White blood cells (leukocytes) found in a urine sample can be concerning, often leading to fears about serious disease. The presence of these cells, medically termed leukocyturia or pyuria, signals an active inflammatory response somewhere along the urinary tract, which includes the kidneys, ureters, bladder, and urethra. Leukocytes are the body’s immune system cells, and their appearance confirms the body is dispatching defenses to the genitourinary system. While cancer is a possible cause, it is extremely uncommon; the majority of cases are caused by far less dangerous conditions like infection or inflammation.
Understanding Leukocyturia: The Non-Malignant Causes
The primary reason leukocytes appear in urine is to combat an infection, making infectious diseases the most frequent diagnosis. A urinary tract infection (UTI) is the most common cause, typically involving bacteria like E. coli. The bacteria multiply, triggering a migration of white blood cells that often leads to symptoms such as pain during urination and frequent urgency. If a UTI progresses without treatment, the bacteria can travel upward, resulting in a kidney infection (pyelonephritis).
This inflammation of the kidney tissue causes a significant increase in leukocytes in the urine, often accompanied by systemic symptoms like fever and flank pain. Non-infectious conditions also cause irritation and inflammation that draw leukocytes to the urinary tract. Kidney stones, hard masses of minerals and salts, can physically damage the urinary lining as they pass.
This irritation prompts an inflammatory response, leading to leukocyturia even without a bacterial infection. Blockages caused by these stones can also cause urine to become stagnant, creating an environment where bacteria can grow more easily and trigger an infection.
Inflammatory conditions affecting specific organs also draw leukocytes. Interstitial cystitis is a chronic bladder condition that results in pain and an inflamed bladder wall. Prostatitis, or inflammation of the prostate gland in men, also causes white blood cells to spill into the urine stream. Leukocyturia sometimes occurs without detectable bacteria, a condition known as sterile pyuria. This can be caused by viral infections, including sexually transmitted infections like chlamydia or gonorrhea, which require specialized testing. Additionally, certain medications, such as some nonsteroidal anti-inflammatory drugs (NSAIDs) or specific antibiotics, can cause kidney inflammation, resulting in leukocytes appearing in the urine.
When Cancer is the Underlying Factor
The link between white blood cells in urine and cancer is much less common than infectious or inflammatory causes. When urinary tract cancers are responsible, it is typically due to secondary effects rather than the tumor directly shedding leukocytes. Tumors (such as bladder, kidney, and prostate cancer) cause local tissue damage and inflammation as they grow. This irritation attracts white blood cells as the body attempts to clean up the damaged tissue.
Tumor growth can also cause a physical obstruction in the urinary tract, leading to urine stagnation. Blocked urine flow dramatically increases the risk of a secondary bacterial infection developing behind the blockage. This subsequent infection causes a spike in leukocyturia, making the white blood cells a sign of the complication rather than the cancer itself.
When cancer is the underlying cause of irritation, leukocyturia is often accompanied by gross hematuria (visible blood in the urine). Hematuria is generally considered more suggestive of malignancy than isolated white blood cells. Bladder cancer is the most frequent urinary tract cancer linked to these symptoms. Kidney cancer and advanced prostate cancer can also contribute to leukocyturia, primarily by causing obstruction or local inflammation. In rare cases, blood cancers like leukemia can cause an extremely high number of white blood cells in the bloodstream, which may then spill into the urine.
Diagnostic Procedures and Next Steps
The initial step in investigating white blood cells in urine is a urinalysis, which provides valuable information about the urine’s composition. This analysis involves two parts: a chemical dipstick test and a microscopic examination of the urine sediment. The dipstick checks for leukocyte esterase, an enzyme indicating the presence of white blood cells. The microscopic examination quantifies the degree of leukocyturia by counting the number of white blood cells per high-power field (WBC/HPF), with more than five WBC/HPF generally considered significant.
The urinalysis also checks for other indicators, such as bacteria, nitrites, and red blood cells, which help differentiate between infectious and non-infectious causes. If a bacterial infection is suggested, a urine culture is performed. The culture is the definitive test for confirming a bacterial UTI, identifying the specific bacteria, and determining its sensitivity to antibiotics to guide treatment.
If leukocyturia is persistent and the urine culture is negative (sterile pyuria), the investigation broadens to look for non-bacterial causes. Physicians may order advanced testing, such as urine cytology, where a pathologist examines the urine for abnormal or malignant cells. Cytology is particularly relevant when bladder cancer is suspected.
Imaging studies are also utilized to visualize the urinary tract. These include renal ultrasounds or computed tomography (CT) scans. Imaging looks for physical obstructions like kidney stones or abnormal growths like tumors, helping determine if an anatomical issue is contributing to the inflammation or infection.