Midstream urine sampling is necessary for accurate measurement of the urinary level of neutrophil gelatinase-associated lipocalin in healthy female subjects

•Urinary NGAL levels were higher in the initial-stream urinary samples.•Urinary WBC counts strongly affect levels of initial-stream NGAL causing high levels that do not reflect AKI.•Midstream urine sampling is strongly recommended for urinary NGAL measurement. Urinary neutrophil gelatinase-associated lipocalin is an established biomarker of acute kidney injury, however, the levels are affected by the number of white blood cells in the urine. As we suspected the portion of the urinary stream sampled could also have a significant influence on the urinary NGAL levels in female subjects, we investigated the influence of the urine sampling procedure on the urinary NGAL levels. We collected 25-mL urinary specimens from each of initial-stream and midstream urinary specimens, including 28 healthy adult female volunteers without kidney diseases or UTI. Then we compared the WBC count, NGAL level, and creatinine level between these specimens. We observed that the urinary NGAL levels were significantly higher in the specimens obtained from initial-stream urinary samples than in midstream specimens, and that they were strongly correlated with the leukocyte esterase activity and WBC count. Moreover, the differences in the urinary NGAL levels between the initial- and midstream urine samples were greater for initial-stream samples with higher leukocyte esterase activities, with a significant difference even for the initial-stream samples with no detectable leukocyte esterase activity. Therefore, midstream urine sampling is strongly recommended for accurate measurement of the urinary NGAL levels.