White blood cells (leukocytes) are the body’s primary defense system, circulating throughout the bloodstream to identify and destroy foreign invaders like bacteria and viruses. Finding these cells in the urine, known medically as pyuria, is a direct sign of an active immune response or inflammation occurring somewhere along the urinary tract. The presence of pyuria indicates that the body is attempting to fight off an irritant or infection in the kidneys, bladder, ureters, or urethra. Since urine is typically a sterile fluid, the appearance of a significant number of these immune cells signals an underlying health issue.
Understanding Pyuria: What Counts as Elevated?
Pyuria is quantitatively defined by the concentration of leukocytes detected during a urinalysis. While a small number of white blood cells is normal, a count is considered elevated when it exceeds a specific threshold. This threshold is generally defined as 10 or more white blood cells per cubic millimeter of unspun urine, or 5 to 10 cells per high-power field (HPF) when viewed under a microscope.
A simpler screening method involves a urine dipstick test, which detects leukocyte esterase, an enzyme found in most white blood cells. If pyuria is confirmed, the next distinction is between non-sterile and sterile pyuria. Non-sterile pyuria is the presence of white blood cells along with a high concentration of bacteria, typically indicating a straightforward bacterial infection. Sterile pyuria is characterized by elevated white blood cells but no bacteria growing in a standard urine culture. This result suggests either an infection caused by organisms that do not grow on routine culture media, or a non-infectious inflammatory process.
Infectious Causes of High White Blood Cells
The most frequent cause of pyuria is a bacterial urinary tract infection (UTI), where pathogens like Escherichia coli colonize the lower urinary tract. In cystitis, an infection localized to the bladder, symptoms often include a burning sensation during urination, frequent urges to urinate, and lower abdominal discomfort. The immune response to these bacteria causes a large infiltration of white blood cells into the urine.
Infections can also ascend to the kidneys, leading to pyelonephritis, a more serious condition. Pyelonephritis typically presents with systemic symptoms, such as a high fever, chills, nausea, and pain in the flank or back. The inflammatory reaction in the kidney tissue causes a substantial shedding of white blood cells into the urine.
Other infectious agents that do not thrive on standard culture media can also cause pyuria, often resulting in sterile pyuria. Common culprits include sexually transmitted infections (STIs) such as Chlamydia trachomatis and Neisseria gonorrhoeae. Atypical infections, like genitourinary tuberculosis, fungi, or viruses, may also be responsible for pyuria missed by routine bacterial culture. Recent antibiotic use can also suppress bacteria, leading to a sterile pyuria presentation despite a recent bacterial source.
Non-Infectious Reasons for Pyuria
When a bacterial infection is ruled out by culture, pyuria may be due to a variety of inflammatory and structural issues within the urinary system. Structural abnormalities, such as kidney stones (renal calculi), can cause mechanical irritation as they pass through the urinary tract. This physical irritation triggers a localized inflammatory response, drawing white blood cells to the area, which then appear in the urine.
Inflammatory conditions affecting the bladder wall, like interstitial cystitis, can cause chronic bladder pain and persistent pyuria without an infection. Inflammation within the kidney itself, known as nephritis, can also cause pyuria, sometimes as a side effect of certain medications. For instance, some nonsteroidal anti-inflammatory drugs (NSAIDs) or certain antibiotics are known to cause tubulointerstitial nephritis.
Pyuria can also be a manifestation of a systemic autoimmune disease, where the body’s immune system mistakenly attacks its own tissues, including those in the kidneys. Conditions such as systemic lupus erythematosus (SLE) or Kawasaki disease are known to cause inflammation that results in pyuria. Finally, contamination of the urine sample from vaginal discharge or external genital inflammation can introduce white blood cells, leading to a false-positive result.
Diagnostic Testing and Treatment Approach
The medical investigation of pyuria begins with a comprehensive urinalysis, followed by a urine culture to determine if bacteria are present and to identify the specific strain. If a bacterial infection is confirmed, sensitivity testing is performed to determine which antibiotics will be most effective. If the initial culture is negative despite pyuria, further specialized testing is required to look for atypical pathogens.
Specific tests for STIs like Chlamydia and Gonorrhea, often involving nucleic acid amplification testing (NAAT), may be ordered, especially in sexually active patients. Blood tests may also be used to check for systemic markers of inflammation or to evaluate kidney function. If structural issues like kidney stones or tumors are suspected, imaging tests, such as an ultrasound or a computed tomography (CT) scan, can visualize the kidneys and bladder.
Treatment is directly focused on the underlying cause identified through this diagnostic process. A bacterial infection is managed with a targeted course of antibiotics, while fungal infections require antifungal medication. For pyuria caused by non-infectious sources, such as structural irritation or systemic disease, treatment involves addressing the root problem, which may include anti-inflammatory medications or specific therapies for autoimmune conditions.