Pyuria is the presence of white blood cells (leukocytes) in urine, indicating an inflammatory response within the urinary tract. When this occurs without a bacterial infection detected by standard urine culture, it is termed sterile pyuria.
Sterile pyuria is not a diagnosis itself, but a symptom signaling an underlying issue requiring medical investigation. The presence of white blood cells suggests the body is responding to irritation or a process in the urinary system. Identifying the root cause is important for appropriate management, as conditions vary widely.
How Sterile Pyuria is Identified
Sterile pyuria is identified through urinalysis, a common urine test involving dipstick analysis and microscopic examination. The dipstick detects leukocyte esterase, an enzyme in white blood cells, indicating their presence.
Microscopic analysis counts white blood cells in the urine. Sterile pyuria is identified when there are more than 10 white blood cells per cubic millimeter or more than 3 per high-power field in an unspun sample. After white blood cells are detected, a urine culture is performed. If no significant bacterial growth (less than 100,000 colony-forming units per milliliter) is found, the pyuria is considered “sterile.” This distinguishes it from a typical urinary tract infection, which would show significant bacterial growth.
Potential Underlying Conditions
Sterile pyuria can stem from various sources, including infections difficult to culture, inflammatory processes, medication side effects, and structural problems. These diverse origins often require thorough evaluation.
Some infections do not grow on standard urine cultures. These include sexually transmitted infections (STIs) like Chlamydia trachomatis, Neisseria gonorrhoeae, Mycoplasma, Ureaplasma, and Trichomonas vaginalis. Fungal infections (e.g., Candida species) and certain viral infections (e.g., Herpes simplex virus, HIV, cytomegalovirus, Epstein-Barr virus, BK virus, adenovirus) can also cause white blood cells in urine. Additionally, a recently or inadequately treated urinary tract infection might present as sterile pyuria if bacterial counts are reduced but inflammation persists.
Inflammatory and autoimmune conditions often lead to sterile pyuria. Interstitial cystitis (bladder pain syndrome) is a chronic condition causing bladder wall inflammation. Kidney inflammation, such as glomerulonephritis or interstitial nephritis, can also result in white blood cells in the urine. Systemic inflammatory diseases, including systemic lupus erythematosus (SLE), Kawasaki disease, and sarcoidosis, can affect the kidneys and urinary tract, causing inflammation.
Certain medications can induce sterile pyuria. These include some antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), diuretics, proton pump inhibitors, olsalazine, and nitrofurantoin. These can cause tubulointerstitial nephritis, inflammation in kidney tubules and surrounding tissue, leading to white blood cells in the urine.
Structural or obstructive issues within the urinary system also contribute to sterile pyuria. Kidney stones can irritate the urinary tract, causing inflammation. Prostate inflammation or enlargement (e.g., prostatitis, benign prostatic hyperplasia) can also cause white blood cells in urine. Tumors in the urinary tract (bladder, kidney, prostate) may induce a local inflammatory response.
Other causes include radiation cystitis (bladder inflammation from pelvic radiation therapy) and foreign bodies in the urinary tract (e.g., catheters, stents) which cause irritation. Intense physical exercise can lead to temporary pyuria. Physiological changes like pregnancy or post-menopausal alterations may also be associated with this finding.
Diagnosis and Management Approaches
Once sterile pyuria is identified, a comprehensive diagnostic process uncovers the underlying cause. This begins with a detailed medical history and physical examination. Depending on the initial assessment, various specialized tests may be recommended.
Imaging studies (ultrasound, CT scans, MRI) visualize the urinary tract, identifying structural abnormalities, kidney stones, or tumors. Specialized urine tests include nucleic acid amplification tests (NAAT) for difficult-to-culture pathogens (e.g., Chlamydia, Mycobacterium tuberculosis). Urine cytology, which examines urine for abnormal cells, may be performed if cancer is suspected.
Cystoscopy, involving a flexible tube with a camera inserted into the bladder, may be performed to visualize its lining and collect tissue samples. Blood tests, including complete blood counts, inflammatory markers (CRP, ESR), kidney function tests, or autoimmune markers, provide further insights into systemic conditions.
Treatment for sterile pyuria targets the specific underlying condition. For example, if an atypical infection is identified, targeted antibiotics or antifungals are prescribed. Inflammatory conditions might be managed with anti-inflammatory medications or specific therapies for autoimmune diseases. If kidney stones are the cause, treatment focuses on their removal or prevention. Consulting a healthcare professional is important for accurate diagnosis and a personalized management plan.