A standard urinalysis can reveal several clues that a kidney stone is present, but it cannot confirm one on its own. It detects indirect signs like blood in the urine, crystals, abnormal pH levels, and markers of infection. Around 84% of people with confirmed kidney stones show microscopic blood in their urine sample, which means roughly 15% of stone patients will have a completely normal urinalysis. Imaging, particularly a CT scan, is the tool that actually locates and confirms a stone.
What a Urinalysis Actually Detects
A urinalysis checks three things relevant to kidney stones: the chemical composition of your urine (via a dipstick), the presence of cells and crystals (via microscopic exam), and signs of infection. None of these directly “see” a stone sitting in your kidney or ureter. Instead, they pick up the damage and chemical environment a stone creates.
The most common finding is blood. When a stone moves through the urinary tract, it scrapes the lining and releases red blood cells into the urine. Sometimes the blood is visible to the naked eye, but more often it’s microscopic, meaning only detectable under a microscope or on a dipstick test. A large meta-analysis in BMC Urology found that 84% of people with confirmed stones had microscopic blood. Dipstick testing caught it slightly more often (around 90%) than microscopic examination alone (78%).
The second useful finding is crystals. When a lab technician examines your urine under a microscope, they may spot crystals that correspond to specific stone types. Calcium oxalate crystals are the most common, but certain crystal shapes are so distinctive they point to a specific diagnosis immediately. Cystine crystals, for example, are a hallmark of a genetic condition called cystinuria, and their presence in urine is essentially diagnostic.
How Urine pH Points to Stone Type
Your urine’s acidity level, measured as pH, is a surprisingly useful piece of the puzzle. Different stone types form in different pH environments, so a single pH reading can help your doctor narrow down what kind of stone you’re dealing with.
Low urine pH (acidic, around 5.0 to 5.5) is the single most important factor in uric acid stone formation. At a pH of 5.3, uric acid can crystallize and form stones even when urine volume is a relatively healthy two liters per day. If your urinalysis shows persistently acidic urine alongside other stone symptoms, uric acid stones become a leading suspect.
High urine pH (alkaline, above 6.0) creates the opposite problem. Calcium phosphate becomes increasingly likely to crystallize as pH climbs above 6, especially when combined with high calcium and low citrate levels in the urine. The prevalence of calcium phosphate stones has been rising in recent years, and alkaline urine is a key contributor. Calcium oxalate, the most common stone material, tends to crystallize most aggressively at very low pH levels (around 4.0) and least at higher pH.
Screening for Stone-Related Infections
One of the most important things a urinalysis does during a stone episode is check for infection. A kidney stone that blocks urine flow while an infection is brewing behind it is a urological emergency. The dipstick portion of the test screens for nitrites and leukocyte esterase (a byproduct of white blood cells), both of which suggest a urinary tract infection. Under the microscope, the lab checks for bacteria and elevated white blood cells.
This matters because an infected, obstructed kidney can deteriorate quickly, and the treatment path changes entirely. If your urinalysis flags infection markers alongside stone symptoms, your medical team will typically prioritize draining the kidney before addressing the stone itself.
Why a Normal Urinalysis Doesn’t Rule Out Stones
About 15% of people with kidney stones show no blood on urinalysis at all. This can happen when a stone is sitting quietly in the kidney without actively moving, when a stone has completely blocked the ureter (preventing blood-tinged urine from reaching the bladder), or simply due to timing. If the sample was collected during a lull between pain episodes, the urine may look perfectly normal.
The overall sensitivity of urinalysis for detecting stones is only about 69%, with a specificity of just 27%. In practical terms, that means urinalysis misses roughly one in three stone patients, and it frequently flags people who don’t have stones at all (since blood in the urine has many other causes, from infections to vigorous exercise). By comparison, a non-contrast CT scan has a sensitivity of about 91% and is the standard for confirming a stone’s size, location, and whether it’s causing an obstruction.
Urinalysis vs. Imaging vs. 24-Hour Testing
Think of these three tools as answering different questions. A standard urinalysis asks: “Are there signs that a stone might be present right now?” A CT scan asks: “Is there a stone, where is it, and how big is it?” A 24-hour urine collection asks: “Why is this person forming stones, and how do we prevent the next one?”
The 24-hour test is a much deeper dive than a routine urinalysis. You collect all of your urine over a full day, and the lab measures calcium, oxalate, citrate, uric acid, phosphate, sodium, and total volume. These results can reveal underlying metabolic problems that a single spot urinalysis would never catch, things like abnormally high oxalate levels, low citrate (which normally inhibits stone formation), or excess calcium excretion. This test is typically ordered after a stone has been confirmed, especially for people who’ve had more than one episode, because the results guide long-term prevention strategies like dietary changes or targeted treatment.
For someone in the emergency room with sudden flank pain, the standard urinalysis serves as a quick screening step. It takes minutes, costs very little, and can flag blood, infection, or crystal patterns that support the suspicion of a stone. But it’s the CT scan that provides the definitive answer, and the 24-hour urine collection that prevents the next stone from forming.