Flank pain is discomfort or sharp pain felt below the ribs and above the hip, typically along the side or back of the body. It originates when structures in or near the kidney area are stretched, inflamed, or irritated. While kidney stones are the most recognized cause, flank pain can stem from infections, muscle injuries, nerve conditions, and occasionally serious vascular problems that need urgent attention.
Where Exactly Is the Flank?
The flank sits between the bottom edge of your rib cage and the top of your hip bone, wrapping from your spine around to the side of your abdomen. This area houses the kidneys, the upper portion of the ureters (the tubes that carry urine from the kidneys to the bladder), and layers of muscle along the spine and abdominal wall. Pain here can come from any of those structures, which is why pinpointing the cause sometimes takes detective work.
Kidney Stones: The Most Common Cause
Kidney stones affect roughly 1 in 11 U.S. adults, making them the single most frequent reason people experience flank pain. The pain happens when a stone moves out of the kidney and into the ureter, stretching and irritating the narrow tube. It tends to be severe and sharp, centered in the side and back below the ribs, and it often radiates downward toward the lower abdomen and groin as the stone travels.
Unlike a steady ache, kidney stone pain typically comes in waves. You may feel intense cramping for several minutes, followed by a brief lull, then another surge. The location of the pain can shift as the stone progresses through the urinary tract. Many people also notice nausea, vomiting, blood-tinged urine, or a persistent urge to urinate. Stones are more common in men, older adults, non-Hispanic White individuals, and people with high blood pressure or diabetes.
For acute episodes, anti-inflammatory pain relievers are the guideline-recommended first choice, though many clinicians still reach for stronger pain medications in practice. Small stones often pass on their own within days to weeks with hydration and pain control. Larger stones may require a procedure to break them apart or remove them.
Kidney Infections
A kidney infection, called pyelonephritis, usually starts as a bladder infection that climbs upward. The classic combination is fever, flank pain, and nausea or vomiting, though not everyone gets all three. Fevers can spike above 103 °F (39.4 °C), and you may also have chills, loss of appetite, burning during urination, and a frequent or urgent need to go. Women are more likely to notice bladder-related symptoms like painful urination and visible blood in the urine alongside the flank pain.
What sets an infection apart from a stone is the systemic feeling of illness. You don’t just hurt on one side; you feel genuinely sick, often with fatigue, body aches, and a high fever. Kidney infections require antibiotics, and severe cases sometimes need intravenous treatment in a hospital to prevent the infection from spreading to the bloodstream.
Muscle Strain and Other Non-Kidney Causes
Not all flank pain comes from the kidneys. The muscles running alongside the spine and wrapping around the torso can strain from heavy lifting, twisting, or even prolonged awkward posture. Muscular flank pain tends to worsen with specific movements or when you press on the sore area, and it often improves with rest. There’s usually no fever, no change in urination, and no nausea. If your pain clearly ties to a physical activity or bending, a muscle spasm or strain is a likely explanation.
Rib injuries, pinched nerves in the mid-to-lower spine, and inflammation of the cartilage connecting ribs to the breastbone can all produce pain that lands in the flank region. These tend to be positional, meaning certain movements or postures make them noticeably better or worse.
Shingles Before the Rash Appears
Shingles can cause flank pain days before any visible skin changes show up, which makes it a surprisingly tricky diagnosis. The virus reactivates along a single nerve pathway and produces burning, tingling, or stabbing pain in a band-like pattern on one side of the body. This prodromal pain phase typically lasts three to five days before the characteristic blistering rash erupts, but in some cases the gap stretches to two weeks or longer.
During that pain-only window, shingles can easily be mistaken for a kidney problem, a muscle strain, or a pinched nerve. One reported case involved a man in his 70s who was evaluated for spinal stenosis for 10 days before the rash finally appeared and clarified the diagnosis. If you have a burning, one-sided pain in the flank that doesn’t match a urinary pattern and no rash is visible yet, shingles is worth considering, especially if you’re over 50 or have a weakened immune system.
Rare but Serious Vascular Causes
In uncommon situations, flank pain signals a blood clot in the artery supplying the kidney. Renal artery thrombosis causes sudden, sharp, unrelenting pain in the flank or upper abdomen. It may come with fever, nausea, and vomiting, and it can look a lot like a kidney stone on initial impression. The key difference is that the pain doesn’t come and go in waves; it stays constant and severe.
This condition is rare and typically occurs in people with predisposing factors like blood clotting disorders, sickle cell disease, significant blood vessel inflammation, or recent trauma. Spontaneous clots in an otherwise healthy artery are extremely uncommon but have been documented. Rapid diagnosis matters because restoring blood flow quickly is the only way to preserve kidney function.
How Flank Pain Is Diagnosed
When kidney stones are suspected, a CT scan without contrast dye is the preferred first-line imaging study. It picks up stones with high accuracy, shows their size and location, and can reveal alternative explanations like an enlarged kidney or abscess. For people with recurrent stones and familiar symptoms, the same type of CT scan remains the go-to choice. If the initial scan is inconclusive, contrast-enhanced CT or MRI can provide additional detail.
Pregnant individuals are an important exception. Because of radiation concerns, ultrasound is recommended as the starting point, with MRI as a backup if more information is needed. Urine tests, blood work, and a physical exam round out the evaluation regardless of the suspected cause. Urine samples can reveal blood, bacteria, or white blood cells that point toward stones or infection, while blood tests help flag signs of reduced kidney function or systemic inflammation.
Warning Signs That Need Prompt Attention
Certain combinations of symptoms with flank pain suggest a problem that shouldn’t wait. Flank pain paired with a high fever and chills points toward a kidney infection that could become dangerous if untreated. Blood in your urine, whether bright red or a brownish color, signals possible kidney or urinary tract involvement that warrants evaluation. Persistent nausea and vomiting alongside flank pain can lead to dehydration and may indicate a stone that isn’t passing or an infection worsening.
Unexplained flank pain that doesn’t improve over several days also deserves investigation, even without dramatic symptoms. Conditions like slow-growing obstructions or early infections can present subtly at first and become more serious over time.