Dr wants me to do creatnine/gfr and bun but he wants me to drink 1 litter water b4 test. why the water? can water mask abnormal results 4 early ckd ?

Best guess...: When dehydrated, blood tests can suggest a degree of kidney failure. One clue that dehydration may be contributing to the abnormal tests is a high BUN/Cr ratio. Drinking plenty of water before the next test will clarify the issue rather than confuse it. It is likely that follow-up tests will be done to be sure that your kidney function is stable. It is important to follow pre-test instructions.