Tubular function urinary biomarkers

Cystatin C is nearly completely metabolized by proximal renal tubular cells. As a consequence, under ordinary circumstances there is little to no detectable cystatin C present in the urine. Thus, a true clearance of cystatin C cannot be determined. However, in the presence of tubular damage, cystatin C may be detected in the urine [147,148] and may be more sensitive to early and mild changes of kidney function compared with creatinine [149,150]. In this regard, elevation in serum cystatin C consistent with AKI, defined by at least a 50% increase from baseline, was evident 1-2 days prior to changes in SCr [151]. Finally, in patients with AKI, elevated urinary cystatin C was highly predictive of subsequent need for acute renal replacement therapy and outperformed several other urinary biomarkers in some studies [152]., but not in others [152a]... 

Flerget-Rosenthal S, van Wijk JA, Brocker-Preuss M, Bokenkamp A. Increased urinary cystatin C reflects structural and functional renal tubular impairment independent of glomerular filtration rate. Clin Biochem 2007 27 April [Epub ahead of print].This study provides evidence for urinary cystatin C as a biomarker for tubular injury. 

Urinary Total Protein and Albumin Urinary total protein and albumin have been nsed for decades as glomerular injury biomarkers and, more recently, were qualified as measurements of glomerular filtration and tubular reabsorption function (Ferguson et al., 2008 Bonventre et al., 2010). Compared with blood concentrations of protein/albumin, a small amount of protein and albumin (microalbumin, which is below the albumin detection threshold by the conventional urinary dipstick 30-300 mg/L) enters the filtrate by the glomerulus and is reabsorbed and subsequently catabolized in the normal kidney proximal tubnle (Vaidya et al., 2008 Charlton et al., 2014). Therefore, increased urinary protein/albumin can reflect glomerular injury, tubular injury, or combined effects, though albuminuria can be observed in rats secondary to other effects such as dehydration or hypertensive conditions (Haschek et al., 2013). 

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