Kidney stones are diagnosed through a combination of imaging, urine tests, and blood work, with a non-contrast CT scan being the single most accurate test available. Most people arrive at the emergency room or urgent care with sudden, severe flank pain, and the diagnostic process typically moves quickly from a physical exam to imaging to confirm whether a stone is present, how big it is, and where it’s lodged.
What Happens During the Physical Exam
The first step is a hands-on evaluation. Your doctor will press on your back near the lower ribs, checking for what’s called costovertebral angle tenderness. Sharp pain in this area is a hallmark of kidney stones. Interestingly, the rest of the abdominal exam is usually unremarkable. Your belly won’t feel rigid or tender to the touch the way it would with appendicitis or other abdominal emergencies, and this distinction helps narrow things down before any imaging is ordered.
Your bowel sounds may be quieter than normal. Severe pain from a stone can temporarily slow gut activity, which is common and not a cause for concern on its own. The absence of signs pointing to the abdominal lining being inflamed is one of the key clues that separates kidney stone pain from surgical emergencies.
Imaging: CT Scans vs. Ultrasound
A non-contrast CT scan is the gold standard for diagnosing kidney stones. It detects stones with about 98% sensitivity and 97% specificity, meaning it catches nearly every stone and rarely produces a false alarm. The scan takes just minutes, requires no contrast dye injection, and shows the stone’s exact size and location. It also reveals whether the stone is blocking urine flow, which changes how urgently it needs to be treated.
Low-dose CT protocols, which use significantly less radiation than a standard scan, still perform remarkably well, catching about 97% of stones. Many hospitals now default to low-dose scans for suspected kidney stones, especially in younger patients where cumulative radiation exposure is a concern.
Ultrasound is less accurate, detecting only 50 to 60% of stones in the ureter (the tube connecting the kidney to the bladder). But it plays an important role in two situations. If you’re a known stone former who has had visible stones on prior imaging, ultrasound paired with a simple abdominal X-ray can be enough to track what’s happening without another CT. Ultrasound is also the first choice for children, where avoiding radiation matters most.
Imaging During Pregnancy
For pregnant patients, ultrasound is the recommended first-line test. Both the American College of Radiology and the American College of Obstetricians and Gynecologists agree on this approach. A transvaginal ultrasound can sometimes spot stones in the lower ureter that a standard kidney ultrasound misses. Doppler ultrasound, which measures blood flow patterns in the kidney, can detect increased resistance that signals a blockage even when a stone isn’t directly visible.
One complication: up to 35% of pregnant patients with a confirmed ureteral blockage show no obvious swelling of the kidney on ultrasound, making diagnosis trickier. When ultrasound results are unclear, MRI without contrast is the next step. CT and X-rays aren’t absolutely off-limits during pregnancy (the radiation dose is generally below the threshold for fetal harm), but they’re reserved for situations where non-radiation options haven’t provided a clear answer.
Blood and Urine Tests
Imaging confirms a stone exists, but blood and urine tests reveal what’s going on around it. A urinalysis checks for blood in the urine, which is present in most kidney stone episodes. It also screens for white blood cells and bacteria, which would indicate an infection. A stone that’s blocking urine flow while an infection is present is a urological emergency requiring immediate drainage, so this test carries real urgency.
Blood tests measure kidney function and check for elevated levels of minerals like calcium and uric acid. High blood calcium, for instance, can point to an overactive parathyroid gland as the underlying cause of repeated stones. These results help your doctor understand not just the current episode but whether there’s a metabolic reason you’re forming stones in the first place.
Why Stone Size Matters in Diagnosis
Once imaging identifies a stone, its size in millimeters largely determines what happens next. The relationship between size and the odds of passing naturally is well established:
- Smaller than 2 mm: 85 to 90% chance of passing on its own
- 2 to 4 mm: about 80% chance
- 4 to 6 mm: 60 to 70% chance
- 6 to 10 mm: 30 to 50% chance
- Larger than 10 mm: less than 10% chance
Once a stone exceeds 6 to 7 mm, the likelihood of it passing without intervention drops below 50%. Stones larger than 10 mm almost always require a procedure. This is why precise measurement on imaging matters so much. It’s not just confirming a stone is there; it’s determining whether you’ll be sent home with pain management or scheduled for treatment.
The 24-Hour Urine Collection
If you’ve had more than one kidney stone, your doctor will likely order a 24-hour urine collection. This isn’t part of the acute diagnosis. It’s a prevention tool, usually done weeks after the stone episode has resolved. You collect every drop of urine over a full day into a container, and the lab measures levels of calcium, oxalate, citrate, uric acid, sodium, phosphorus, potassium, and magnesium.
The results create a chemical profile of your urine that explains why stones are forming. High calcium, high oxalate, high phosphorus, and high sodium all increase stone risk. On the protective side, higher levels of citrate, potassium, magnesium, and greater urine volume are associated with fewer stones. Low urine volume is one of the most common and most fixable risk factors. Even urine pH matters: overly acidic urine promotes uric acid stones, while overly alkaline urine can encourage calcium phosphate stones.
This collection gives your doctor specific targets for dietary changes or medications. Without it, prevention advice is generic. With it, recommendations can be tailored to whatever imbalance is driving your stone formation.
Conditions That Mimic Kidney Stones
Several conditions produce pain that feels nearly identical to a kidney stone, which is part of why imaging is so important rather than diagnosing based on symptoms alone. Appendicitis can cause right-sided flank pain that overlaps with stone pain. Gallbladder attacks produce upper abdominal pain that sometimes radiates to the back. Diverticulitis, ovarian cysts, and pelvic inflammatory disease can all present similarly, particularly in women.
The most dangerous mimic is a leaking abdominal aortic aneurysm, which is frequently misdiagnosed as kidney stone pain in older adults. This is a life-threatening emergency, and it’s one of the main reasons emergency physicians rely on CT imaging rather than treating flank pain presumptively. A urinary tract infection can also coexist with a stone, and missing that combination can lead to a rapidly worsening infection that backs up behind the blockage.
Preparing for Your Tests
A non-contrast CT scan for kidney stones requires almost no preparation. You won’t need to fast, drink contrast liquid, or have an IV placed. You’ll lie on a table, hold still for a few minutes, and the scan is done.
Ultrasound preparation depends on what your doctor needs to see. In some cases, you’ll be asked to drink 30 to 32 ounces of water an hour before the test and avoid using the restroom so your bladder is full during imaging. Your technician may then ask you to empty your bladder partway through so they can compare images with it full and empty. Occasionally, you may be asked to skip eating after midnight the night before, though this varies by facility.