It is commonly written in textbooks that in case of AKI ,if someone wants to differentiate between pre renal failure and ATN, ratio of BUN or urea to Cr is important.ie if we take urea(not BUN), ratio urea to Cr of 40 to 1 means pre renal failure and less than that means ATN. my question is does that rule apply in case of ckd as well? For example a ckd patient aged 40 with serum baseline Cr 2 will have eGFR of 37 by MDRD equation and therefore in ckd stage 3 in baseline.If this patient presents acutely in ER dept with Cr 2.5 and urea 120 ie a ratio greater than 40:1 can it be called acute on chronic kidney disease due to pre renal causes suppose history is not available? my confusion is that in case of ckd are the filtration secretion reabsorption processes for urea and Cr effected equally?if not how can this ratio apply as it does in a previously normal kidney effected by AKI