Flank pain is discomfort or pain felt on one side of your body between your lower ribs and the top of your hip bone, toward your back. It’s one of the most common reasons people search for answers online because the sensation can range from a dull ache to an intense, sharp pain, and the causes span from a simple muscle strain to a kidney stone or infection. Understanding where the pain is, what it feels like, and what other symptoms come with it helps narrow down what’s going on.
Where Exactly Is the Flank?
Your flanks are the two areas on either side of your spine, sitting below your rib cage and above your hips. If you put your hands on your waist and slide them toward your back, you’re touching your flanks. This region sits directly over your kidneys, which is why kidney problems are the first thing most people (and most doctors) think of when flank pain shows up. But the area also overlaps with muscles, the lower ribs, the ureters (tubes connecting each kidney to the bladder), and portions of the colon and spleen on the left or liver on the right.
Kidney Stones: The Most Common Culprit
Kidney stones are the leading cause of sudden, severe flank pain. A stone sitting quietly inside your kidney may cause no symptoms at all. Pain typically starts when a stone moves into one of the ureters and partially or fully blocks urine flow. That blockage stretches the ureter and kidney, producing intense waves of pain that many people describe as the worst they’ve ever felt.
The pain often starts in the flank and radiates downward toward the lower abdomen, groin, or inner thigh. It comes in waves rather than staying constant, and it doesn’t get better or worse when you change position. Nausea, vomiting, and blood in the urine are common alongside it. A non-contrast CT scan of the abdomen and pelvis is the standard imaging test, with sensitivity as high as 97% for detecting stones. Ultrasound is less reliable at spotting stones directly (24% to 57% sensitivity), though it’s very good at picking up the secondary signs of blockage like fluid buildup around the kidney.
Small stones (under about 5 mm) often pass on their own with plenty of fluids and pain management at home. Larger stones may need medical procedures to break them up or remove them.
Kidney Infection
A kidney infection, called pyelonephritis, is another major cause. It usually develops when bacteria travel up from the bladder. The hallmark combination is flank pain plus fever, chills, nausea, and sometimes painful or frequent urination. The pain tends to be steady rather than coming in waves, and tapping on the back over the affected kidney typically produces sharp tenderness.
Unlike kidney stones, infection involves systemic illness. You feel genuinely sick, not just in pain. Fever is a key diagnostic marker. If you have flank pain with a temperature above 100.4°F (38°C), that combination points strongly toward infection and needs prompt treatment with antibiotics.
Muscle and Skeletal Causes
Not all flank pain comes from internal organs. Muscle strain, poor posture, prolonged sitting, or an injury to the lower ribs can all produce pain in the same area. The key difference is how the pain behaves. Musculoskeletal flank pain tends to feel like a dull ache, soreness, or stiffness. It gets worse with certain movements, like twisting, bending, or lifting, and improves when you find a comfortable position or rest.
Kidney pain, by contrast, does not change with movement. It stays the same whether you’re lying down, standing, or shifting around. It also doesn’t improve on its own without addressing the underlying cause. If your flank pain clearly worsens when you move a certain way and eases when you stop, a muscle or rib issue is more likely than a kidney problem.
Less Common but Serious Causes
Several rarer conditions can produce flank pain and are worth knowing about, especially if the more common explanations don’t fit.
A blood clot in the renal artery (renal artery thrombosis) causes sudden flank pain and tenderness, often with fever, blood in the urine, nausea, a sudden spike in blood pressure, and a rapid decline in kidney function. This is a vascular emergency. People with atrial fibrillation, blood clotting disorders, or recent abdominal surgery are at higher risk.
Shingles can cause flank pain days before any rash appears. The virus reactivates along a single nerve, producing pain, tingling, or itching in a band-like pattern on one side of the body. Because the rash hasn’t appeared yet, it’s easy to mistake this early pain for a kidney or muscle problem. If a blistering rash follows the pain a few days later, wrapping around one side of your torso, shingles is the answer.
Spontaneous bleeding around the kidney, known as Wunderlich syndrome, is rare but life-threatening. It presents with flank pain, a palpable mass in the side, and dropping blood pressure. People with kidney tumors or blood vessel abnormalities are most at risk.
How to Tell What’s Causing Your Pain
A few questions can help you sort through the possibilities before you even see a doctor:
- Does it come in waves? Wave-like pain that peaks and fades suggests a kidney stone.
- Do you have a fever? Fever plus flank pain points toward infection.
- Does movement change the pain? Pain that worsens with twisting or bending is more likely musculoskeletal. Pain unaffected by position is more likely from an organ.
- Is there blood in your urine? Visible or microscopic blood supports a kidney stone, infection, or vascular problem.
- Did the pain start with tingling or skin sensitivity? This pattern, especially on just one side, raises the possibility of shingles.
A basic urine dipstick test can check for blood cells, white blood cells (a sign of infection), and nitrites (produced by bacteria). This simple, fast test gives doctors an early clue about whether the urinary tract is involved.
When Flank Pain Needs Urgent Attention
Mild flank pain that lasts a day or two and responds to rest or over-the-counter pain relief is generally not an emergency. But certain combinations of symptoms warrant prompt medical care:
- Fever or chills alongside flank pain suggest infection that needs antibiotics.
- Inability to urinate may indicate complete blockage, which can damage the kidney.
- Severe pain that doesn’t respond to any position change or basic pain relief often means a stone or other obstruction needs intervention.
- Lightheadedness, rapid heartbeat, or dropping blood pressure with flank pain could signal internal bleeding.
- Pain lasting more than a few days without improvement deserves evaluation even if it’s not severe.
Imaging and Testing
If your doctor suspects a kidney stone, the first-line test is a CT scan of the abdomen and pelvis without contrast dye. Contrast can actually obscure stones by lighting up the surrounding kidney tissue, so the plain scan is preferred. For pregnant patients, MRI is the go-to alternative to avoid radiation. MRI alone catches about 50% of stones directly, but when combined with signs of blockage like swelling of the kidney and fluid around it, its accuracy rises to about 86%.
Ultrasound is often used as an initial screening tool, especially in emergency settings or for pregnant patients. It’s excellent at detecting hydronephrosis (swelling of the kidney from backed-up urine), which indirectly confirms that something is blocking the ureter, even if the stone itself isn’t visible on the image.