Pyuria Refers to White Blood Cells (WBCs), Not Leukocyte Esterase
Pyuria is defined as the presence of white blood cells in the urine, typically ≥10 WBCs per mm³ in an enhanced urinalysis or ≥5 WBCs per high-power field on centrifuged urine specimens. 1 Leukocyte esterase is merely a detection method for pyuria—it's an enzyme released by WBCs that can be measured on a dipstick test. 1
Understanding the Distinction
- Pyuria = WBCs themselves in the urine, representing the actual inflammatory cells present in the urinary tract 1
- Leukocyte esterase = a biochemical marker that detects the presence of these WBCs through a chemical reaction on dipstick testing 2
The American Academy of Pediatrics defines pyuria based on actual WBC counts, not on the leukocyte esterase test result. 1 Think of leukocyte esterase as a screening tool to detect pyuria, not as pyuria itself.
Detection Methods for Pyuria
Pyuria can be detected through multiple approaches: 1
- Microscopic examination of urine sediment (direct visualization of WBCs)
- Hemocytometer cell counts of uncentrifuged urine (quantitative WBC measurement)
- Leukocyte esterase dipstick (indirect detection via enzyme)
- Automated urinalysis using flow imaging technology
Clinical Significance
- The presence of pyuria indicates the host's inflammatory response in the urinary tract, which is what causes tissue damage and scarring in UTIs 1
- Pyuria is a hallmark of true urinary tract infection and helps distinguish UTI from asymptomatic bacteriuria 1
- The absence of pyuria has excellent negative predictive value approaching 100% for ruling out true UTI in most cases 1
Diagnostic Performance of Leukocyte Esterase
- Leukocyte esterase testing has moderate sensitivity (83%) but limited specificity (78%) for detecting UTIs 1
- When combined with nitrite testing, sensitivity increases to 93% 1
- A negative leukocyte esterase test effectively rules out the presence of pyuria and thus UTI in most populations 2
Common Pitfall to Avoid
Do not confuse a positive leukocyte esterase test with the definition of pyuria itself. 2 Leukocyte esterase is simply detecting the WBCs—the pyuria is the WBCs themselves. When interpreting urinalysis results, you're looking for evidence of WBCs (pyuria), and leukocyte esterase is one tool among several to identify their presence. 1, 2