Fever with flank pain, blood in the urine, or pain so strong that you cannot sit still needs emergency care now. Call 911 or your local emergency number.
The kidneys sit toward the back of the upper abdomen, one on each side of the spine, roughly level with the lowest ribs, which is why pain from them is felt in the flank rather than in the low back. If you put your hands on your waist with your thumbs pointing backwards, your thumbs land close to the area where kidney pain is usually described.
They are not inside the main abdominal cavity. They sit behind it, packed in fat and muscle, with the lower ribs covering their upper halves. The right kidney normally sits slightly lower than the left because the liver occupies the space above it. That depth matters for how pain feels. A structure buried under several layers of tissue produces pain that is dull, deep and hard to pinpoint with one finger, unlike a muscle knot you can press on and reproduce.
Each kidney drains into a thin tube called the ureter, which runs down behind the abdomen toward the bladder. A review of renal innervation and pain in autosomal dominant polycystic kidney disease describes how nerve fibres from the kidney and ureter enter the spinal cord at the same levels that serve the flank, lower abdomen and groin, which is why pain arising in the kidney is often felt some distance from the organ itself 1. That shared wiring explains a classic pattern, pain that begins high in the flank and travels down and forward toward the groin.
Most pain in the flank is not coming from the kidney. The muscles, ribs, spinal joints and nerves of the mid and lower back sit directly over the kidney area, and irritation in any of them produces pain in the same zone. Heavy lifting, a hard training block, a long drive, a night on an unfamiliar mattress or a twist while carrying something on one side are all common starting points.
Several everyday factors stack up before any of this becomes a lab question. Short sleep and high stress lower pain tolerance and increase muscle guarding. A sudden jump in training load, especially deadlifts, rowing, running on cambered roads or anything done one sided, strains the muscles that attach along the lower ribs and pelvis. Low fluid intake concentrates the urine and is a well known setup for stone formation. Alcohol dehydrates and often produces a dull one sided ache the next day. Life stage plays a part too, since pregnancy changes how the kidneys drain and shifts load onto the lower back. Some medicines can also affect the kidneys or the markers used to assess them, so ask your prescriber about anything you take.
When pain really does come from the urinary tract, three patterns come up most often. The first is a stone. Stone pain, also called renal colic, typically starts in the flank and moves down toward the lower abdomen, groin or genitals as the stone travels along the ureter, it comes in waves, and people often pace or change position constantly because nothing makes it comfortable. A multispecialty consensus review on imaging in suspected renal colic indicates that imaging, usually computed tomography or ultrasound rather than bloodwork, is what confirms or rules out a stone 2, and the same consensus was published for radiology practice 3. Studies suggest that microscopic blood in the urine is common in renal colic and urolithiasis, although it is not present in everyone with a stone 4.
The second pattern is infection. Research suggests that acute pyelonephritis in adults typically presents with fever and flank pain, often with nausea, and that urinalysis and urine culture are central to identifying it 5. A clinical review of urinary tract infection describes the distinction clearly, since a bladder infection tends to cause burning and urgency low down, while involvement of the kidney adds flank pain, fever and feeling systemically unwell 6.
The third pattern is structural. Cysts in the kidney can stretch the capsule and produce chronic one sided or two sided flank pain, which is why pain is a recognised problem in people with autosomal dominant polycystic kidney disease 1. Rarer causes exist as well. Loin pain hematuria syndrome is described as severe flank pain with blood in the urine that persists after other causes have been excluded 7. Flank pain with high blood pressure and blood in the urine is associated with renal artery emergencies such as infarction or dissection, which cross sectional imaging can identify 8.
The most useful difference is movement. Muscle and joint pain usually changes when you bend, twist or shift position and settles with rest, while pain coming from the kidney tends to persist no matter how you sit or lie. Position change is not a perfect test, but it is the question clinicians ask early.
| What to notice | More typical of muscle or joint pain | More typical of pain coming from the kidney |
|---|---|---|
| Where it sits | Lower back, across the spine or over a sore spot you can press | Higher and deeper, under the back of the lower ribs, usually one sided |
| Response to movement | Worse with bending, twisting or standing up, easier with rest | Much the same whatever position you take |
| Quality | Tight, achy, sometimes sharp with one specific movement | Deep and dull, or coming in waves, hard to localise with one finger |
| Where it spreads | Into the buttock or down the back of the leg | Around the side toward the lower abdomen, groin or genitals |
| What comes with it | Stiffness after sitting, soreness after training | Fever, chills, nausea, burning on urination, blood in the urine |
These patterns are typical rather than definitive, and the two can overlap. Someone can strain a back muscle and pass a stone in the same month. The combinations that matter most are flank pain with fever, flank pain with visible blood in the urine, and flank pain severe enough that you cannot keep still.
