Kidney stone testing typically starts with a combination of imaging and lab work. The gold standard is a non-contrast CT scan, which catches up to 97% of stones regardless of size or location. But depending on your situation, your doctor may use ultrasound, X-rays, blood tests, or urine tests, either alone or together, to confirm a stone and figure out what’s causing it.
CT Scans: The Most Reliable Test
A non-contrast CT scan (meaning no dye is injected) is the reference standard for detecting kidney stones. It works by taking rapid cross-sectional images of your abdomen and pelvis, and it can spot stones as small as 1 millimeter. A large meta-analysis found that even low-dose CT scans have a pooled sensitivity of 97% and specificity of 95%, meaning they catch almost every stone and rarely produce false alarms.
The scan itself takes only a few minutes. You lie on a table that slides through a doughnut-shaped machine, and you’re done. No special preparation is needed, and you don’t need to drink contrast fluid beforehand. Beyond finding the stone, a CT scan also reveals the stone’s size, its exact position in the urinary tract, and whether it’s blocking urine flow. It can also identify other conditions that mimic stone pain, like appendicitis or ovarian problems, which is especially useful in an emergency room setting where the diagnosis isn’t yet clear.
Ultrasound: The Radiation-Free Option
Ultrasound is the first-choice test for pregnant women and is often preferred for children, since it uses sound waves rather than radiation. It’s also commonly used as an initial screening tool in adults when a doctor wants to avoid CT radiation exposure.
The trade-off is accuracy. Earlier studies found ultrasound detects only 24% to 57% of stones overall, with particular difficulty spotting small stones and stones lodged in the ureter (the tube connecting the kidney to the bladder). More recent research puts the sensitivity higher, around 89% for detecting at least one stone in a kidney, but performance still drops for stones under 5 millimeters. Ultrasound is better at picking up indirect signs of a stone, like swelling of the kidney from backed-up urine, which can raise confidence in the diagnosis even when the stone itself isn’t visible.
X-Rays: Limited but Still Useful
A plain abdominal X-ray, sometimes called a KUB (kidneys, ureters, bladder), is a quick and inexpensive test, but it misses a lot. Its sensitivity is only about 49%, meaning it catches roughly half the stones a CT scan would find. The main problem is that certain stone types, particularly uric acid stones, are radiolucent, meaning they don’t show up on a standard X-ray at all. Even stones that are visible on X-ray become hard to detect when they’re smaller than 5 millimeters.
X-rays are more useful for tracking a stone you already know about. If a CT scan has confirmed a stone and it’s the type that shows up on X-ray, follow-up KUB films can monitor whether it’s moved or passed without repeating the higher-radiation CT.
Urine Tests
A basic urinalysis is almost always part of the workup. You provide a urine sample, and the lab checks it for blood (which is present in most active stone episodes), signs of infection, and crystals that hint at what the stone is made of. The urine’s pH level also provides clues: very acidic urine is associated with uric acid stones, while alkaline urine can point toward certain calcium-based or infection-related stones.
If you’ve had stones before or your doctor suspects an underlying metabolic problem, a more involved test called a 24-hour urine collection may be ordered. This means you collect every drop of urine over a full day in a provided container. The lab then measures levels of calcium, oxalate, citrate, uric acid, sodium, potassium, magnesium, phosphorus, and creatinine, along with total volume and pH. The results create a detailed map of your stone risk factors. For example, high calcium or oxalate levels point to the most common stone types, while low citrate (a natural stone inhibitor) suggests your urine isn’t doing enough to prevent crystals from forming. The American Urological Association recommends this testing for anyone with recurrent stones and for first-time stone formers who want to understand their risk.
Blood Tests
Blood work serves two purposes: checking how well your kidneys are functioning right now, and looking for metabolic conditions that cause stones to form in the first place. A standard panel includes creatinine (which reflects kidney function), calcium, uric acid, sodium, and potassium. A complete blood count and markers of inflammation may be added to check for infection, especially if you have a fever alongside stone symptoms.
Certain blood results trigger further investigation. High calcium in the blood, for instance, can signal a parathyroid gland problem that drives stone formation. Elevated uric acid may point toward gout-related stones. These findings don’t just confirm the current stone; they guide a prevention plan so you’re less likely to develop another one.
Stone Analysis After You Pass One
If you pass a stone or have one removed surgically, your doctor will likely send it to a lab for composition analysis. The gold-standard method uses infrared spectroscopy or X-ray diffraction, techniques that identify the exact minerals in the stone. Older chemical analysis methods are less precise and are being phased out in favor of these approaches.
Knowing what a stone is made of changes everything about prevention. Calcium oxalate stones (the most common type) call for different dietary and medical strategies than uric acid stones or the rarer struvite stones linked to chronic urinary infections. If you’re given a strainer to use at home while waiting for a stone to pass, using it consistently is one of the most valuable things you can do for your long-term care.
MRI: A Less Common Alternative
MRI is rarely used for kidney stones because it doesn’t image stones directly the way CT does. In one study comparing the two, MRI detected stones in only 50% of patients versus 91% for CT. However, MRI can detect the secondary effects of a stone, like fluid buildup around the kidney and a swollen ureter. When those indirect signs are factored in, MRI’s accuracy improves to about 86%. It’s generally reserved for situations where both CT and ultrasound are unsuitable.
What to Expect During a Typical Workup
If you go to an emergency room or urgent care with sudden, severe flank pain, the typical sequence looks like this: a urine sample and blood draw happen almost immediately, and imaging (usually a CT scan) follows shortly after. Results from these tests together confirm whether a stone is present, how big it is, where it’s stuck, and whether it’s causing complications like infection or significant blockage.
For smaller stones likely to pass on their own (generally under 5 to 6 millimeters), the focus shifts to pain management and monitoring. For larger stones or those causing obstruction, the imaging details guide decisions about procedures to break up or remove the stone. If this is your first stone and you want to reduce the chance of a second one, expect your doctor to recommend a 24-hour urine collection and stone analysis once the acute episode is over. About half of people who form one kidney stone will form another within 5 to 10 years, so the metabolic workup is worth completing.