Kidney stones are typically diagnosed through a combination of symptom evaluation, imaging, and lab tests. The process usually starts with recognizing a characteristic pain pattern, then confirming the stone’s presence and location with a scan. In most cases, a non-contrast CT scan is the gold standard, detecting stones with up to 97% sensitivity. But depending on your situation, your doctor may start with ultrasound or use blood and urine tests to understand what’s causing the stones in the first place.
What the Pain Pattern Tells Your Doctor
Before any scan or blood draw, the way your pain behaves gives a strong initial clue. The classic kidney stone presentation is sudden, severe pain in the flank (the side of your back below the ribs) that comes in waves and radiates downward toward the groin. Where exactly the pain travels depends on where the stone is lodged. A stone stuck near the top of the ureter (the tube connecting your kidney to your bladder) tends to cause deep flank pain. A stone in the middle section sends pain toward the front of the abdomen. A stone near the bladder radiates into the groin, testicle, or labia.
One detail that helps doctors distinguish kidney stones from other emergencies: people with stone pain can’t get comfortable. They pace, shift positions constantly, and writhe. This is the opposite of conditions like appendicitis or a perforated organ, where patients lie perfectly still because any movement makes the pain worse. Elevated heart rate and blood pressure are also common during an episode, even in people with no history of either. Other symptoms like nausea, blood in the urine, or a sudden urge to urinate frequently all reinforce the suspicion.
Non-Contrast CT: The Reference Standard
A non-contrast CT scan (meaning no dye is injected) is the most accurate imaging test for kidney stones. It picks up stones of virtually any size and composition, with a pooled sensitivity of 97% and specificity of 95% across studies. It also reveals other potential causes of your pain if the culprit turns out not to be a stone.
Radiation exposure used to be a significant concern with CT, but modern low-dose protocols have dramatically reduced it. A low-dose kidney stone CT delivers roughly 1.6 to 2.1 millisieverts of radiation, compared to 9.6 to 12.6 millisieverts with a standard-dose scan. That low-dose approach maintains the same high detection accuracy while cutting radiation by more than 80%. If you’re someone who has had multiple CT scans or is concerned about cumulative exposure, it’s worth knowing that most facilities now default to these reduced-dose protocols for stone evaluation.
When Ultrasound Comes First
Ultrasound is often the first imaging test ordered, particularly for pregnant women, children, and people who should avoid radiation. It uses no radiation, costs less than CT, and can reliably detect signs that a stone is blocking urine flow. For identifying the obstruction itself (swelling of the kidney, fluid buildup), ultrasound is up to 100% sensitive and 90% specific.
Its limitation is in directly seeing the stones. Overall sensitivity for detecting stones ranges from about 24% to 57%, and it struggles with smaller stones and those lodged in the middle portion of the ureter. A large study from the University of California San Francisco found that starting with ultrasound instead of CT led to no difference in serious complications, pain outcomes, return emergency visits, or hospitalizations. The takeaway: ultrasound is a reasonable first step. If it doesn’t give a clear answer, your doctor can follow up with a CT scan based on how your symptoms evolve.
Other Imaging Options
A plain abdominal X-ray (sometimes called a KUB) is quick and inexpensive but misses most small stones. It detects only about 8% of stones 5 mm or smaller. For larger stones over 5 mm, detection improves to around 78%. X-rays are more useful for tracking a known stone over time than for making an initial diagnosis.
MRI without contrast can identify signs of obstruction and surrounding fluid, reaching about 84% sensitivity when those indirect signs are present. It’s not a first-line choice for stone detection, but it fills a gap for patients who can’t have CT and whose ultrasound was inconclusive, particularly pregnant patients where both radiation and iodine-based contrast are concerns.
Blood and Urine Tests
Imaging confirms a stone is there, but blood and urine tests help explain why it formed. A basic blood draw checks kidney function and looks for elevated calcium or uric acid, both of which promote stone formation. These results also help your doctor assess whether the stone is affecting your kidney’s ability to filter waste.
A urinalysis (a quick dip test on a urine sample) can reveal blood in the urine, infection, or abnormal pH levels. If crystals are visible under the microscope, they can hint at what type of stone you’re dealing with.
For people with recurrent stones, a 24-hour urine collection provides much deeper insight. You collect all your urine over a full day, and the lab measures concentrations of calcium, oxalate, uric acid, citrate, magnesium, sodium, phosphorus, and pH, among other markers. A computer model then calculates your urine’s “supersaturation” for different crystal types, essentially quantifying how likely your urine chemistry is to produce each kind of stone. This test can predict whether you’re prone to calcium oxalate stones, calcium phosphate stones, uric acid stones, or other types, and it identifies specific factors (low fluid intake, high sodium, low citrate) that your treatment plan can target.
Analyzing a Passed Stone
If you pass a stone, saving it for analysis is one of the most valuable things you can do. Labs use techniques like infrared spectroscopy and X-ray diffraction to break down the stone’s exact mineral composition. This is more precise than any blood or urine test because it tells you definitively what the stone was made of.
Knowing the composition changes everything about prevention. Calcium oxalate stones (the most common type) call for different dietary and medical strategies than uric acid stones, which are heavily influenced by urine acidity. Your doctor will typically give you a strainer to use when urinating so you can catch the stone as it passes. Even a fragment is enough for the lab to work with.
Conditions That Mimic Kidney Stones
Several other conditions produce pain that feels similar to a kidney stone, which is part of why imaging matters so much. Appendicitis, ovarian cysts, ectopic pregnancy, and abdominal aortic aneurysm can all cause sudden flank or lower abdominal pain. Musculoskeletal problems and gallbladder disease sometimes overlap in location as well.
A few features help narrow things down before imaging results are back. Kidney stone pain is typically unilateral (one side only), comes in intense waves, and isn’t triggered by movement or eating. The pain often migrates downward over hours as the stone moves. Fever alongside stone symptoms raises concern for an infected, obstructed kidney, which is a more urgent situation that changes the treatment timeline significantly.