The flank is the stretch of your side between the bottom of your ribs and the top of your hip bone, curving around toward your back. It is the outside wall over your kidneys, which sit deep in the back of the belly.
Pain here is usually a kidney or muscle problem, which is why doctors treat the words “flank pain” as a clue to where to look rather than a diagnosis. Because so many organs share nerves with this spot, the pain can also come from somewhere else entirely.1PubMed Central. Non-urologic flank pain: a diagnostic approach
Where the Flank Is on Your Body
The flank has clear bony landmarks you can feel. It sits between the bottom edge of your rib cage and the top rim of your hip bone.2Journal of Cutaneous Medicine and Surgery. Female Hips and Back and Male Flanks Front to back, it reaches roughly from the front bony point of the hip to the back of the hip crest.3Journal of Cutaneous Medicine and Surgery. Female Hips and Back and Male Flanks In the back, the lowest two ribs, the 11th and 12th, form part of the top boundary. If you put your hands on your hips with your thumbs pointing back, your hands rest just below the flank.
To find your own, press a fingertip into the soft side of your waist and slide it down until you meet the hard top rim of the hip bone. Then slide it up until you meet the lowest ribs.
These landmarks also separate it from its neighbours. Lower back pain sits low and near the spine, in the middle. Belly pain is in front. Flank pain is off to one side and wraps backward, usually under the ribs. The zones overlap, so people often describe the same pain differently, and a pain can start in the flank and reach toward the front. If so, describe the whole path to the doctor rather than forcing it into one label.
What Lies Beneath the Flank
The deepest muscle here is the quadratus lumborum, a muscle that is part of the back wall of the belly rather than the back itself.4Kenhub. Quadratus lumborum muscle It runs from the top of the hip bone to the lowest rib and the sides of the lower spine bones.5PubMed Central. Anterior Quadratus Lumborum Block: A Scoping Review of Anatomical Rationale, Techniques, and Clinical Applications It helps hold you upright and bend sideways, which is why it can ache after lifting or twisting. The long spine muscles lie just behind it.
Deeper still, the kidneys, their blood vessels, the tubes draining urine, and the adrenal glands sit together in one fat-filled space behind the belly lining.6Study.com. What Is the Flank Area Anatomy Organs and Pain Just in front of that space lie parts of the colon, the first part of the small bowel, and the pancreas.7Study.com. What Is the Flank Area Anatomy Organs and Pain Behind the kidneys is a layer with no organs, mostly fat and vessels.8Study.com. What Is the Flank Area Anatomy Organs and Pain The kidneys rest against the diaphragm, the lowest ribs, and the deep muscles of the back wall.9TeachMeAnatomy. The Kidneys – Position – Structure – Vasculature – TeachMeAnatomy
Many people picture the kidneys low in the back near the belt line. They actually sit higher, tucked partly under the lowest ribs on either side of the spine. The right kidney usually sits slightly lower than the left because the liver takes up room above it. So kidney pain is often felt just beneath the rib edge or in the angle between the ribs and the spine, rather than down by the hips.
Each side has different neighbours:
- Right side: the liver, the first part of the small bowel, and a bend of the colon sit near the kidney.10TeachMeAnatomy. The Kidneys – Position – Structure – Vasculature – TeachMeAnatomy
- Left side: the spleen, stomach, pancreas, and the other bend of the colon sit near the kidney.11TeachMeAnatomy. The Kidneys – Position – Structure – Vasculature – TeachMeAnatomy
- Both sides: nerves running to the groin and lower belly pass behind the kidneys.12TeachMeAnatomy. The Kidneys – Position – Structure – Vasculature – TeachMeAnatomy
Those nerves explain a lot. Because they travel on toward the groin, kidney pain can spread down that way. It also means a nerve or spine problem can feel like kidney pain.
