Kidney stones are diagnosed through a combination of imaging, urine tests, and blood work. The single most accurate test is a non-contrast CT scan, which catches 97% of stones regardless of size or type. But the full picture of testing depends on whether you’re in acute pain right now or trying to figure out why you keep forming stones. Here’s what each test does and what to expect.
CT Scans: The Most Reliable Test
A non-contrast CT scan (meaning no dye is injected) is the gold standard for finding kidney stones. A meta-analysis of seven studies found it has 97% sensitivity and 95% specificity, which means it catches nearly every stone and rarely flags something that isn’t one. The scan takes just a few minutes, and because no contrast dye is needed, preparation is minimal. You can eat, drink, and take your medications beforehand as normal.
Most hospitals now use low-dose CT protocols that deliver significantly less radiation than a standard scan. For stones 3 millimeters or larger, low-dose CT performs nearly as well as the full-dose version, with sensitivity around 97% for mid-size stones and 100% for stones 5 millimeters and up. The one weakness: very small stones under 3 millimeters are harder to spot on low-dose scans, with detection dropping to about 83%. For most people, though, these tiny stones are the ones most likely to pass on their own, so missing them rarely changes treatment.
If you have a higher body weight (BMI over 30), low-dose scans lose some accuracy for detecting stones in the ureter, the tube connecting the kidney to the bladder. Your doctor may opt for a standard-dose scan in that case to get a clearer picture.
Ultrasound as a First Step
Ultrasound uses no radiation at all, which makes it the preferred starting point for pregnant women, children, and people who’ve had repeated imaging. It’s good at finding stones still sitting in the kidney, especially larger ones, but it’s less reliable for spotting smaller stones or those that have moved into the ureter. When ultrasound results are inconclusive and symptoms point strongly toward a stone, a CT scan usually follows.
X-Rays and Their Limits
A standard abdominal X-ray (sometimes called a KUB, for kidneys-ureters-bladder) can show calcium-based stones, which are the most common type. But it misses uric acid stones entirely because they don’t show up on plain film. It also can’t distinguish a stone from other calcifications in the body. X-rays are sometimes used for follow-up, to track a known stone’s position over time, but they aren’t reliable enough to be the primary diagnostic tool.
Urine Tests
A basic urinalysis is one of the first things ordered when a stone is suspected. The lab checks for blood in the urine (which isn’t always visible to the naked eye), crystals, and signs of infection. Blood in the urine supports the diagnosis, though not every stone causes it, and not every case of blood in the urine means a stone.
If you’ve already passed a stone or had one removed, a 24-hour urine collection gives a much deeper look at why you’re forming stones in the first place. You collect every drop of urine over a full day into a container provided by your lab. The sample is analyzed for several key substances: calcium (excess calcium is the most common abnormality in stone formers), oxalate (which combines with calcium to form the most common stone type), citrate (a natural stone inhibitor, so low levels are a red flag), uric acid, and overall urine volume and pH. Uric acid stones tend to form when urine is too acidic, while calcium phosphate stones are more likely when urine is too alkaline. These results help your doctor recommend specific dietary changes or preventive treatment tailored to your chemistry.
Blood Tests
Blood work during a kidney stone evaluation typically checks calcium levels, uric acid, and kidney function markers like creatinine. High blood calcium can point to an overactive parathyroid gland, four small glands behind the thyroid that regulate calcium and phosphorus. When these glands produce too much hormone, calcium builds up in the blood and eventually in the kidneys, forming stones. Identifying this pattern matters because it means the underlying cause is a hormonal problem that can be treated, not just a dietary one. Elevated uric acid in the blood suggests a tendency toward uric acid stones and may also signal gout.
Stone Analysis After Passing
If you pass a stone or have one removed surgically, the actual stone itself is one of the most valuable diagnostic tools. Labs analyze its composition using infrared spectroscopy, which identifies the precise mineral makeup. This is considered more reliable than older chemical analysis methods. Knowing whether your stone is calcium oxalate, calcium phosphate, uric acid, or a less common type like struvite directly shapes prevention strategies. Your doctor will likely ask you to strain your urine through a fine mesh filter to catch any fragments on their way out.
Can You Test at Home?
Urine pH test strips are cheap and available at most pharmacies, and some stone formers use them to monitor whether their urine is too acidic or too alkaline. In practice, their usefulness is limited. A study of stone-forming patients found that only 59% had dipstick pH readings within 0.5 units of their actual 24-hour urine pH. That margin of error is too wide to guide treatment decisions reliably. Home strips can give you a rough sense of trends if you test multiple times a day over several days, but they aren’t a substitute for a lab-grade 24-hour collection.
There is no reliable at-home test that can confirm whether you currently have a kidney stone. If you’re experiencing sharp flank pain, pain that radiates to your lower abdomen or groin, nausea, or visible blood in your urine, imaging and lab work are the only way to know for sure what’s happening.