When a routine urine test is performed, laboratory technicians examine “pus cells,” the common term for leukocytes or white blood cells (WBCs). These cells are fundamental components of the body’s immune system, and their general role is to defend against foreign invaders. Finding WBCs in the urine, known medically as pyuria, signals inflammation or infection somewhere along the urinary tract, from the kidneys down to the urethra. A small, trace amount of these cells is considered normal, reflecting natural cell turnover. However, an elevated count serves as an important indicator that an underlying condition requires medical investigation.
Defining the Normal Range and Measurement
The normal count for these cells is determined during a standard urinalysis, where a urine sample is analyzed under a microscope. This microscopic analysis is performed after the sample has been centrifuged, which concentrates cellular material in the urine sediment for easier viewing. The concentration of white blood cells is reported as the number of cells observed per high-power field (HPF).
For adult males, a count of zero to five white blood cells per HPF is generally considered normal. The accepted normal level for adult females is often slightly higher, sometimes up to zero to ten cells per HPF. This difference is largely due to the potential for external contamination during sample collection, as the female urethra is shorter and closer to the vagina.
To ensure an accurate reading, proper sample collection is necessary. Patients are instructed to provide a “clean-catch midstream” sample, cleansing the genital area and collecting only the middle portion of the urine stream. Contamination from surrounding tissues can artificially inflate the cell count, potentially leading to a false-positive result for pyuria. If the count exceeds these established thresholds, it signals the presence of an inflammatory response that medical professionals must evaluate further.
Common Causes of Elevated Pus Cells (Pyuria)
Pyuria, the presence of an abnormally high number of white blood cells in the urine, primarily signals an active inflammatory process within the urinary system.
Infectious Causes
The most frequent cause is a bacterial Urinary Tract Infection (UTI), which can be localized in the bladder (cystitis) or the urethra (urethritis). A more serious infection, known as pyelonephritis, occurs when bacteria ascend to the kidneys, often resulting in a significantly higher cell count and systemic symptoms.
Infectious causes also include Sexually Transmitted Infections (STIs) such as chlamydia and gonorrhea, which infect the urethra. Non-bacterial infections, such as tuberculosis affecting the urinary tract, may also lead to elevated WBC counts. If white blood cells are present but routine cultures do not detect bacteria, the condition is called “sterile pyuria,” indicating a non-standard cause.
Non-Infectious Causes
Non-infectious conditions are also significant contributors to pyuria by causing irritation and inflammation. Kidney stones, for example, can irritate the lining of the urinary tract as they pass or block urine flow, leading to an inflammatory response. Inflammation of the kidney tissue (interstitial nephritis) or chronic conditions like interstitial cystitis can also cause persistent pyuria. Sterile pyuria can also result from systemic autoimmune disorders or the long-term use of certain medications, including nonsteroidal anti-inflammatory drugs (NSAIDs) or specific antibiotics.
Symptoms and Necessary Follow-Up
The physical signs accompanying pyuria are often related to the underlying cause, especially if it is a urinary tract infection. Common symptoms include dysuria (a painful or burning sensation while urinating), alongside an increased frequency and sudden urgency to pass urine. The presence of white blood cells and debris can also make the urine appear cloudy, or it may have a strong, foul odor. Severe symptoms, such as fever, chills, nausea, vomiting, or pain in the flank region, may suggest the infection has progressed to the kidneys.
When urinalysis detects an elevated number of pus cells, professional medical follow-up is needed to identify the source of the inflammation. The next step typically involves ordering a urine culture, a test designed to grow and identify any bacteria present in the sample. Identifying the specific microorganism determines if an antibiotic is appropriate and which one will be most effective. If the culture is negative, indicating sterile pyuria, further testing is required to look for non-bacterial causes, such as STIs, kidney stones, or other inflammatory disorders.