Blood tests cannot locate pain, but two of them add useful context when flank pain is being assessed. They are creatinine and the total white blood cell count. A urine test usually sits alongside them, because the urinary tract tells its story most directly in the urine.
| Marker | Why it is linked to this symptom | What a result can and cannot tell you |
|---|---|---|
| Creatinine | A waste product of normal muscle metabolism that the kidneys clear, so blood levels rise when filtration slows | Can show that filtration has changed and give an estimated filtration rate, cannot show why, which side is involved, or whether the pain is coming from the kidney at all |
| Total white blood cells | Rise with infection and with inflammation generally, which matters when flank pain comes with fever or chills | Can support a suspicion of infection and track how someone is doing, cannot say where the infection is or confirm the kidney is involved |
Typical adult reference ranges for these two markers look like this.
| Marker | Typical lab reference range | Units |
|---|---|---|
| Creatinine | about 0.6 to 1.3 | mg/dL |
| Total white blood cells | about 4.0 to 11.0 | x10^9/L, equivalent to 4,000 to 11,000 cells per microlitre |
These are typical lab reference ranges. Ranges vary by lab, age and sex, and the range printed on your own report is the one that applies to your sample.
Creatinine gives an indirect but useful read on filtration, which is why it is one of the first blood markers checked when flank pain is evaluated. Muscle produces creatinine at a fairly steady rate, the kidneys filter it out, and the amount left in the blood reflects how much filtering work is getting done. When filtration slows, creatinine climbs. Labs usually report an estimated glomerular filtration rate alongside it, calculated from creatinine, age and sex. You can read more about what the marker measures on the Creatinine page.
What creatinine cannot do is tell you where pain is coming from. A blocking stone, an infection or a cyst can all be present with a creatinine value sitting squarely inside the standard range, particularly when the other kidney is compensating. That is why imaging has the central role in suspected renal colic rather than bloodwork 2, and why flank pain with high blood pressure and blood in the urine is associated with renal artery problems that only imaging will show.
Plenty of things other than kidney function move creatinine. More muscle mass raises it, so a heavily trained person can sit above the standard range with completely normal filtration. A large meat meal, a hard session the day before, dehydration and creatine supplementation can all nudge it upward. Age, sex and pregnancy shift the expected value, and some medicines change either creatinine itself or how the kidneys handle it, so ask your prescriber about anything you take. A single reading is a snapshot, and the direction of travel across repeat tests usually says more than one number.
The practical way to read it is in three steps. The value shows how much creatinine is in your blood. A higher than expected value can reflect reduced filtration, high muscle mass, a recent hard training block or low hydration. A result outside the standard range, or one that has moved noticeably since your last test, is worth discussing with a clinician who can see it next to your history and examination.
A raised white blood cell count signals that the immune system is responding somewhere in the body, but it does not say where, so it supports rather than settles the question of a kidney infection. White cells rise as part of the response to bacterial infection, which is why the count is checked when someone has flank pain together with fever or chills. The Total WBCs page covers what the count includes and how the subtypes differ.
The location question is answered elsewhere. Research suggests that acute pyelonephritis in adults is identified mainly through the clinical picture of fever and flank pain together with urinalysis and urine culture, rather than through a blood count on its own 5. A clinical review makes the same point when separating bladder infection from kidney involvement, where symptoms and urine findings carry the weight 6. Blood in the urine is a related clue, and research suggests that microscopic blood is frequently found in renal colic, so it can accompany a stone as well as an infection 4.
Many ordinary things move the white cell count. Hard exercise raises it for hours afterwards. So do acute stress, smoking, pregnancy, a recent viral illness and simply the time of day the sample was drawn. Some medicines raise or lower it, so ask your prescriber about anything you take. A count slightly outside the standard range in someone who feels well is a different situation from the same count in someone with fever and one sided flank pain, and that context is what a clinician adds.
