Leukocytes are white blood cells, and their presence in urine is a common finding during a standard urinalysis. The technical term for an elevated count of these cells is pyuria, which suggests an inflammatory reaction is occurring somewhere along the urinary tract. While the detection of leukocytes often leads to a suspicion of a Urinary Tract Infection (UTI), this finding alone is not definitive proof of a bacterial infection. The immune response can be triggered by many types of irritation or inflammation, meaning a high leukocyte count may have a non-infectious origin. Determining the underlying cause of the inflammation is key, especially when traditional signs of a UTI are absent.
What Leukocytes in Urine Mean
The detection of white blood cells in urine signifies that the immune system has dispatched cells to the urinary tract. Leukocytes mobilize to an area when they sense an invading organism or tissue damage. Their appearance in urine serves as a direct indicator of inflammation or irritation within the kidneys, ureters, bladder, or urethra.
Healthcare providers assess the level of leukocytes through two main methods: a urine dipstick test and microscopic analysis. The dipstick test checks for leukocyte esterase, an enzyme released by these immune cells, which provides a rapid screening result. Microscopic analysis is performed for a more precise measurement, quantifying the number of white blood cells per unit of volume or per high-power field (HPF).
A count of five or more white blood cells per HPF in a centrifuged urine sample is considered the threshold for pyuria. Since a bacterial UTI is the most frequent cause of localized inflammation, a positive leukocyte result usually points toward an infection. However, when a urine culture fails to grow significant amounts of bacteria, the condition is termed “sterile pyuria,” confirming inflammation without a common bacterial culprit.
Common Causes Not Related to Infection
The finding of sterile pyuria requires deeper investigation, as many conditions other than a classic bacterial infection can trigger a leukocyte response. One common non-infectious cause is the physical irritation from a kidney stone (renal calculi). As a stone travels or remains lodged, it causes mechanical damage and inflammation to the lining of the urinary passages, prompting an influx of white blood cells.
Certain medications are also causes of pyuria by inducing interstitial nephritis, an inflammation of the tissues surrounding the kidney tubules. Nonsteroidal anti-inflammatory drugs (NSAIDs), proton pump inhibitors (PPIs), and some penicillin-based antibiotics have been implicated. The immune reaction to the drug can cause damage and inflammation in the kidney, leading to a spillover of leukocytes into the urine.
Systemic inflammatory and autoimmune diseases can also manifest with sterile pyuria. Conditions such as Systemic Lupus Erythematosus (SLE) or Kawasaki disease involve widespread inflammation, and the kidneys can become a target of the immune system’s overactivity. The leukocytes found in the urine are a reflection of an underlying, body-wide disorder rather than a localized urinary problem.
Chronic inflammation that elevates leukocyte levels can be caused by interstitial cystitis, a chronic bladder condition, or tumors within the urinary tract. Other factors, such as a recent urological procedure or the presence of a foreign object like a catheter, can also induce inflammation. A common non-medical cause involves sample collection itself, where vaginal leukocytes contaminate a urine specimen, leading to a false-positive reading for pyuria.
Diagnostic Testing and When to Consult a Doctor
When a urinalysis is positive for leukocytes but the urine culture is negative for bacteria, a physician begins the process of differentiation. The first step involves a repeat urinalysis using a properly collected, clean-catch midstream sample to rule out external contamination. If pyuria persists despite a negative culture, the focus shifts to identifying atypical infections or non-infectious diseases.
Specific nucleic acid amplification tests (NAAT) may be ordered to check for fastidious organisms that do not grow on standard culture media, such as Chlamydia trachomatis or Neisseria gonorrhoeae. If risk factors are present, testing for Genitourinary Tuberculosis (GU-TB), which requires specialized techniques, might also be considered. Blood tests, including a complete blood count (CBC) and markers for systemic inflammation, are often ordered to check for underlying systemic disease.
Imaging studies, such as a renal ultrasound or Computed Tomography (CT) urography, evaluate the urinary tract for structural issues. These tests help identify kidney stones, tumors, or other obstructions that could be causing inflammation. The diagnostic plan is tailored to the patient’s symptoms and medical history.
A person should consult a doctor anytime they notice concerning urinary symptoms, such as pain or burning during urination, persistent pelvic discomfort, or blood in the urine. If a urine test has confirmed pyuria, follow-up is necessary regardless of whether a UTI is initially suspected. Immediate medical attention is warranted if pyuria is accompanied by severe symptoms like fever, flank pain, or signs of systemic illness, as these may indicate a more serious condition.