Diagnostic Value of Leukocyte Esterase in Sputum Quality Grading: Comparison with Neutrophil Elastase

Background: Current sputum quality grading relies on the relative proportion of leukocytes and squamous epithelial cells.However, rejection rates vary depending on the criteria used, and no consensus standard exists.Leukocyte esterase (LE) testing, widely used in the diagnosis of urinary tract infections and periprosthetic joint infections, may serve as a surrogate marker of inflammation.However, LE is qualitative and visually interpreted, and its relationship to conventional sputum grading remains unclear.To address this, we evaluated the diagnostic utility of neutrophil elastase (NE) as a quantitative marker in relation to sputum grading and LE testing.Methods: Between July and October 2023, a total of 119 sputum specimens submitted for culture were retrospectively analyzed.Sputum quality was determined using the Murray-Washington grading system (grades 1-3, inadequate; grades 4-6, adequate).LE testing was performed using Multistix 10 SG reagent strips and scored from negative to 3+.NE concentration was measured in 97 specimens using the Human Neutrophil Elastase SimpleStep ELISA Kit following the manufacturer's protocol.Results: Of 119 specimens, LE positivity was significantly higher in adequate than inadequate specimens (66/67 [98.5%] vs. 35/52 [67.3%],P<0.001).Among the 97 evaluable specimens, NE levels were significantly higher in LE-positive than LE-negative specimens and were also associated with higher leukocyte counts (white blood cell [WBC] >25/low-power field [LPF] vs. WBC <10/LPF, P<0.001).Conclusions: NE reflected leukocyte burden and correlated with sputum grading and LE positivity, supporting its role as a quantitative adjunctive marker for evaluating sputum quality, complementing LE testing.