Kidney Causes of Flank Pain
Kidney stones are a common cause of sudden, severe flank pain.13SAGE Journals (InnovAiT). Renal colic About 1 in 11 people will be diagnosed with stone pain at some point in their life.14SAGE Journals (InnovAiT). Renal colic More than half who have it once get it again within 5 to 10 years.15SAGE Journals (InnovAiT). Renal colic
Stone pain comes in waves that rise and fade rather than staying at one level, as the urine tube squeezes around the stone. Many people can’t get comfortable and keep shifting, pacing, or curling up, unlike pain from a strained muscle, where people often prefer to stay still. The pain may travel toward the groin as the stone moves down the tube, and nausea often comes with it. Blood in the urine, which may look pink, red, or brown, can appear, though a stone can also be present without visible blood.
A kidney infection, called pyelonephritis, typically shows up as flank pain, fever, and nausea together.16PubMed. The diagnosis and management of a patient with acute pyelonephritis The pain of an infection is usually a steady ache rather than waves. It often follows burning when urinating or a bladder infection, and chills and feeling very unwell point toward infection. A stone and an infection together is an emergency. Vomiting that stops you keeping fluids down shouldn’t be ignored.
Muscle, Spine, and Nerve Causes
Pain that feels exactly like kidney pain can come from any nearby organ or any part with the same nerve supply.17PubMed Central. Non-urologic flank pain: a diagnostic approach Examples include a slipped disc in the upper back, an irritated nerve at the lowest rib level, arthritis where the ribs meet the spine, and tight, knotted muscle.18PubMed Central. Non-urologic flank pain: a diagnostic approach Even a leg-length difference can strain this area enough to cause it.19PubMed Central. Non-urologic flank pain: a diagnostic approach
Muscle pain often changes with movement. Twisting, bending sideways, or standing up from a chair makes it worse, and a certain position eases it. Pressing on the sore spot from the outside often reproduces the pain, which is less typical of a kidney stone. It often follows an obvious trigger such as lifting, a new exercise, or a long stretch of sitting. There is usually no fever and the urine looks normal. These are clues rather than proof, so a doctor still needs to rule out other causes if you are unsure.
For a likely strain, gentle movement usually beats bed rest. Heat and over-the-counter pain relief can help. See a doctor if it isn’t improving within a couple of weeks or if new symptoms appear.
Some flank pain lasts months. For stubborn pain of that kind, an injection around the quadratus lumborum gave lasting relief at 6 months to about two-thirds of people in one group.20PubMed Central. Interventional pathway in the management of refractory chronic flank pain: Prospective series in 45 patients Chronic flank pain without a kidney cause is worth raising with a pain specialist.
Other Organs That Can Cause Flank Pain
About 30% of people have an appendix tucked behind the bowel toward the back.21Provider Practice Essentials. Appendicitis In these people, appendicitis can cause right flank pain that is mistaken for a kidney stone.22Provider Practice Essentials. Appendicitis Appendix pain often starts near the belly button and then shifts toward the right side, though an appendix lying toward the back may not follow this pattern.
In women, an ovarian cyst can twist, bleed, or burst, which needs fast treatment to save the ovary.23Surgery (United Kingdom). Gynaecological causes of abdominal pain A burst cyst can bleed heavily and look very like an ectopic pregnancy.24PubMed. Ovarian cyst rupture causing hemoperitoneum: imaging features and the potential for misdiagnosis About 1 in 100 pregnancies in the UK is ectopic, growing outside the womb.25Surgery (United Kingdom). Gynaecological causes of abdominal pain Anyone who could be pregnant and has sudden one-sided pain should seek urgent care. Pelvic causes can come with a missed period or vaginal bleeding, which should be mentioned straight away.
Inflammation of the pancreas can also produce flank pain.26PubMed Central. Non-urologic flank pain: a diagnostic approach This pain often bores through to the back and may get worse after eating or drinking alcohol.
