When a urine sample is analyzed, the presence of various cells and substances is checked to assess overall health. Finding an elevated level of leukocytes, often called white blood cells, is a common result that indicates the body’s defense system has been activated. This finding in a routine urinalysis suggests that inflammation or an immune response is occurring somewhere along the urinary tract.
What Are Leukocytes?
Leukocytes are specialized cells made in the bone marrow that circulate as the primary component of the immune system. Their role is to act as the body’s defensive force, neutralizing foreign invaders like bacteria, viruses, or fungi. When tissue injury or infection occurs, these cells rush to the site of the problem to destroy harmful agents. Their appearance in urine signals that this protective immune activity is taking place within the urinary system.
Defining Pyuria: Normal versus Elevated Levels
The medical term for an excessive presence of leukocytes in the urine is pyuria. A small, trace amount is considered normal, often reflecting minor daily cellular turnover. Generally, a healthy person will have fewer than five white blood cells per high-power field (WBCs/HPF) when a urine sample is examined microscopically.
When the number of cells consistently exceeds this threshold (usually defined as 10 or more WBCs per cubic millimeter or over 5 WBCs/HPF), it is classified as pyuria. This elevated count indicates a significant inflammatory process somewhere along the urinary tract, from the kidneys down to the urethra. Interpreting this result requires considering the patient’s symptoms and other findings from the urinalysis.
Common Reasons for Leukocytes in Urine
The most frequent cause is a bacterial urinary tract infection (UTI), affecting the bladder or the urethra. When bacteria invade, the immune system sends a large number of leukocytes, primarily neutrophils, to fight the infection. If the infection progresses upward to the kidneys, pyelonephritis occurs, which results in a very high leukocyte count.
Leukocytes can also be present without a detectable bacterial infection, a condition referred to as sterile pyuria. This finding points to inflammatory conditions not caused by common bacteria. Causes of sterile pyuria include:
- Kidney stones that irritate the urinary lining as they pass.
- Inflammation of the bladder wall, known as interstitial cystitis.
- Certain sexually transmitted infections (STIs), such as chlamydia and gonorrhea, because the organisms do not grow on standard lab cultures.
- Autoimmune diseases, like lupus, which can trigger inflammation in the kidneys.
- The use of certain medications, such as some nonsteroidal anti-inflammatory drugs.
How Leukocytes in Urine Are Tested
Leukocytes are typically first detected during a standard urinalysis, which utilizes two main methods for screening and confirmation. The initial screening is often performed using a chemical dipstick, a small strip dipped into the urine sample. This strip contains a test pad for leukocyte esterase (LE), an enzyme present in the white blood cells, particularly neutrophils.
A color change on the LE pad indicates a positive result, suggesting the presence of leukocytes in the sample. While quick, the dipstick only indicates the presence of the enzyme, giving results like “trace” or “1+” rather than an exact cell count. This result is highly sensitive for detecting pyuria but is not a definitive measure.
For a confirmed diagnosis and precise count, the sample undergoes a microscopic examination. A lab technician concentrates the urine by spinning it in a centrifuge, then examines the resulting sediment under high magnification to physically count the white blood cells. This process provides the definitive count of WBCs/HPF to confirm pyuria.
If pyuria is confirmed, the next step is often a urine culture, especially if an infection is suspected. This test attempts to grow any bacteria present in the sample, which identifies the specific microbe and determines the appropriate antibiotic treatment.