White blood cells in urine, called pyuria, signal that your body is fighting inflammation or infection somewhere in your urinary tract. A normal urine sample contains very few white blood cells. On a standard microscopic exam, 8 or more white blood cells per high-power field reliably points to a problem, while automated analyzers flag anything above 2 per high-power field as significant. If your lab results show elevated levels, it usually means your immune system has sent these cells to deal with a threat in your kidneys, bladder, or urethra.
Why White Blood Cells Show Up in Urine
White blood cells normally patrol your bloodstream. When tissue in or near the urinary tract becomes inflamed, the blood vessels at that site slow their flow and display sticky molecules on their inner walls. White blood cells latch onto these molecules, squeeze through the vessel wall in an amoeba-like motion, and push through surrounding tissue until they reach the source of trouble. Once they arrive, they work to neutralize bacteria, viruses, or other threats. When enough of them accumulate, they spill into the urine and show up on a test.
This process is tightly regulated. Your body doesn’t send white blood cells everywhere at once. It directs them only to the specific spot where inflammation is occurring, which is why their presence in urine points specifically to a problem in or near the urinary tract rather than a general illness.
Most Common Causes
Urinary tract infections are by far the leading cause. Bacteria, most often E. coli, enter the urethra, multiply in the bladder, and trigger an immune response that floods the area with white blood cells. Beyond UTIs, several other conditions can elevate the count:
- Kidney stones: Stones irritate and inflame the tissue they contact, drawing white blood cells even without infection.
- Sexually transmitted infections: Gonorrhea, syphilis, and HIV can all produce pyuria.
- Interstitial cystitis: This chronic bladder pain condition causes ongoing inflammation that keeps white blood cell levels elevated.
- Kidney infection (pyelonephritis): A more serious infection that has traveled from the bladder up to the kidneys.
- Tuberculosis: TB can affect the kidneys and urinary tract, sometimes producing white blood cells in urine with no ordinary bacteria found on culture.
When No Bacteria Are Found
Sometimes a urine test shows plenty of white blood cells, but the culture comes back negative for bacteria. This is called sterile pyuria, and it has its own set of explanations. Certain medications are a surprisingly common culprit. NSAIDs like ibuprofen, some antibiotics, proton pump inhibitors used for acid reflux, and diuretics can all inflame kidney tissue and trigger white blood cell release without any infection being present.
Systemic diseases can also be responsible. Lupus frequently causes pyuria, often without symptoms. In one long-term study of nearly 950 lupus patients, 23% had at least one episode of sterile pyuria. Sarcoidosis, severe high blood pressure, and diabetes are other systemic conditions linked to it. Pelvic inflammation from a nearby issue, like appendicitis affecting tissue close to the bladder, can cause it too. People with urinary catheters or stents almost always show elevated white blood cells regardless of infection.
Postmenopausal hormonal changes and pregnancy are both physiological causes. In these cases, the pyuria itself isn’t necessarily dangerous, but it still warrants investigation to rule out infection.
How the Test Works
There are two main ways white blood cells get detected. The first is a urine dipstick, a plastic strip dipped into your sample that changes color when it detects an enzyme released by white blood cells called leukocyte esterase. It’s fast and cheap, with about 79% sensitivity and 84% specificity. That means it catches most cases but occasionally misses some or flags false positives. The dipstick also checks for nitrites, which certain bacteria produce, giving a quick two-part screen for infection.
The second method is microscopic urinalysis, where a lab technician or automated machine actually counts the white blood cells in a urine sample under magnification. This is more precise than the dipstick and is typically used as a follow-up when the dipstick shows abnormalities. If infection is suspected, a urine culture is the definitive test. A sample is placed in conditions that encourage bacterial growth, and results come back in one to three days showing exactly which organism is present and which treatments will work against it.
What You Might Notice
Elevated white blood cells alone don’t always produce obvious symptoms. When they do, the signs usually come from the underlying cause rather than the white blood cells themselves. With a UTI, you’ll typically feel burning during urination, a frequent urgent need to go, and lower abdominal pressure. Your urine may look cloudy or have an unusual smell. If a kidney infection is involved, fever, flank pain, and nausea often develop. With kidney stones, sharp pain radiating from the back to the groin is the hallmark symptom. In cases of sterile pyuria from medications or chronic conditions, you may have no urinary symptoms at all, and the finding only shows up on routine testing.
Pyuria During Pregnancy
Pregnancy increases the risk of urinary tract infections for specific anatomical reasons. Hormonal shifts relax and widen the tubes connecting the kidneys to the bladder, while the growing uterus compresses them. Both changes cause urine to pool rather than drain efficiently, creating an environment where bacteria thrive. Kidney infections during pregnancy peak in the second trimester and are one of the most common reasons pregnant people are hospitalized for a medical condition.
The stakes are higher during pregnancy. UTIs have been linked to increased rates of preterm delivery and low birth weight. Kidney infections can lead to serious complications including sepsis. This is why prenatal care includes a urine culture early in pregnancy to catch even symptom-free bacterial colonization. Treatment typically involves a 5 to 7 day course of antibiotics selected based on culture results, with a follow-up culture one to two weeks after finishing to confirm the infection has cleared.
How Elevated White Blood Cells Are Treated
Treatment targets whatever is causing the white blood cells to appear, not the white blood cells themselves. For a straightforward UTI, a short course of antibiotics (generally no longer than seven days) is standard. The choice of antibiotic depends on local resistance patterns, since bacteria in different regions have developed resistance to different drugs. Once the infection clears, the white blood cell count returns to normal on its own.
For kidney stones, treatment depends on the stone’s size and location, ranging from waiting for it to pass naturally to procedures that break it apart. For medication-induced pyuria, stopping or switching the offending drug usually resolves it. Chronic conditions like interstitial cystitis or lupus require longer-term management of the underlying disease. In all cases, the white blood cells are the messenger, not the problem. Finding out why they’re there is what matters.
Pyuria in Children
Children can develop white blood cells in urine from UTIs just like adults, but some causes are more specific to pediatric populations. Kawasaki disease, an inflammatory condition affecting blood vessels in young children, is the most common non-infectious cause of sterile pyuria in kids. One study of 135 children with Kawasaki disease found sterile pyuria in 79% of them. Certain medications cause kidney tissue inflammation in children more often than recognized, particularly NSAIDs, anticonvulsants, and proton pump inhibitors. Children with sickle cell disease also show higher rates of sterile pyuria, likely due to repeated small areas of tissue damage in the kidneys. In any child with unexplained pyuria, especially alongside protein or blood in the urine, further investigation is important to catch kidney involvement early.