Acute kidney injury (AKI) and chronic kidney disease (CKD) are common and profound issues for human health. Functional biomarkers such as serum creatinine (Scr) have been used as diagnostic indexes for AKI and CKD. In AKI, however, kidney damage precedes functional change. Therefore, novel biomarker candidates have been explored using urine sample. Among these candidates, neutrophil gelatinase-associated lipocalin, liver-type fatty acid-binding protein, interleukin-18, and kidney injury molecule-1 are well examined in patients with AKI in specific clinical settings. Furthermore, besides albuminuria known as the established biomarker in CKD, the usefulness of these urinary biomarkers in CKD is also being recognized. The general use of urinary biomarkers for AKI and CKD has not been qualified, but the incorporation must help understand the renal condition. Meanwhile, when the alteration in urinary biomarkers is evaluated, the fluctuation of urine volume (UV) should be corrected because the fluctuation causes the varied concentration of urinary biomarkers. Urinary biomarker excretion rate (UBER) corrected by UV itself is known as a gold standard method and is calculated as a product of urinary biomarker concentration and UV, which requires timed urine collection. Alternatively, urinary biomarker-to-creatinine ratio (UBCR) corrected by urinary creatinine is used to substitute UBER and is calculated as a quotient of dividing urinary biomarker concentration by urinary creatinine concentration, for which spot urine is available. In the case that creatinine kinetics is under a nonsteady state, UBCR is influenced in a positive and negative way. As a positive way, the reliability of UBCR to detect kidney damage can be enhanced due to the decreased urinary creatinine. As a negative way, the change of UBCR is offset due to the increased urinary creatinine, resulting in the overlook of kidney damage. Therefore, the influence of the nonsteady state of creatinine on the kinetics should be considered when the alteration in urinary biomarkers is evaluated.
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