Kidney stones are diagnosed through a combination of symptoms, urine and blood tests, and imaging. The process typically starts when you show up with severe flank pain, and a non-contrast CT scan is the gold standard for confirming a stone is present. But the full diagnostic picture involves several steps, each designed to answer a different question: Is it actually a stone? Where is it? How big is it? And what’s it made of?
What Symptoms Point to a Kidney Stone
The classic presentation is a sudden onset of severe pain that starts in your flank (the side of your back, below your ribs) and radiates down toward your groin. This happens when a stone moves into the ureter, the narrow tube connecting your kidney to your bladder, and partially or fully blocks urine flow. The pain can shift to your upper or lower abdomen as the stone travels.
One telling sign is how you respond to the pain. People with kidney stones tend to pace, shift positions constantly, and writhe, unable to get comfortable. This is distinctly different from conditions like appendicitis, where patients lie perfectly still because any movement makes the pain worse. That behavioral difference is something doctors look for during an initial assessment.
At least half of people with an active stone episode experience nausea and vomiting. Elevated heart rate and blood pressure are also common, even in people with no history of cardiac or blood pressure problems. During a physical exam, pressing on the area where the lower ribs meet the spine (called the costovertebral angle) typically produces dramatic tenderness. Beyond that, the rest of the physical exam is often unremarkable, which is why imaging and lab tests are so important.
Urine and Blood Tests
A urinalysis is one of the first things ordered when a kidney stone is suspected. The main thing it looks for is blood in your urine. About 85 to 94 percent of people with a confirmed stone will have at least microscopic amounts of blood, meaning it’s detectable under a microscope even if your urine looks normal to the naked eye. That still leaves 6 to 15 percent of stone patients with no blood in their urine at all, so a clean urinalysis doesn’t rule out a stone.
Blood tests round out the initial workup. A standard panel includes electrolytes, calcium, creatinine (which measures kidney function), and uric acid. These results help identify metabolic issues that may have caused the stone in the first place. High calcium or uric acid levels, for instance, point toward specific stone types and can shape both treatment and prevention strategies.
CT Scans: The Primary Imaging Tool
A non-contrast CT scan is the preferred first imaging study for suspected kidney stones, backed by the highest level of clinical evidence from the American Urological Association. “Non-contrast” means no dye is injected. Stones show up clearly on their own because they’re denser than surrounding tissue.
This type of CT has a sensitivity as high as 97 percent, meaning it catches nearly every stone present. A meta-analysis of seven studies found that even low-dose CT protocols (which use less radiation) achieved 97 percent sensitivity and 95 percent specificity. The scan also reveals stone size, location, and whether it’s causing any blockage, all of which determine what happens next. Sensitivity does drop for very small stones, but for clinically significant ones, CT is the most reliable tool available.
A newer technique called dual-energy CT can go a step further by identifying what a stone is made of before it’s ever passed or removed. It works by scanning at two different energy levels and comparing how the stone absorbs each one. Uric acid stones produce a distinctly different ratio than calcium-based stones. This matters because uric acid stones can sometimes be dissolved with medication, while calcium stones cannot.
When Ultrasound Is Used Instead
Ultrasound uses no radiation, which makes it the go-to option for pregnant patients and a reasonable first choice for children. It’s also commonly used in follow-up appointments to monitor known stones over time.
The trade-off is accuracy. Ultrasound detects kidney stones with a sensitivity of about 77 percent, compared to CT’s 97 percent. It also tends to overestimate stone size, measuring stones at an average of 8.7 mm when the same stones measured 5.5 mm on CT. This overestimation is more pronounced with smaller stones and in patients with a higher body mass index. In practical terms, that size inflation could lead to unnecessary procedures in up to 40 percent of cases where ultrasound shows a stone larger than 4 mm. For initial diagnosis, ultrasound can confirm a stone if one is found, but a negative result is less reassuring than a negative CT.
The 24-Hour Urine Collection
If you’ve had a kidney stone, especially more than one, your doctor will likely order a 24-hour urine collection. This test isn’t about finding the stone itself. It’s about figuring out why you’re forming stones so you can prevent the next one.
You collect all of your urine over a full day into a provided container. The lab then measures a panel of substances: calcium, oxalate, citrate, uric acid, sodium, magnesium, potassium, phosphorus, creatinine, total volume, and pH. Each of these tells a different part of the story. Low citrate levels, for example, mean your urine lacks a natural stone inhibitor. High calcium or oxalate levels mean there’s too much raw material available to form stones. Low urine volume means everything is too concentrated. The results guide specific dietary changes or medications tailored to your individual chemistry.
Identifying the Stone Itself
Whenever a stone is passed naturally or removed surgically, it should be sent to a lab for composition analysis. This is the most direct way to know exactly what type of stone you had. The most common types are calcium oxalate (accounting for the majority of stones), calcium phosphate, uric acid, and struvite (which are linked to urinary tract infections).
Knowing the composition changes prevention advice significantly. A person with calcium oxalate stones might need to reduce high-oxalate foods like spinach and almonds, while someone with uric acid stones might benefit from making their urine less acidic. If you pass a stone at home, straining your urine through a fine mesh or filter and saving anything solid is one of the most useful things you can do for your long-term care.
Signs That Require Emergency Evaluation
Most kidney stone episodes, while extremely painful, are managed on an outpatient basis. But certain combinations of symptoms signal a true emergency. A fever alongside an obstructing stone suggests infection trapped behind the blockage, which can progress to sepsis rapidly. This requires urgent intervention to drain the kidney.
Other emergency scenarios include complete inability to urinate, stones blocking both kidneys simultaneously, or a stone blocking the only functioning kidney in someone who has just one. Hospital admission may also be necessary if you can’t keep fluids down due to persistent vomiting, if pain doesn’t respond to strong medications, or if you’re very young, elderly, or medically fragile. In these cases, imaging and intervention happen on an accelerated timeline.