A urinalysis is a common diagnostic test that provides a detailed look at the health of the body’s urinary system. This test often includes checking for the presence of leukocytes, which are white blood cells (WBCs) that act as the body’s primary defense against foreign invaders. The detection of these cells in urine assesses the level of inflammation or infection within the urinary tract. While a minimal number of WBCs is expected in healthy urine, an elevated count suggests the body is actively mounting an immune response. A higher-than-normal concentration signals the need for further medical investigation.
The Role of Leukocytes in the Urinary Tract
Leukocytes are an integral part of the immune system, constantly circulating in the bloodstream to patrol for threats. Their primary function is to neutralize bacteria, viruses, or other pathogens. When infection or inflammation occurs in the urinary tract, these cells migrate from the blood vessels into the affected tissue.
This migration is a protective measure, allowing leukocytes to contain and eliminate the threat at its source. As they fight, some white blood cells are shed into the urine. Therefore, the presence of leukocytes in a urine sample serves as direct evidence of an ongoing fight against infection or inflammation in the kidneys, bladder, or urethra.
How Leukocytes Are Measured and Defined as Normal
The determination of leukocyte levels is done through two main methods during a urinalysis. The first is a rapid screening tool called the dipstick test, which detects leukocyte esterase, an enzyme produced by neutrophils. This test provides a quick result, often reported as negative, trace, or positive, but it does not give an exact cell count.
The definitive measurement is performed through microscopic analysis of the urine sediment. A concentrated amount of the sample is examined under high magnification, and the number of white blood cells per high-power field (WBC/HPF) is counted. For a healthy adult, a count of zero to five WBCs per HPF is considered a normal finding. In women, a count slightly higher than five may be accepted due to possible external contamination from the genital area.
When the count significantly exceeds this threshold, it is defined as pyuria, meaning an elevated number of leukocytes in the urine. Pyuria is generally diagnosed when microscopic analysis reveals more than five to ten WBCs per HPF. This elevated count signals an inflammatory process within the urinary system that requires medical attention.
Primary Conditions Leading to Elevated Counts (Pyuria)
The most frequent cause of pyuria is a bacterial urinary tract infection (UTI), which can affect the bladder (cystitis) or the urethra (urethritis). The high leukocyte count results from the immune system sending white blood cells to the infected site to eliminate bacteria. If the infection travels upward to the kidneys, known as pyelonephritis, the resulting inflammation leads to a significant increase in leukocytes.
Sterile Pyuria
Other conditions can also trigger pyuria, even in the absence of a typical bacterial infection, a state often called sterile pyuria. Kidney stones can cause irritation and inflammation as they move or block the urinary tract, prompting an immune response. Certain sexually transmitted infections (STIs), such as chlamydia or gonorrhea, cause inflammation of the urethra, resulting in pyuria that may show a negative bacterial culture.
Non-Infectious Causes
Non-infectious inflammatory diseases can also cause an elevated leukocyte count. These include interstitial nephritis, an inflammation of the kidney tubules, or systemic autoimmune conditions like lupus. Some medications, including certain antibiotics and nonsteroidal anti-inflammatory drugs (NSAIDs), can occasionally induce inflammation in the kidney tissue, leading to the presence of leukocytes in the urine.
Ensuring Accurate Results and Subsequent Diagnosis
To prevent a misleadingly high leukocyte count, proper collection of the urine sample is essential. The “clean catch” or midstream technique is used to minimize contamination from external bacteria and cells that could skew the result. The patient must clean the genital area and begin urinating into the toilet before collecting the middle portion of the stream into the sterile cup.
Even with careful collection, certain factors can interfere with the test results. A false-negative result on the dipstick test can occur if the urine contains high levels of Vitamin C. Conversely, a sample that is not processed quickly can have a falsely low leukocyte count as the cells break down over time. If pyuria is confirmed, the physician typically orders a urine culture and sensitivity test to identify any specific pathogen and determine the most effective antibiotic treatment. For cases of sterile pyuria, where no bacteria are found, further testing, such as imaging studies or specialized STI screening, may be necessary to identify non-infectious causes.