What are the causes of white blood cells in urine?

Causes of White Blood Cells in Urine

The presence of white blood cells (leukocytes) in urine primarily indicates inflammation in the urinary tract, with urinary tract infection being the most common cause, but other inflammatory and non-infectious conditions can also lead to pyuria.

Infectious Causes

• Urinary Tract Infection (UTI): The most common cause of pyuria, characterized by bacteria in the urine along with white blood cells. UTIs typically present with symptoms such as dysuria, frequency, urgency, and sometimes fever 1.

• Pyelonephritis: Upper urinary tract infection involving the kidneys, often presenting with fever, flank pain, and significant pyuria 2.

• Asymptomatic Bacteriuria: Presence of bacteria in urine without symptoms, commonly found in elderly patients. Despite bacterial growth, this condition may not require treatment in many cases 2.

• Catheter-Associated UTI: Patients with indwelling catheters frequently develop bacteriuria with associated pyuria, though the correlation between pyuria and infection is weaker than in non-catheterized patients 3.

• Non-bacterial Infections: Infections caused by:

  • Chlamydia trachomatis (particularly in women with pyuria but without significant bacteriuria) 4
  • Mycobacterium tuberculosis (renal TB)
  • Fungal infections (candidiasis) 3

Non-Infectious Inflammatory Causes

• Interstitial Cystitis: Chronic bladder inflammation that can cause pyuria without infection 1.

• Urinary Stone Disease: Kidney stones or bladder stones can cause irritation and inflammation of the urinary tract 2.

• Chemical or Drug-Induced Cystitis: Certain medications or chemicals can irritate the bladder lining, leading to inflammation and pyuria 1.

• Radiation Cystitis: Inflammation of the bladder due to radiation therapy 1.

Systemic Conditions

• Glomerulonephritis: Inflammation of the kidney's filtering units can result in pyuria along with other urinary abnormalities 2.

• Systemic Lupus Erythematosus: Can cause lupus nephritis with associated pyuria 1.

• Vasculitis: Various forms of vasculitis affecting the kidneys can lead to pyuria 2.

Other Causes

• Contamination: Improper collection techniques can lead to contamination with vaginal or perineal white blood cells, especially in bag specimens from infants or poorly collected samples 2.

• Recent Urologic Procedures: Instrumentation of the urinary tract can cause transient pyuria 1.

• Tumor or Malignancy: Urinary tract tumors can cause inflammation and pyuria 1.

Diagnostic Considerations

• Significant Pyuria: Generally defined as ≥10 WBCs/mm³ in uncentrifuged urine using a hemocytometer, or ≥5 WBCs per high-power field in centrifuged specimens 1, 4.

• Leukocyte Esterase: A positive test indicates the presence of WBCs in urine with moderate sensitivity (83%) but limited specificity (78%) for UTI 1, 5.

• Combined Testing: When leukocyte esterase is combined with nitrite testing, the sensitivity increases to 93% with a specificity of 96% for UTI diagnosis 1.

• Pyuria Without Bacteriuria: May indicate:

  • Non-bacterial infections (TB, fungal, viral)
  • Partially treated bacterial infection
  • Interstitial cystitis
  • Urinary stone disease
  • Renal tuberculosis 4

Age-Specific Considerations

In Children

• Pyuria is a hallmark of true UTI in children and helps distinguish UTI from asymptomatic bacteriuria 2.

• In febrile infants and young children, the absence of pyuria on initial urinalysis does not rule out UTI, as it may be absent in up to 20% of cases with culture-proven pyelonephritis 2.

• The American Academy of Pediatrics recommends using both pyuria and positive urine culture (≥50,000 CFU/mL) for diagnosing UTI in children 2.

In Elderly

• Asymptomatic bacteriuria is common in elderly patients (prevalence 15-50% in long-term care facilities) and often does not require treatment 2, 1.

• Non-specific symptoms like confusion or functional decline alone should not trigger UTI treatment without specific urinary symptoms in older adults 1.

Clinical Pearls

• The absence of pyuria has excellent negative predictive value for ruling out UTI in most patient populations 1, 4.

• Pyuria is more strongly associated with gram-negative bacterial infections than with gram-positive or fungal infections 3.

• In patients with indwelling catheters, pyuria with a WBC count >10/μL has a specificity of 90% but a sensitivity of only 37% for predicting catheter-associated UTI 3.

• The optimal cutoff for pyuria to predict bacteriuria in acute care settings appears to be >25 WBCs/hpf, which provides the best combination of sensitivity and specificity 6.