A completely normal blood panel does not rule out a cause for flank pain, because many causes never show up in blood at all. Muscle strain, rib and spinal joint irritation, and nerve irritation such as shingles before the rash appears all produce real pain with unremarkable bloodwork.
Stones are the clearest example in the urinary tract. Loin pain hematuria syndrome is described as a situation where standard investigations keep coming back unremarkable despite severe pain 7.
Things outside the kidney matter too. Gallbladder problems, digestive conditions, pelvic and gynaecological causes and referred pain from the spine can all be felt in the flank. Blood markers are one input. The history, the examination, a urine sample and sometimes imaging are the rest, and only a clinician can assemble them.
Some combinations need to be seen the same day or sooner rather than monitored at home. Go to an emergency department or call 911 if flank pain comes with a fever or shaking chills, if there is visible blood in the urine, or if the pain is so severe that you cannot sit still or cannot keep fluids down.
Also seek care promptly for flank pain after a fall or a blow to the back, flank pain with a sudden rise in blood pressure, a sharp drop in how much urine you are passing, flank pain during pregnancy, and flank pain in people with a single kidney, a kidney transplant or known kidney disease. Pain that has been grumbling on for weeks, or that keeps returning on the same side, deserves an appointment even without any of the above.
Do not try to work out which cause you have from where it hurts. Location narrows the possibilities, it does not settle them, and the same spot can be served by a muscle, a rib joint, a nerve and a kidney. Bring the details that help, which are when it started, what makes it better or worse, whether urination has changed, and whether you have had a fever. Never ignore a result that sits outside the standard range, even if you feel fine.
If you are having thoughts of self-harm, call or text 988 (US) or your local emergency number now.
Both markers discussed here are measured in routine bloodwork, and the FitScript Essential panel includes creatinine along with a complete blood count that reports total white blood cells. Testing is a membership built around 150+ biomarkers tested per year, so values can be followed over time instead of read as a single snapshot.
Atlas, FitScript's AI health advisor, explains what each result means in plain language and shows how it sits against the standard reference range and against your own previous results. Interpretation is not the same as a decision. A licensed provider reviews the picture, orders any further testing such as a urine test or imaging, and treatment decisions belong to a licensed provider. This page is educational and is not medical advice. Take any result outside the standard range, and any flank pain that is new, severe or persistent, to a clinician or licensed provider who can examine you.
For more on the two urinary tract patterns described above, see the related guides on kidney stone symptoms and kidney infection symptoms.
The kidneys sit at the back of the upper abdomen, one on each side of the spine, roughly level with the lowest ribs. They lie behind the abdominal organs rather than inside the main abdominal cavity. The right kidney usually sits a little lower than the left because the liver takes up space above it.
Kidney pain is usually felt in the flank, the area between the lowest rib and the top of the hip, a hand's width out from the spine. It is often one sided, deep and hard to point to with a single finger. It can spread forward toward the lower abdomen or groin.
In the body, the kidneys are positioned high in the back of the abdomen, tucked under the lower ribs on either side of the spine and closer to the back than the belly. Each one connects to the bladder through a thin tube called the ureter, which runs down toward the pelvis.
The clearest difference is how it responds to movement. Back muscle and joint pain usually changes when you bend, twist or shift position and eases with rest. Pain from the kidney tends to stay much the same whatever you do, sits higher and deeper, and is often one sided. Overlap happens, so a clinician should sort it out.
Stone pain usually begins high in the flank and travels downward and forward toward the lower abdomen, groin or genitals as the stone moves. It often comes in waves rather than staying steady, and people commonly cannot find a comfortable position. Severe pain of this kind needs same day medical assessment.
Yes, though one sided pain is more common. Pain on both sides can come from a cause affecting both kidneys, or from a back or muscle problem being felt across the whole lower rib area. Two sided flank pain with fever, vomiting or changes in urination should be assessed promptly.
Not on their own. Creatinine shows how well the kidneys are filtering, and the white blood cell count shows whether the immune system is responding somewhere, but neither points to a location. A urine test and, when needed, imaging answer the location question. A clinician decides which tests make sense.
Treat it as urgent and seek care straight away rather than waiting to see if it settles. Flank pain with fever or shaking chills, visible blood in the urine, or pain severe enough that you cannot sit still needs emergency care. Call 911 or your local emergency number if those signs are present.