Urinary kidney injury molecule-1 (KIM-1) and clusterin, serum cystatin C, and plasma fibroblast growth factor 23 (pFGF23) are biomarkers that appear earlier and are more sensitive than sCr in the setting of kidney injury. In fact, a detectable increase in sCr does not occur until 24-48 hours after a primary renal injury [2]. KIM-1, a type 1 transmembrane glycoprotein, is not expressed in normal kidneys, but is exhibited at high levels in proximal tubule epithelial cells and is excreted into the urine within 6-12 hours after toxic or ischemic injury [10, 11, 12]. Similarly, clusterin expression is induced by glomerular, tubular, or papillary kidney injury, with increased urine concentrations at 8, 24, and 48 hours post-toxic exposure [12, 13]. Cystatin C, a cysteine protease inhibitor, is not affected by age, gender, race, or overall muscle mass, making it a useful glomerular filtration marker in early KI. In a prospective study, cystatin C rise preceded an increase in sCr by 1-2 days [14]. FGF23 increases as early as within one hour of acute KI induction in mice and remains elevated in chronically compromised GFR, thereby serving as a useful biomarker in both acute and chronic KI [15]. In addition, plasma FGF23 has recently been show to be a strong predictor for risk of adverse outcomes in adults with acute kidney injury (AKI), with the median FGF 23 level obtained within 24 hours of admission being 5.5 times higher in patients with AKI compared to "control" ICU patients without kidney injury and almost 20 times higher than the upper limit of normal [16]. Therefore, KIM-1, clusterin, cystatin C, and FGF23 could provide earlier detection of kidney injury related to HDMTX when compared with sCr measurements. The utility of these biomarkers in identifying early kidney injury following treatment with HDMTX has not been previously studied. This study will compare these biomarkers with sCr pre-, intra-, and post-HDMTX treatment to determine their utility in detecting early kidney injury.