I think it takes a while for the lone kidney to deal with its suddenly doubled workload. Not to mention that your body just had to deal with a bunch of chemicals and medications from the operation. I bet you that when they re-do the tests in a few weeks, there will be a substantial improvement. Roughly speaking, I was told that the remaining kidney generally makes up half of what was lost. So if after donation a figure drops from 100 to 50, it may eventually settle at around 75. But it may take a while. In the meantime, take it easy, rest a lot, eat healthy foods and drink loads of fluids.