Kidney stones are typically diagnosed through a combination of imaging, urine tests, and blood work. The single most accurate test is a non-contrast CT scan of the abdomen and pelvis, which detects up to 95% of stones regardless of size or location. But the full diagnostic picture usually involves several steps, starting from the moment you arrive with pain and continuing well after a stone has passed.
The Physical Exam: What Happens First
Before any imaging or lab work, a doctor will assess your symptoms and perform a physical exam. The hallmark check is for costovertebral angle tenderness, where the examiner places a hand on your back just below the ribs near the spine and taps it with a closed fist. If this reproduces or worsens your pain on one side, it strongly suggests a stone or kidney issue. One study found this test has a specificity of 99%, meaning a positive result almost always points to a real problem, though it misses many cases on its own (sensitivity around 15%). The location and pattern of your pain, along with symptoms like nausea or blood in your urine, help guide which tests come next.
CT Scan: The Most Reliable Test
A non-contrast CT scan of the abdomen and pelvis is the gold standard for kidney stone detection. “Non-contrast” means no dye is injected into your veins, which makes the scan quick and straightforward. You lie on a table that slides through the scanner, and the whole process takes only a few minutes. The American Urological Association considers it the most reliable imaging tool for evaluating stone size, location, number, and density.
Early studies found sensitivity and specificity approaching 100% for detecting stones of all sizes. Even in broader clinical use, detection rates exceed 95%. The scan also reveals other potential causes of your pain, like appendicitis or an ovarian cyst, which can mimic stone symptoms. The main downside is radiation exposure, which is why doctors weigh the benefits carefully, especially for younger patients or those who need repeated imaging.
Ultrasound: When CT Isn’t Ideal
Ultrasound uses sound waves instead of radiation, making it the preferred first choice for pregnant patients, children, and people who need frequent monitoring. It’s also widely available and can be done at the bedside. The tradeoff is accuracy. Pooled data puts ultrasound sensitivity at roughly 45%, though individual studies range from 24% to 93% depending on technique and stone size.
Larger stones are much easier to spot. Ultrasound has a positive predictive value of about 94% for stones over 10 mm, meaning when it identifies a big stone, it’s almost always right. For small stones (1 to 4 mm), that predictive value drops to around 50%. There’s also a sizing problem: among stones that ultrasound measured at 5 to 7 mm, only 28% actually measured at least 5 mm on a follow-up CT. This matters because stone size influences treatment decisions. For routine surveillance of known stones, ultrasound works well enough to track major changes, but CT remains more precise when surgical planning is involved.
Plain X-Ray and Its Limits
A standard abdominal X-ray (sometimes called a KUB, for kidneys, ureters, and bladder) is fast and low-cost, but it misses a significant number of stones. Only about 60% of kidney stones are radiopaque, meaning they show up on a plain X-ray. Calcium-based stones, which account for over 75% of all stones, generally appear. Uric acid stones, however, are radiolucent. They contain only lightweight elements like carbon, nitrogen, and oxygen, which don’t absorb X-rays well enough to create a visible image. This means a clear X-ray doesn’t rule out a stone. Plain films are most useful for tracking stones that have already been identified on CT, not for initial diagnosis.
Urine Tests
Urinalysis
A basic urinalysis is one of the first tests ordered when a stone is suspected. The main finding doctors look for is blood in the urine (hematuria), which is present in the vast majority of stone cases. Studies have found microscopic hematuria in 95% to 100% of patients with confirmed stones. Urinalysis can also reveal crystals that hint at the stone type, signs of infection, and urine pH, which influences what kinds of stones are likely to form.
24-Hour Urine Collection
This test is less about diagnosing a current stone and more about understanding why you’re forming them, especially if you’ve had more than one episode. You collect all of your urine over a full 24-hour period in a provided container, which is then sent to a lab. The analysis measures a long list of substances: total urine volume, calcium, oxalate, citrate, uric acid, pH, magnesium, phosphate, sodium, potassium, and sulfate. Some labs also calculate supersaturation values, which estimate how likely your urine is to form crystals.
Low citrate levels, for instance, suggest your urine lacks a natural inhibitor of stone formation. High oxalate or calcium points toward the most common stone types. High uric acid paired with acidic urine pH favors uric acid stones. If you have a history of cystine stones or a positive screening test for cystine, the collection can measure cystine excretion specifically. The results give your doctor a metabolic roadmap for preventing future stones through diet changes, fluid intake targets, or medication.
Blood Tests
Blood work during a stone episode serves two purposes: checking kidney function and looking for metabolic clues. A basic panel typically includes calcium, uric acid, creatinine (a marker of kidney function), and blood urea. Electrolytes like sodium, potassium, and chloride round out the picture.
The results can point toward the type of stone you’re dealing with. People who form calcium oxalate stones, the most common variety, tend to have significantly higher serum calcium levels. Those who form uric acid stones often show elevated blood sugar, uric acid, blood urea, and creatinine. Abnormal results may also uncover an underlying condition driving stone formation, such as overactive parathyroid glands pushing calcium levels too high.
Analyzing a Passed or Removed Stone
If you manage to catch a stone after passing it, or if one is removed surgically, lab analysis of the stone itself provides the most definitive answer about its composition. The stone is washed, dried, and ground into a powder. Two main techniques are used. Infrared spectroscopy shines infrared light through the powdered sample and matches the resulting pattern against a library of known stone compositions, identifying specific compounds like calcium oxalate, calcium phosphate, uric acid, struvite, or cystine. Chemical spot testing takes a different approach, dissolving the powder and adding reagents that produce specific colors, precipitates, or bubbles to indicate which ions are present.
Knowing the exact composition shapes prevention. A uric acid stone calls for alkalinizing the urine and reducing purine-rich foods. A calcium oxalate stone may call for limiting high-oxalate foods and ensuring adequate calcium intake (counterintuitively, dietary calcium actually helps). Struvite stones signal chronic urinary infections that need to be addressed. This is why doctors strongly encourage you to strain your urine through a fine mesh or filter during a stone episode. Even a fragment a few millimeters wide is enough for analysis.
Testing During Pregnancy
Pregnant patients present a unique challenge because the most accurate test, CT, involves ionizing radiation. The American College of Obstetricians and Gynecologists recommends ultrasound as the first-line imaging choice during pregnancy, since it poses no known risk to the fetus. If ultrasound is inconclusive and more information is needed, non-contrast MRI is the preferred next step. MRI avoids radiation entirely, though gadolinium contrast should be used only when it would significantly change the diagnosis or outcome.
CT is not strictly off-limits during pregnancy. The radiation dose from a single scan is well below levels associated with fetal harm. But when ultrasound or MRI can answer the clinical question, they’re the safer path. If a pregnant patient does need multiple imaging studies involving radiation, consulting a radiation physicist to calculate the total fetal dose is a reasonable precaution.