Flank Pain: What It Means and What Causes It

Flank pain meaning and location

The flank is the area of your side between the lowest rib and the top of the pelvis, running from the side of the belly round toward the spine. Doctors use the term to point to the kidneys, which sit high in the back of the abdomen on each side of the spine, partly protected by the lower ribs.

The spot where the lowest rib meets the spine is the costovertebral angle (CVA). Tenderness when a doctor taps there ("CVA tenderness") suggests a kidney problem.

Flank pain is different from:

• Low back pain, which is lower and centered on the spine.
• Side pain across the front or side of the abdomen, which more often comes from the bowel, ovaries or liver. See side pain.

What causes flank pain

Kidney stones

Kidney stones are the classic cause. When a stone moves into the ureter (the tube from kidney to bladder), it blocks urine flow and stretches the kidney. The pain, called renal colic:

• starts suddenly and is often severe, coming in waves every 20 to 60 minutes
• moves from the flank round to the lower belly and groin, testicle or labia as the stone travels down
• makes you restless, unable to find a comfortable position
• comes with nausea, vomiting, blood in the urine (pink, red or only on testing) and urgency to pee

Most stones under 5 mm pass on their own within a few weeks.

Kidney infection (pyelonephritis)

A bladder infection that travels up to the kidney causes pyelonephritis. Expect a constant, dull or aching flank pain with fever, chills, nausea, and often burning or frequent urination in the days before. It needs antibiotics, and severe cases need hospital treatment.

Muscle and spine problems

• Muscle strain: pain on one side after lifting, twisting or sport. It is tender to press, worse with movement and eases with rest.
• Rib or spine problems: arthritis of the spine, a slipped disc or a pinched nerve root can send pain around the side. A rib fracture or bruised rib causes sharp pain on breathing.
• Costochondritis or slipping rib syndrome: pain at the lower rib edge.

Other kidney causes

• Hydronephrosis: a kidney swollen with trapped urine from a stone, tumor, enlarged prostate or pregnancy.
• Kidney cysts and polycystic kidney disease: a cyst that bleeds or becomes infected causes flank pain.
• Kidney cancer: usually painless early. Late signs are flank pain, a lump and blood in the urine.
• Kidney infarction or renal vein clot: rare, sudden and severe.

Causes outside the kidneys

• Shingles: burning or tingling pain in a band on one side, followed 2 to 5 days later by a blistering rash.
• Abdominal aortic aneurysm: a leaking aneurysm can mimic a kidney stone in older adults.
• Appendicitis, gallbladder disease, pancreatitis, diverticulitis: pain can spread to the flank.
• Lower lobe pneumonia or pleurisy: pain with breathing, cough and fever.
• Ovarian cyst or ectopic pregnancy: lower flank and pelvic pain in women.

Right side vs left side flank pain

Kidney stones, infections and muscle strains cause pain on either side. Other organs make one side more likely:

Pain that moves from side to side, or is on both sides, is less likely to be a stone and more likely to come from muscles, the spine, or a condition affecting both kidneys.

Kidney pain vs back muscle pain

Pain that is worse when you lie on one side and press on the area tends to be muscular. Lying on your right side can also change discomfort from the liver, gallbladder or a muscle on that side.

Flank pain lasting for weeks

A dull flank ache that persists for weeks is less often an emergency, but it should be checked. Possible causes include a stone sitting in the kidney, chronic hydronephrosis, a kidney cyst, a muscle or spine problem, or, rarely, a tumor. Tell your doctor about any blood in the urine, weight loss, night sweats, or pain that wakes you at night.

How doctors diagnose flank pain

Your doctor will ask when and how the pain started, whether it comes in waves, where it spreads, and about fever, urine changes, previous stones, and in women, the possibility of pregnancy. They will tap the CVA area, examine the belly, check the skin for a shingles rash, and in older adults feel for a pulsating mass.

Tests often include:

• Urine test: blood suggests a stone, white cells and nitrites suggest infection
• Blood tests: kidney function, white cell count, CRP, and a pregnancy test where relevant
• Non-contrast CT scan: the best test for kidney stones
• Ultrasound: first choice in pregnancy and children, and shows a swollen kidney
• Bedside ultrasound or CT angiography if an aortic aneurysm is suspected

Treatment and what you can do

Treatment depends on the cause:

• Kidney stones: pain relief (NSAIDs such as ibuprofen are effective for renal colic), anti-nausea medicine, fluids, and sometimes a medicine that relaxes the ureter. Large stones may need shock-wave lithotripsy, ureteroscopy, or removal through the back.
• Blocked, infected kidney: emergency drainage with a stent or a tube through the skin, plus antibiotics.
• Pyelonephritis: antibiotics for 7 to 14 days, depending on the medicine. Hospital care if you cannot keep fluids down.
• Muscle strain: stay active, use heat, and take over-the-counter pain relievers. Most improve within 1 to 2 weeks.
• Shingles: antiviral medicine started within 3 days of the rash works best.

At home, for mild pain while you arrange care:

• Drink enough fluid to keep your urine pale, unless a doctor has restricted fluids.
• Use a heating pad on the flank for 15 to 20 minutes.
• Take acetaminophen (paracetamol) or ibuprofen as directed on the label. Avoid NSAIDs if you have kidney disease, are pregnant, or have stomach ulcers.

Frequently asked questions

Flank pain is a symptom of what?

Flank pain is most often a symptom of a kidney stone or kidney infection. It can also come from strained back muscles, spine problems, shingles, gallbladder or pancreas disease, lower lung infections and, rarely, a leaking aortic aneurysm or kidney tumor.

How do I know if flank pain is my kidney?

Kidney pain is deep, high under the ribs on one side, and does not get better or worse with moving. Stones cause severe waves of pain spreading to the groin. Infection causes a steady ache with fever and burning urination. A urine test can quickly show blood or infection.

What does stabbing flank pain mean?

Sudden, stabbing pain that comes in waves is typical of a kidney stone. Sharp pain on breathing suggests a rib, lung or pleura problem. Stabbing pain that follows a band around one side may be shingles before the rash. Severe sudden pain in older adults needs emergency assessment.

Why does my flank hurt when I sleep on my right side?

Pressure on a strained muscle, a bruised rib, or an irritated nerve often makes flank pain worse when you lie on that side. Liver or gallbladder problems can also feel worse lying on the right. If pain persists beyond a week or comes with fever or urine changes, get checked.

Can flank pain last for weeks?

Yes. A muscle strain can ache for several weeks, and a stone that stays in the kidney can cause a dull, recurring ache. Persistent flank pain should be assessed with urine tests and imaging, especially if you notice blood in your urine or weight loss.

Should I go to the ER for flank pain?

Go to the ER for severe pain you cannot control, flank pain with fever and chills, vomiting that stops you keeping fluids down, fainting, or very little urine. Sudden severe pain in an older adult with high blood pressure or a history of smoking needs emergency care.