Does hydration affect egfr rate?

Maybe if very ill: If a person is either mildly under-hydrated because he hadn't drank much fluids that day, normally hydrated, or mildly over-hydrated because he had drank a lot of fluids, his kidneys still should be working normally, so his blood test to estimate his GFR should still be valid. If a person is very ill, and has quite abnormal hydration, his abnormal blood test results may not reflect his usual gfr.

Not usually: If you are dehydrated to the extent of being sick from it, it usually does not affect eGFR. However, it is a good idea to drink plenty of water the morning of the test.