Kidney pain is a deep, steady ache felt in your flank, the area on either side of your spine just below the rib cage and above the hips. Unlike muscle-related back pain, it doesn’t get better or worse when you shift position, and it won’t resolve on its own without treatment. The most common causes are kidney stones, infections, and cysts, though dehydration and other conditions can also be responsible.
Kidney Pain vs. Back Pain
Because your kidneys sit against the back muscles just below the rib cage, it’s easy to confuse kidney pain with a pulled muscle or spinal issue. The key difference is movement. Musculoskeletal back pain changes with your body: it flares when you bend, twist, or lift, and it eases when you find a comfortable position. It often radiates down the legs if a nerve is involved, and it tends to feel like stiffness or soreness.
Kidney pain behaves differently. It stays in one area of the flank regardless of how you move. It can spread to the lower abdomen or inner thighs, but it doesn’t shoot down the legs. It also tends to come with other symptoms, like changes in urination, fever, or nausea, that pure back pain wouldn’t cause. If you’re unsure which you’re dealing with, those accompanying symptoms are your best clue.
Kidney Stones
Kidney stones are the most recognizable cause of sudden, severe kidney pain. The pain isn’t actually from the stone sitting in your kidney. It comes from the stone moving into the ureter, the narrow tube connecting the kidney to the bladder. When a stone blocks that tube, urine backs up, stretching the kidney and causing the tissue around it to swell. That stretching and spasm is what produces the intense, wave-like pain people describe as some of the worst they’ve ever felt.
A stone that’s actively moving down the ureter can be more painful than one that’s stuck in place. That sounds counterintuitive, but a motionless stone allows your body to activate pressure-relief mechanisms: fluid reroutes through the kidney’s lymphatic system, and internal pressure gradually drops. A stone bouncing along creates intermittent blockages that restart the whole pain cycle each time.
Small stones often pass on their own with fluids and pain management, though the process can take days to weeks. Larger stones may need medical intervention to break them up or remove them. Either way, the pain typically comes in waves rather than staying constant, and you may notice blood in your urine or a burning sensation when you urinate.
Kidney Infection
A kidney infection, called pyelonephritis, usually starts as a bladder infection that travels upward. The pain is more of a constant, deep ache in one flank rather than the sharp waves of a kidney stone. What sets an infection apart is systemic illness: fever, chills, nausea, vomiting, and feeling generally terrible in a way that a stone alone wouldn’t cause.
Your urine may look cloudy or smell unusual, and you’ll likely feel a frequent, urgent need to urinate even when your bladder isn’t full. Doctors diagnose kidney infections through a urinalysis that checks for white blood cells and bacteria in your urine, along with blood tests to gauge how well your kidneys are functioning and whether the infection has spread. Kidney infections require prompt treatment with antibiotics because an untreated infection can cause permanent kidney damage or enter the bloodstream.
Polycystic Kidney Disease
Polycystic kidney disease (PKD) causes fluid-filled cysts to grow on the kidneys over time. As the cysts enlarge, they stretch the kidney’s outer capsule and press on surrounding tissue, producing a dull, persistent pain in the side or back. That pain can come and go or become constant depending on what’s happening inside the kidney. A cyst that bleeds internally, a urinary tract infection developing alongside the cysts, or a kidney stone forming within the altered kidney structure can all trigger flare-ups of sharper pain.
PKD is a genetic condition, so if you have a family history and are experiencing unexplained flank pain, imaging can reveal whether cysts are present.
Dehydration and Stone Formation
Not drinking enough water doesn’t directly cause pain in the kidney, but it sets the stage for problems that do. When you’re dehydrated, your kidneys produce more concentrated urine. That concentrated urine contains higher levels of minerals and waste products, which can crystallize and eventually form kidney stones. Chronic dehydration is one of the most common and most preventable risk factors for stones.
If you’ve had a kidney stone before, hydration becomes even more important. Keeping sodium intake below 2,300 mg per day also helps, because excess sodium increases the amount of calcium your kidneys filter, raising the concentration of stone-forming minerals in your urine. The combination of drinking enough water to produce pale yellow urine and moderating salt intake significantly reduces recurrence risk.
What Doctors Check For
When you see a provider for kidney pain, the evaluation typically involves blood tests, urine tests, and sometimes imaging. Blood tests measure creatinine, a waste product from normal muscle breakdown. Your kidneys are supposed to clear creatinine efficiently, so rising levels suggest they’re struggling. Doctors use your creatinine level to estimate your glomerular filtration rate (GFR), which tells them how well your kidneys are filtering blood. A GFR of 60 or above is normal. Below 60 suggests kidney disease, and below 15 indicates kidney failure.
Urine tests check for two things: signs of infection (white blood cells, bacteria) and signs of kidney damage (a protein called albumin that healthy kidneys keep out of urine). A urine albumin level above 30 mg/g is a potential sign of kidney damage. Imaging, usually ultrasound or CT scan, can reveal stones, cysts, blockages, or structural problems that blood and urine tests can’t detect.
Symptoms That Need Immediate Attention
Some kidney pain warrants a trip to the emergency room rather than waiting for a scheduled appointment. Go in if you experience:
- Pain you can’t manage with over-the-counter medication
- Inability to urinate, which may signal a complete blockage
- Fever or chills alongside flank pain, suggesting infection
- Severe nausea or vomiting, especially if you can’t keep fluids down
- Blood in your urine
- A sudden, urgent need to urinate that’s unusual for you
- Persistent fatigue or general illness that won’t resolve
A complete urinary blockage or an untreated infection can cause permanent kidney damage within hours to days, so these symptoms are worth taking seriously even if the pain itself feels tolerable.