White blood cells, also known as leukocytes, are a fundamental part of the body’s immune system, primarily responsible for defending against foreign invaders and disease. These cells are produced in the bone marrow and circulate throughout the bloodstream, rushing to any site of inflammation or infection. The presence of an elevated number of these cells in the urine is a condition called pyuria or leukocyturia, and it is a significant finding that suggests an inflammatory response somewhere along the urinary tract. While small, trace amounts of leukocytes may occasionally be present, their consistent or high-level appearance is a direct signal that the immune system is actively engaged. This finding is an indicator requiring further medical investigation to identify the underlying cause.
How White Blood Cells Are Detected
The primary method for detecting leukocytes in urine is through a urinalysis, which involves both chemical and microscopic examinations. The initial chemical screen often uses a urine dipstick, a small strip with pads that change color when dipped into the sample. One of the key indicators on this dipstick is the test for leukocyte esterase, an enzyme released by white blood cells, particularly neutrophils, when they break down. A positive result for leukocyte esterase suggests the likely presence of white blood cells in the urine.
A more definitive count is obtained through a microscopic examination of the urine sediment. After the urine sample is spun down in a centrifuge, the solid components are viewed under a high-power field (HPF) microscope. A count of five or fewer white blood cells per HPF is generally considered within the normal range. Counts consistently above this level, particularly ten or more per HPF, constitute pyuria.
Primary Infectious Causes of Elevation
The most frequent reason for finding an elevated white blood cell count in the urine is a bacterial Urinary Tract Infection (UTI). When bacteria, such as Escherichia coli, successfully colonize the urinary tract, the immune system launches a response, sending leukocytes to the site of infection. These white blood cells then exit the body via the urine, resulting in pyuria. Infections can be categorized by location, with lower tract infections being the most common.
Lower Tract Infections
A bladder infection, or cystitis, causes localized inflammation as the white blood cells attempt to eliminate the bacteria. This leads to symptoms like painful or frequent urination.
Upper Tract Infections
If the infection is left untreated, it may ascend the urinary tract to the kidneys. A kidney infection, known as pyelonephritis, is a more serious upper tract infection that causes significant inflammation within the kidney tissue itself. This condition typically results in a much higher level of pyuria, often accompanied by systemic symptoms like fever, chills, and flank pain. The presence of white blood cell casts—cylindrical structures formed in the kidney tubules—on microscopic examination is a specific indicator of a kidney-level inflammatory process.
Non-Infectious Factors and Other Contributors
While infection is the most common cause, the presence of white blood cells in the urine does not always mean bacteria are involved. This situation is referred to as “sterile pyuria,” where leukocytes are found but a standard urine culture returns a negative result for bacterial growth. Non-bacterial causes of inflammation along the urinary tract can trigger the same immune response.
Causes of Sterile Pyuria
- Kidney stones can cause chronic irritation and inflammation as they move through the urinary system, leading to a release of leukocytes.
- Generalized systemic inflammatory diseases, such as Systemic Lupus Erythematosus (SLE), can affect the kidneys, causing inflammation that allows white blood cells to pass into the urine.
- Certain medications, including some nonsteroidal anti-inflammatory drugs (NSAIDs), can induce an inflammatory condition in the kidney called interstitial nephritis, which also manifests as pyuria.
- Other infectious causes, such as Chlamydia trachomatis or Mycobacterium tuberculosis (genitourinary tuberculosis), may not grow in standard laboratory cultures.
A common source of a false-positive reading is sample contamination. This occurs when white blood cells from external sources, such as vaginal discharge, enter the urine sample during collection. For this reason, proper “clean catch” collection technique is emphasized before a urinalysis.
Determining Treatment and Urgency
The finding of pyuria is a signal for the healthcare provider to conduct further testing to determine the precise underlying cause. The next logical step after a positive urinalysis is typically a urine culture, which attempts to grow and identify the specific bacteria present. If a bacterial infection is confirmed, the standard course of action involves prescribing antibiotics, tailored to the specific type of bacteria identified and its susceptibility to various drugs.
For non-infectious causes, the treatment is directed at the primary condition. This might involve procedures to manage kidney stones, changes to medication regimens, or specific therapies for systemic inflammatory diseases. Monitoring associated symptoms is important in determining the urgency of care. Symptoms such as a high fever, severe pain in the back or side (flank pain), or persistent vomiting suggest a serious upper urinary tract infection or a systemic issue requiring immediate medical attention. Isolated pyuria with no symptoms, often called asymptomatic pyuria, is less urgent but still warrants follow-up.