Leukocytes, or white blood cells (WBCs), are a fundamental part of the immune system, constantly neutralizing threats like bacteria and foreign invaders. Their presence in urine is a common finding during a standard urinalysis and is medically termed leukocyturia. Detecting these cells strongly indicates that the body is mounting an inflammatory response somewhere within the urinary tract. These conditions are often identifiable and treatable, most frequently involving infections.
What White Blood Cells Are Doing in Urine
White blood cells are the body’s primary defense mechanism against infection and inflammation. When a pathogen enters the urinary tract, the body signals these immune cells to migrate to the affected area. Leukocytes squeeze through blood vessel walls to reach the site, where they engulf and destroy invading organisms.
The resulting cellular debris and the white blood cells themselves shed into the urine as it passes through the inflamed tissue. Leukocyturia thus acts as a marker, confirming an inflammatory process localized within the kidneys, ureters, bladder, or urethra. A large concentration of these cells is specifically referred to as pyuria, indicating the presence of pus.
Common Medical Conditions Indicated by Leukocyturia
The most frequent cause of elevated white blood cells in urine is a Urinary Tract Infection (UTI), which can occur anywhere from the urethra up to the kidneys.
Infectious Causes
In a lower UTI (cystitis), bacteria multiply in the bladder, triggering a localized immune response. If the infection travels upward to the kidneys (pyelonephritis), the reaction becomes more severe, often leading to a much higher leukocyte count and systemic symptoms like fever and flank pain. Leukocyturia can also be caused by sexually transmitted infections (STIs) such as chlamydia or gonorrhea.
Non-Infectious Causes
Non-infectious conditions can also cause irritation and inflammation that draw white blood cells into the urine. Kidney stones can physically irritate or obstruct the urinary tract as they pass, leading to inflammation and the appearance of leukocytes, even without a bacterial infection.
Inflammation of the kidney tubules, known as interstitial nephritis, is another potential cause. This condition is often triggered by certain medications or systemic autoimmune diseases, causing immune cells to infiltrate the kidney tissue. Leukocytes may also be present in sterile pyuria, where routine culture detects no bacteria. Note that in women, a urine sample can be contaminated by vaginal secretions, leading to a false positive if a clean-catch midstream sample is not properly collected.
Methods for Measuring Leukocytes in Urine
Leukocytes are typically detected and quantified in a standard urinalysis using a two-step process. The initial screening involves a urine dipstick test, a rapid, qualitative method that checks for leukocyte esterase, an enzyme found within white blood cells. A color change on the dipstick suggests a significant number of white blood cells are present.
The definitive quantification is performed through microscopic examination of the urine sediment. The sample is spun in a centrifuge, and the concentrated sediment is examined under a microscope. The number of white blood cells is then counted and reported as white blood cells per High Power Field (WBCs/HPF). The accepted threshold for an abnormal result, indicating leukocyturia, is greater than five WBCs/HPF.
Follow-Up Diagnosis and Treatment
Once leukocyturia is confirmed, the healthcare provider determines the underlying cause. If a bacterial infection is suspected, a urine culture is ordered to identify the specific bacteria and determine which antibiotics will be most effective (sensitivity testing).
If the cause is confirmed to be bacterial, treatment involves a course of antibiotics based on sensitivity results. For uncomplicated UTIs, a short course of oral antibiotics is often sufficient. Hydration and pain management are common components of care, especially for conditions like kidney stones.
Further diagnostic imaging, such as an ultrasound or a Computed Tomography (CT) scan, may be necessary if a kidney stone, obstruction, or chronic kidney inflammation is suspected. If the workup suggests a non-infectious cause, such as interstitial nephritis or a systemic condition, treatment shifts to managing the underlying disease. In cases of sterile pyuria, investigation may broaden to look for less common causes, including specific STIs or tuberculosis.