Kidney stones are diagnosed through a combination of symptom evaluation, imaging, and lab tests. In most cases, a non-contrast CT scan is the single most reliable tool, with 97% accuracy for detecting stones. But the full diagnostic picture usually involves several steps, from recognizing the classic pain pattern to analyzing blood and urine for underlying causes.
Recognizing the Symptoms
The hallmark of a kidney stone is renal colic: a sudden, severe pain that starts in the flank (the area between your lower ribs and hip on one side of your back) and radiates forward and downward toward the groin or, in men, the testicle. The pain tends to come in waves rather than staying constant, and it’s often intense enough that people can’t find a comfortable position. Nausea and vomiting frequently accompany it.
Blood in the urine is present in about 85% of cases. Sometimes it’s visible to the naked eye; other times it only shows up under a microscope during a urinalysis. That said, the absence of blood doesn’t rule out a stone. The combination of sudden flank-to-groin pain with nausea and blood in the urine is distinctive enough that most clinicians will suspect a kidney stone quickly, but imaging is needed to confirm it.
Why CT Scans Are the Gold Standard
A non-contrast CT scan of the abdomen and pelvis is the most reliable way to confirm a kidney stone. “Non-contrast” means no dye is injected beforehand, which makes it quick and straightforward. In studies, this type of scan correctly identifies stones with 97% sensitivity and 96% specificity, meaning it catches nearly all real stones and rarely flags something that isn’t one.
CT is preferred because it reveals details that directly affect treatment decisions: the stone’s exact size, location, number, and density. Size matters enormously. Stones smaller than 3 mm pass on their own about 98% of the time. At 4 mm, the rate drops to 81%. At 5 mm, roughly 65% will pass. Once a stone reaches 6 mm or larger, the odds of spontaneous passage fall to 33% or lower, and stones 6.5 mm and above pass on their own only about 9% of the time. Knowing these numbers helps you and your doctor decide between waiting it out and pursuing a procedure.
When Ultrasound Is Used Instead
CT scans involve radiation, which makes them a poor choice for two groups: pregnant women and children. For both, ultrasound is the first-line imaging tool.
In pregnancy, ultrasound avoids radiation exposure to the fetus entirely. The trade-off is accuracy. Its sensitivity for detecting stones during pregnancy ranges from 34% to 86%, depending heavily on the skill of the operator, the position of the fetus, and where the stone is located. Pregnancy also causes the kidneys to swell naturally (a condition called physiologic hydronephrosis), which can look similar to the swelling caused by a blocking stone, making interpretation trickier.
In children, ultrasound detects stones with 59% to 78% sensitivity and nearly 100% specificity. It may miss stones lodged in the ureter (the tube connecting kidney to bladder), but it’s good at picking up the swelling that suggests a stone is causing a blockage. Pairing ultrasound with a plain X-ray of the abdomen can improve detection of certain stone types. MRI is another radiation-free option, though it doesn’t reliably visualize stones in the upper urinary tract or measure stone density, so it plays a limited role.
Blood and Urine Tests
Imaging tells you where a stone is. Lab work tells you why it formed and whether anything else is going on. A basic blood panel typically checks creatinine (to assess kidney function), calcium, uric acid, and electrolytes like sodium, potassium, and bicarbonate. Elevated calcium can point to a parathyroid problem. High uric acid suggests gout-related stones. Abnormal creatinine may signal that a stone is impairing kidney function.
A urinalysis examines urine for blood, signs of infection, and crystals. The type of crystal offers clues about stone composition. Calcium oxalate and calcium phosphate crystals are the most common. Uric acid crystals tend to form when urine is too acidic, often in people with gout or type 2 diabetes. Struvite crystals are linked to repeated urinary tract infections and can grow into very large stones. Urine pH is part of this picture too: acidic urine favors uric acid stones, while alkaline urine favors calcium phosphate and struvite formation.
Conditions That Mimic Kidney Stones
Several other conditions cause sudden abdominal or flank pain and can look like a kidney stone before imaging is done. Appendicitis, particularly when the appendix sits in an unusual position, can produce right-sided pain that overlaps with stone symptoms. Diverticulitis typically causes left-sided lower abdominal pain. Ovarian cysts or ectopic pregnancy can mimic stones in women. A kidney infection can cause flank pain with fever. Even musculoskeletal problems or a pinched nerve can occasionally create confusing symptoms. This is one reason imaging is so important: the clinical picture alone, while suggestive, isn’t enough for a definitive diagnosis.
Analyzing the Stone Itself
If you pass a stone or have one removed surgically, your doctor will likely send it for composition analysis. This is one of the most valuable diagnostic steps because knowing exactly what a stone is made of shapes long-term prevention. The preferred methods are infrared spectroscopy and X-ray diffraction. In both techniques, the stone is ground into a fine powder, then analyzed to identify its chemical components with high precision.
Older chemical analysis kits dissolve the stone and test for individual ions, then work backward to deduce the original compounds. These kits are still used in some settings but are less precise than modern spectroscopy. More advanced research tools like micro-CT can produce detailed three-dimensional images of a stone’s internal structure, though this isn’t standard clinical practice.
The 24-Hour Urine Collection
For people who’ve had more than one stone, or whose first stone suggests an underlying metabolic issue, a 24-hour urine collection is the key diagnostic step for prevention. It measures the specific substances in your urine that promote or inhibit stone formation.
The process is straightforward but requires discipline. You discard your first morning void on day one, then collect every drop of urine for the next 24 hours, including the first void the following morning. The collection container should stay refrigerated. A preservative solution is typically included to keep the sample stable.
The lab measures total urine volume, pH, calcium, oxalate, citrate, uric acid, magnesium, phosphate, and sulfate. Each value tells a specific part of the story. Low citrate (a natural crystallization inhibitor) is one of the most common findings in calcium stone formers, showing up in roughly one-third of cases. Optimal citrate levels are around 600 mg per day. Elevated oxalate, found in another third of calcium stone patients, is flagged when levels exceed about 40 mg per day, with a treatment goal of 25 mg or lower. High urine calcium, low urine volume, and abnormal pH each point to different dietary or metabolic corrections.
The results from this test are what allow a tailored prevention plan, whether that means increasing fluid intake, adjusting diet, or in some cases starting medication to change urine chemistry. Without it, prevention advice stays generic. With it, your doctor can target the exact imbalance driving your stone formation.