
Hip pain is another common source of confusion, but it typically sits lower. If pressing on the muscles along your spine reproduces the pain, or if it changes when you shift position, the source is more likely musculoskeletal. Kidney pain generally stays constant regardless of how you move, and it may come with urinary symptoms like blood in your urine, painful urination, or fever.
Kidney Stones
Kidney stones are by far the most common reason for sudden, severe kidney pain. About 1 in 10 adults in the U.S. will have a stone at some point, and prevalence has been climbing steadily, particularly among men over 65, where rates rose from 4.3% to 6.4% between 2012 and 2021.
The pain doesn’t come from the stone itself cutting tissue. It comes from blockage. When a stone plugs the ureter (the narrow tube connecting a kidney to the bladder), urine backs up and stretches the kidney’s outer capsule. That sudden stretching activates pain receptors and produces the intense, wave-like pain known as renal colic. Interestingly, a stone that’s actively moving down the ureter and causing intermittent blockage is often more painful than one sitting still, because the pressure fluctuates unpredictably.
The pain typically starts in the flank, then shifts lower as the stone moves. If it reaches the lower ureter near the bladder, you may feel it in your lower abdomen or groin, along with a strong, constant urge to urinate. Many small stones pass on their own with fluids and pain management over a few days to a couple of weeks. Larger stones that won’t pass may need a procedure to break them up or remove them.
Kidney Infections
A kidney infection, called pyelonephritis, usually starts as a bladder infection that travels upward. Bacteria climb from the bladder through the ureter and into the kidney, triggering inflammation and swelling. The result is a steady, aching flank pain that doesn’t come in waves like stone pain does.
What sets a kidney infection apart from a simple bladder infection is the systemic symptoms. A bladder infection gives you burning with urination and frequency. A kidney infection adds fever, chills, nausea, and sometimes vomiting on top of those urinary symptoms. The flank pain is usually on one side and may feel tender when someone presses on it. Kidney infections require prompt treatment with antibiotics, because untreated infections can spread to the bloodstream.
Hydronephrosis (Kidney Swelling)
Hydronephrosis is the medical term for a swollen kidney, and it happens whenever urine can’t drain properly and backs up. Kidney stones are one cause, but it can also result from a tumor pressing on the ureter, scar tissue from surgery, an enlarged prostate, or even a structural difference you were born with. The internal pressure stretches the renal pelvis, the funnel-shaped collection area inside the kidney, and that stretch is what produces the pain.
The pain from hydronephrosis is usually a dull, persistent ache in the flank that may travel to the lower abdomen or groin. If the blockage develops slowly, you might not notice pain for a long time, but a sudden obstruction can cause intense discomfort similar to kidney stones.
Polycystic Kidney Disease
Polycystic kidney disease (PKD) is a genetic condition in which fluid-filled cysts grow throughout the kidneys, gradually enlarging them. The most common form, autosomal dominant PKD, affects roughly 1 in 400 to 1,000 people. Pain from PKD tends to be a chronic, dull heaviness in the flank or abdomen as the kidneys grow larger. However, sudden sharp pain can signal a complication: bleeding into a cyst, an infected cyst, a kidney stone (which people with PKD are more prone to), or twisting of the enlarged kidney. If your pain shifts from a familiar dull ache to something sharp and new, imaging is typically used to check for a large or problematic cyst.
Pain From Common Medications
Over-the-counter pain relievers like ibuprofen and naproxen can themselves cause kidney problems, especially with regular use. These drugs work by blocking enzymes that produce inflammation, but those same enzymes also help keep blood flowing to the kidneys. In healthy, well-hydrated people this usually isn’t an issue. But if you’re dehydrated, older, or already have reduced kidney function, these medications can significantly cut blood flow to the kidneys, leading to acute kidney injury.
The tricky part is that symptoms of this kind of kidney damage are nonspecific: fatigue, nausea, decreased urine output, and swelling in the ankles or legs. You won’t always feel classic flank pain. In some cases, the kidneys develop an immune-mediated reaction that causes fever, joint pain, and a rash. The damage is usually reversible once you stop the medication, but it’s something to be aware of if you rely on these drugs regularly.
How Kidney Pain Is Diagnosed
When you show up with flank pain, the workup typically starts with a urine test (looking for blood, bacteria, or crystals) and blood work to check kidney function. Imaging comes next, and the choice depends on what your doctor suspects.
- CT scan without contrast is the gold standard for kidney stones, with sensitivity as high as 97%. It can detect nearly all types of stones, even very small ones, and reveals secondary signs like swelling of the ureter or fluid around the kidney.
- Ultrasound is the first choice for pregnant patients and is highly effective at detecting hydronephrosis, with up to 100% sensitivity for ureteral obstruction. Adding color Doppler can boost stone detection to as high as 99% for stones under 5 millimeters.
- MRI is sometimes used when radiation needs to be avoided, particularly during pregnancy, and can identify blockages and other structural problems.
Warning Signs That Need Immediate Attention
Most kidney pain has a treatable, non-life-threatening cause. But certain combinations of symptoms signal something more urgent. Flank pain with a high fever and chills suggests a kidney infection that could spread to the bloodstream. Pain with visible blood in the urine, especially if you’ve never had it before, warrants prompt evaluation. An inability to urinate at all alongside flank pain points to a complete blockage that can damage the kidney if it isn’t relieved. Severe nausea and vomiting with kidney pain can also lead to dangerous dehydration, making it harder for your body to manage whatever is causing the problem.
If your pain is mild and comes and goes without fever or urinary symptoms, it’s reasonable to hydrate well and monitor it for a day or two. But pain that is severe, worsening, or accompanied by any of the symptoms above should be evaluated the same day.