White blood cells (WBCs), also known as leukocytes, are a key component of the immune system that protects the body against foreign invaders like bacteria and viruses. When undergoing a urine test (urinalysis), a small number of these cells is normally expected. However, a significantly increased number of WBCs in a urine sample is a key diagnostic indicator, signaling inflammation or an active immune response within the urinary tract. This finding prompts further investigation to pinpoint the precise cause.
Understanding White Blood Cells in Urine
The presence of white blood cells in the urine is medically termed pyuria or leukocyturia. Assessment typically uses two primary methods during a standard urinalysis. The initial rapid screening involves a urine dipstick test, which detects the enzyme leukocyte esterase. Since this enzyme is released by neutrophils, a positive result strongly suggests a high concentration of leukocytes is present.
The more precise method is a microscopic examination of the spun urine sediment. A laboratory technician physically counts the number of white blood cells per high-powered field (HPF). A count of zero to five WBCs per HPF is generally considered the normal range for healthy individuals. An elevated count, defined as greater than five WBCs per HPF, confirms pyuria and necessitates further investigation.
Common Causes of Elevated Counts
The most frequent reason for elevated white blood cells in the urine is an infectious process, specifically a urinary tract infection (UTI). When bacteria, most commonly Escherichia coli, colonize the urinary system, the body initiates a rapid immune defense. This response involves the swift recruitment of neutrophils from the bloodstream into the infected tissues.
These neutrophils migrate into the bladder or kidney tissue and then shed into the urine while attempting to neutralize the pathogens. In a bladder infection (cystitis), this immune activity causes symptoms like a burning sensation during urination and increased frequency. If the infection ascends into the kidneys, causing pyelonephritis, the immune surge is more intense and is often accompanied by systemic symptoms such as fever, flank pain, and chills.
Non-Infectious Reasons for High WBCs
While infection is the most common cause, elevated white blood cells can occur even when a bacterial culture is negative, a condition referred to as sterile pyuria. One non-infectious cause is the physical irritation and inflammation induced by kidney stones. As these mineral deposits travel through the urinary tract, they cause trauma to the lining, triggering a localized inflammatory response that draws leukocytes.
Another potential cause is interstitial nephritis, an inflammatory condition of the kidney’s tubules often reacting to certain medications like antibiotics. In this scenario, immune cells infiltrate the kidney tissue itself, and their presence is detected in the urine without a bacterial infection. Furthermore, a poorly collected sample contaminated by external genital skin cells or vaginal discharge can lead to a falsely elevated white blood cell count.
Next Steps After an Abnormal Result
Following the detection of pyuria, the next step is to determine if an active bacterial infection is the underlying cause. The physician will usually order a urine culture, a laboratory test that attempts to grow and identify the specific microorganism present. This culture confirms the presence of bacteria and indicates which antibiotics will be most effective for treatment.
If the urine culture is negative or symptoms are persistent, further testing may be warranted. Imaging studies, such as an ultrasound or CT scan, may be used to look for non-infectious causes like kidney stones or structural abnormalities. The treatment plan is then tailored to the confirmed diagnosis, involving antibiotics for infection or specific management for inflammatory conditions.