Do small leukocytes in the urine contribute to a positive urinalysis (UA)?

Small Leukocytes in Urinalysis

Yes, small leukocytes (white blood cells) in urine contribute to a positive urinalysis (UA) and are a key indicator used in diagnosing urinary tract infections (UTIs). 1, 2

How Leukocytes Contribute to Positive Urinalysis

• Leukocytes in urine (pyuria) are detected through either leukocyte esterase on dipstick testing or direct microscopic visualization of white blood cells (WBCs) 1
• A positive urinalysis for UTI can be indicated by any of the following: positive leukocyte esterase, positive nitrites, microscopy positive for WBCs, or microscopy positive for bacteria 2
• Leukocyte esterase has moderate sensitivity (83%, range 67-94%) but limited specificity (78%, range 64-92%) for detecting UTIs 1, 2
• Microscopic examination for WBCs (pyuria) has a sensitivity of 73% and specificity of 81% for UTI diagnosis 2

Diagnostic Performance of Leukocytes in Urinalysis

• The presence of leukocytes combined with symptoms such as dysuria, frequency, urgency, fever, or gross hematuria strongly suggests a UTI 1
• When leukocytes are accompanied by a positive nitrite test, the specificity for UTI increases to 96% with a combined sensitivity of 93% 1, 2
• The absence of pyuria (negative leukocyte esterase and no microscopic WBCs) has excellent negative predictive value (100% in some studies) for ruling out UTI 1, 3
• A WBC count of >5 WBC/HPF has a sensitivity of 90-96% and a specificity of 47-50% for UTI 2

Clinical Interpretation of Leukocytes in Urinalysis

• Leukocyte esterase is detected through dipstick testing, which uses a chemical reaction that produces a color change, with results typically reported as negative, trace, 1+, 2+, or 3+ based on the intensity 1
• A positive leukocyte esterase test indicates the presence of pyuria but requires clinical correlation with symptoms and urine culture for definitive UTI diagnosis 1, 4
• Pyuria is absent in approximately 20% of febrile infants with culture-proven pyelonephritis, highlighting the importance of not ruling out UTI based solely on negative leukocyte findings in certain populations 2, 5
• Different pathogens may produce varying levels of pyuria - E. coli infections more commonly cause leukocyturia and pyuria compared to Klebsiella spp. and Enterococcus spp. infections 5

Diagnostic Algorithm for UTI Using Leukocytes

1. Initial screening: Dipstick urinalysis for leukocyte esterase and nitrite 1, 2
2. If leukocyte esterase positive: Proceed with microscopic examination for WBCs 1
3. If pyuria (≥10 WBCs/high-power field or positive leukocyte esterase) is present AND symptoms suggest UTI: Obtain urine culture 1
4. Combined approach: The most sensitive approach combines multiple parameters: leukocyte esterase test AND nitrite test OR microscopy positive, which achieves 99.8% sensitivity 2

Important Considerations and Pitfalls

• False-positive leukocyte esterase results can occur with contaminated specimens, certain oxidizing agents, and some medications 1
• False-negative results may occur with high urinary glucose, high specific gravity, or certain antibiotics 1
• The test has lower sensitivity in infants who void frequently, as shorter bladder dwell time may result in fewer detectable leukocytes 1
• Do not rule out UTI based solely on negative leukocyte results in patients with strong clinical suspicion, especially in certain populations like infants 2, 5
• Asymptomatic bacteriuria with pyuria is common, especially in older adults (prevalence 15-50% in non-catheterized long-term care facility residents) 1