Hypothesis 1: Directly measured GFR (mGFR, a type of test to measure the filtering rate of kidneys) by iohexol clearance obtained prior to HDMTX will demonstrate greater sensitivity and specificity for prediction of delayed MTX excretion and/or toxicity in children and adolescents with ALL than serum creatinine (sCr) alone or sCr used for eGFR calculation. If this study proves that mGFR is a better predictor of delayed MTX excretion and/or toxicity, then another study will be developed in the future to determine if modifying the HDMTX dose or adjusting supportive care based on mGFR will prevent delayed clearance and toxicity without impacting patient survival.