Leukocytes (white blood cells or WBCs) are a key component of the immune system, defending against pathogens and foreign substances. The presence of these cells in the urine, a condition referred to as pyuria, signals an immune response within the urinary tract. Finding leukocytes indicates the body is actively fighting an irritant or infection, but it does not specify the exact location or cause. A urinalysis detects these cells and serves as an important initial clue for further investigation.
How Leukocytes Are Measured and Clinical Thresholds
The detection of white blood cells in urine typically involves two main laboratory methods. The first is the urine dipstick test, a rapid screening tool that detects leukocyte esterase, an enzyme released by neutrophils. A positive result suggests an elevated number of leukocytes but does not provide an exact count.
The second, more precise method is the microscopic evaluation of the urine sediment, which provides a quantitative count. This involves counting the number of white blood cells seen per high-power field (HPF) under a microscope. The general clinical consensus is that finding more than 5 to 10 WBCs/HPF is considered pyuria, or a high count. For symptomatic patients, a count of 10 WBCs/HPF or more is often used as the standard threshold for diagnosing a urinary tract infection. This measurement indicates inflammation, but it is not a definitive diagnosis on its own.
Primary Causes Related to Infection
Infections are the most frequent reason for elevated leukocytes, as pyuria represents the immune system mobilizing to fight microbial invasion. The most common culprit is a bacterial Urinary Tract Infection (UTI), which can occur in the lower or upper parts of the system. In cystitis (a bladder infection), the body sends white blood cells to the bladder lining to contain the bacteria, which are then shed into the urine.
A more serious infection, pyelonephritis, involves the kidneys and typically presents with a higher degree of pyuria, often accompanied by fever and flank pain. The immune response is intensified because the infection has reached the upper urinary tract, leading to a more pronounced presence of leukocytes. Beyond typical bacterial UTIs, other infections that do not grow on standard cultures can also cause high leukocyte counts.
Sexually Transmitted Infections (STIs) such as Chlamydia trachomatis and Neisseria gonorrhoeae are recognized causes of pyuria, especially when they cause urethritis (inflammation of the urethra). Certain viral infections, including adenovirus, can also lead to inflammation and subsequent pyuria, sometimes resulting in hemorrhagic cystitis. Furthermore, residual pyuria can occur following a recently or partially treated UTI, even if the bacterial count is no longer high, as the body clears the remaining inflammation.
Non-Infectious and Atypical Reasons for Elevation
A high leukocyte count without a positive bacterial culture is known as “sterile pyuria,” indicating inflammation from a non-bacterial source. Conditions causing systemic inflammation can manifest with pyuria, such as interstitial nephritis (an inflammatory disorder affecting the kidney tubules) or autoimmune diseases like lupus. These conditions trigger an immune response involving the kidneys, leading to white blood cells in the urine filtrate.
Physical irritants within the urinary tract, like kidney stones, can also cause localized inflammation. As a stone moves or rests against the lining, it causes irritation and trauma, prompting a defensive white blood cell response. Medications, including some antibiotics and non-steroidal anti-inflammatory drugs, have been associated with drug-induced inflammation in the kidneys, which can present as pyuria.
A common consideration, particularly in women, is sample contamination. Leukocytes from vaginal secretions or poor collection technique can mix with the urine, leading to a false-positive reading. Healthcare providers must consider the quality of the urine sample, especially if a high number of squamous epithelial cells are observed, as this suggests the specimen was not a clean-catch midstream sample.
Diagnostic Follow-Up and Management Strategies
Once pyuria is detected, the next diagnostic step is determining the underlying cause through further testing. The most important follow-up test is the urine culture and sensitivity (C&S), which attempts to grow and identify the specific bacteria present. This culture determines if a bacterial infection is present and, if so, which antibiotics are most effective for treatment.
If the urine culture is negative despite a high leukocyte count, the provider will investigate the non-infectious causes of sterile pyuria. This may involve taking a detailed history to check for recent medication use or symptoms pointing toward systemic inflammatory disorders. When pyelonephritis or an anatomical issue like a kidney stone is suspected, imaging studies like an ultrasound or Computed Tomography (CT) scan may be ordered to visualize the urinary tract.
Management strategies are directly tailored to the final diagnosis. If a bacterial infection is confirmed, a course of antibiotics is typically prescribed based on the culture’s sensitivity results. For non-infectious causes, the focus shifts to addressing the underlying condition, such as managing systemic inflammation or addressing irritation caused by kidney stones. Treatment is always directed at resolving the source of the immune response.