A routine urinalysis often checks for specific cells in the urine. Leukocytes, or white blood cells, are part of the immune system designed to fight off foreign invaders and respond to inflammation. Finding leukocytes in a urine sample usually signals that the body is actively mounting an immune response somewhere within the urinary tract. This finding indicates potential underlying irritation or disease.
What Leukocytes in Urine Indicate
The medical term for an abnormal amount of white blood cells in the urine is pyuria, which essentially means pus is present in the sample. In a healthy person, the urinary tract is nearly sterile, and a normal reading is fewer than five white blood cells per high-power field (WBC/HPF) when examined under a microscope. When a laboratory report uses the term “large” leukocytes, it most often refers to a high concentration of these cells, rather than the physical size of individual cells. A high concentration is clinically defined as 10 or more white blood cells per cubic millimeter of urine.
The urine dipstick test is another common detection method, checking for the presence of leukocyte esterase. Leukocyte esterase is an enzyme released by white blood cells, and a positive result strongly suggests pyuria. While pyuria indicates inflammation or infection, it must be considered alongside a patient’s overall symptoms and other test results.
The Most Common Causes (Infections)
The majority of elevated leukocytes in the urine are caused by bacterial infections within the urinary system. A urinary tract infection (UTI) is the most frequent culprit, occurring when bacteria, most commonly E. coli, ascend the urinary tract. These infections are broadly categorized by location, including urethritis (urethra), cystitis (bladder), and pyelonephritis (kidneys). The presence of bacteria triggers an inflammatory response, causing white blood cells to be shed into the urine.
Pyelonephritis, a kidney infection, generally presents with more severe systemic symptoms, such as fever, flank pain, and chills. Leukocytes in the urine may also stem from sexually transmitted infections (STIs), particularly those causing urethritis, like Chlamydia trachomatis and Neisseria gonorrhoeae. Although these are bacterial, they often result in “sterile pyuria,” meaning white blood cells are present, but the organism does not grow on standard urine culture plates. Other infectious agents, including viruses, fungi, and parasites, can also trigger a localized inflammatory response leading to pyuria.
Non-Infectious Underlying Conditions
Inflammation within the kidney tissue, known as interstitial nephritis, is a significant non-infectious cause of pyuria. This condition is often an immune-mediated allergic reaction to certain medications, including nonsteroidal anti-inflammatory drugs (NSAIDs) or specific antibiotics. The reaction causes the kidney’s interstitium to become inflamed, shedding white blood cells into the urine without signs of systemic infection.
Systemic inflammatory and autoimmune diseases, such as systemic lupus erythematosus or Kawasaki disease, can also cause pyuria when their generalized inflammatory effects extend to the urinary tract. Structural issues, like kidney stones, cause mechanical irritation and inflammation as they pass or obstruct urine flow. This constant irritation signals the immune system, leading to the presence of white blood cells.
Certain physiological factors and external contamination can lead to a positive leukocyte finding in the absence of disease. Contamination from vaginal discharge or menstrual blood in female patients is a common reason for a false positive result. The long-term use of indwelling urinary catheters or recent urological procedures can also cause localized irritation resulting in pyuria. In rare instances, malignancies such as bladder or kidney cancer may be associated with pyuria due to localized inflammation.
Confirming the Diagnosis and Treatment
A positive leukocyte finding requires focused follow-up testing to identify the precise cause and guide appropriate treatment. The next step is usually a urine culture and sensitivity test, which attempts to grow any bacteria present to confirm an infection. This test also determines which antibiotics will be effective. Obtaining the culture before starting any antibiotic treatment is important, as medication can quickly reduce the bacterial count and lead to a false-negative result.
If the culture is negative, confirming sterile pyuria, the investigation shifts to non-infectious causes. The physician may order specific blood tests for markers of systemic inflammation or specialized tests for organisms like Chlamydia or tuberculosis that do not grow on standard culture media. Imaging studies, such as an ultrasound or CT scan, are often necessary if a structural problem like a kidney stone or a tumor is suspected. Treatment depends entirely on the underlying diagnosis: bacterial infections are managed with targeted antibiotics, while non-infectious causes require addressing the specific condition, such as discontinuing an offending medication or managing a systemic disease.