Warning Signs That Need Emergency Care
Flank pain is usually from kidneys or muscles, but a burst bulge in the body’s main artery can cause it too.27PubMed Central. An unusual presentation of ruptured abdominal aorta aneurysm Between 78% and 94% of people with a burst aortic aneurysm die.28PubMed Central. An unusual presentation of ruptured abdominal aorta aneurysm Sudden severe flank or back pain with dizziness or fainting, especially in older smokers, needs emergency services at once.
After a fall, crash, or blow, these are warning signs of kidney injury:
- Blood in the urine29ecosystem.tactuum.com. Renal Trauma – Blunt
- Bruising or scrapes on the flank30ecosystem.tactuum.com. Renal Trauma – Blunt
- Broken ribs31ecosystem.tactuum.com. Renal Trauma – Blunt
- A swollen or tender belly32ecosystem.tactuum.com. Renal Trauma – Blunt
Other red flags include being unable to keep fluids down and passing very little or no urine. Pain in pregnancy needs prompt attention, as does any flank pain if you have only one kidney or a kidney transplant.
If you think it is an emergency, call emergency services rather than driving yourself, especially if you feel faint or confused or the pain began suddenly and severely, because you could get worse on the way. Tell them where the pain is, how suddenly it began, and any relevant history such as kidney stones, one kidney, or blood vessel problems. Don’t eat or drink while waiting, in case you need a scan or an operation.
How Doctors Find the Cause
The exam often includes a gentle tap over the spot where the lowest rib meets the spine.33PubMed Central. Evaluation of the usefulness of costovertebral angle tenderness in patients with suspected ureteral stone The doctor places one hand there and taps it gently with a fist.34PubMed Central. Evaluation of the usefulness of costovertebral angle tenderness in patients with suspected ureteral stone If the tap worsens the pain and one side hurts more than the other, that points toward a kidney problem, though it is one clue among several.35PubMed Central. Evaluation of the usefulness of costovertebral angle tenderness in patients with suspected ureteral stone
At the visit, the doctor will ask when the pain began, whether it comes in waves or is steady, and whether you have had fever or changes when passing urine. You will probably be asked for a urine sample, which can show blood or signs of infection. Blood tests may check how well the kidneys work and whether there are signs of infection. Women who could be pregnant will usually be offered a pregnancy test before any scan.
For imaging, a CT scan without dye finds nearly all stones in the urine tubes.36PubMed. Modern approach of diagnosis and management of acute flank pain: review of all imaging modalities It shows stone size and location, the two things that decide treatment.37PubMed. Acute flank pain: a modern approach to diagnosis and management It also spots a non-kidney cause in about a third of people with sudden flank pain.38PubMed. Modern approach of diagnosis and management of acute flank pain: review of all imaging modalities It takes about 5 minutes and needs no injection.39PubMed. Modern approach of diagnosis and management of acute flank pain: review of all imaging modalities
Because of radiation and cost, many doctors now start with ultrasound and use CT scan only when needed.40PubMed. Imaging of flank pain: readdressing state-of-the-art Ultrasound can detect a swollen, blocked kidney in up to 96% of cases.41PubMed. Modern approach of diagnosis and management of acute flank pain: review of all imaging modalities Low-dose CT is an option that reduces radiation.42PubMed. Imaging of flank pain: readdressing state-of-the-art
Flank Pain After Kidney Surgery
Open kidney surgery through the side can injure the nerves to the abdominal wall muscles.43PubMed Central. Risk factors for the development of flank hernias and bulges following surgical flank approaches to the kidney in adults Those muscles can then weaken, leaving a bulge or hernia on that side.44PubMed Central. Risk factors for the development of flank hernias and bulges following surgical flank approaches to the kidney in adults A new bulge after side surgery is worth showing your surgeon.
A bulge can be slack muscle that has lost its tone because its nerve supply was damaged, without any tear in the wall. A hernia is different: tissue pushes through a gap in the muscle wall, and it may grow larger or ache when you cough or strain. A bulge that becomes painful, red, or hard and will not go back in needs urgent review.