Kidney stones do show up in urine tests, but not in the way you might expect. A standard urinalysis can’t detect a stone directly or tell you its size and location. What it can reveal are telltale signs that a stone is present: blood in your urine, crystals in the sediment, abnormal pH levels, and markers of infection. These clues are useful, but a urine test alone has a sensitivity of only about 69% for confirming stones, meaning it misses roughly one in three cases. Imaging, particularly a CT scan, remains the most reliable way to confirm a stone.
What a Standard Urinalysis Can Detect
When your doctor orders a urinalysis for suspected kidney stones, the lab looks at several things in your urine sample. The most important is blood. Even when you can’t see it yourself, microscopic blood (called hematuria) appears in about 84% of people with confirmed kidney stones. A dipstick test catches it slightly more often, picking up blood in roughly 90% of confirmed cases, while microscopic examination under a lab microscope detects it in about 78%. So while blood in the urine is a strong indicator, its absence doesn’t rule stones out. Around 10 to 20% of people with a confirmed stone have no detectable blood at all.
The lab also checks the urine sediment for crystals. These tiny mineral formations can hint at what type of stone you’re dealing with. Calcium oxalate crystals suggest high calcium or oxalate levels. Uric acid crystals, which tend to be amber-colored, show up in acidic urine and point toward uric acid stones. Cystine crystals indicate a hereditary condition. Struvite crystals are almost always linked to a urinary tract infection caused by certain bacteria. Finding crystals doesn’t guarantee a stone exists, and not finding them doesn’t mean you’re in the clear, but they add a useful piece to the puzzle.
Urine pH is another clue. When pH drops below 5.5, the urine becomes saturated with uric acid, which promotes uric acid stone formation. On the alkaline side, higher pH levels favor calcium phosphate and struvite stones. Knowing your urine pH helps your doctor understand which type of stone you’re likely forming and how to prevent future ones.
Signs of Infection Alongside Stones
A urinalysis also screens for white blood cells and nitrites, both of which signal infection. Nitrites in the urine suggest bacteria are present, specifically the type that convert nitrates to nitrites. White blood cells indicate your immune system is fighting something off. This matters because kidney stones can trap bacteria and lead to urinary tract infections or even kidney infections. Struvite stones, in particular, form because of infections caused by bacteria that produce an enzyme making urine more alkaline. If your urinalysis shows signs of both stones and infection, treatment becomes more urgent, since an infected, obstructed kidney is a medical emergency.
Why Urinalysis Alone Isn’t Enough
A urine test is a helpful screening tool, but it has real limitations for diagnosing kidney stones. In one study of emergency department patients with suspected kidney stones, urinalysis had a sensitivity of 69% and a specificity of just 27%. That low specificity means many people with positive urine findings don’t actually have a stone; blood or crystals in the urine can come from other conditions entirely.
Combining a urine test with a patient’s history of previous stones pushes the sensitivity up to about 94%, which is much better. But for definitive diagnosis, a non-contrast CT scan is the standard. CT scans detect stones with about 91% sensitivity and can show the stone’s exact size, location, and whether it’s blocking urine flow. Both the American Urological Association and the European Association of Urology recommend CT as the imaging method of choice before planning treatment. A urinalysis and urine culture are still recommended before any procedure, but they serve a different role: checking for infection and guiding the overall picture rather than confirming the stone itself.
The 24-Hour Urine Test for Prevention
If you’ve already passed a stone or had one confirmed, your doctor may order a more detailed test called a 24-hour urine collection. This isn’t about finding a stone in real time. It’s a prevention tool that measures the chemistry of your urine over a full day to identify why you’re forming stones in the first place.
A complete 24-hour panel measures calcium, oxalate, citrate, uric acid, phosphate, sodium, and total urine volume, among other values. Each has an optimal range. Normal daily calcium excretion should be under 250 mg, with an optimal target below 200 mg. Oxalate should stay under 40 mg per day, with less than 25 mg being ideal. Citrate, which acts as a natural stone inhibitor, should be around 640 mg or more daily when you’re producing about two liters of urine.
These numbers tell your doctor exactly what’s off in your urine chemistry. If your oxalate is high, dietary changes or supplements may help. If your citrate is low, potassium citrate supplements can raise urine pH and reduce the risk of both uric acid and calcium-based stones. If your urine volume is consistently low, simply drinking more water may be the single most effective change. The 24-hour test turns stone prevention from guesswork into a targeted plan based on your specific risk factors.
Which Test You Actually Need
If you’re in pain right now and wondering whether you have a kidney stone, a urinalysis is a reasonable first step and one your doctor will almost certainly order. Blood in the urine combined with classic symptoms like sharp flank pain, nausea, or pain radiating to the groin raises suspicion significantly. But confirming the stone, seeing how big it is, and deciding whether it needs treatment requires imaging.
If you’ve had stones before and want to understand why they keep coming back, the 24-hour urine collection is the test that provides actionable answers. It won’t show you a stone on a screen, but it reveals the metabolic environment that’s producing them, which is ultimately more valuable for long-term prevention.