After undergoing removal of a kidney, your kidney function tests will be abnormal and the estimated glomerular filtration rate (eGFR) will be low. After removal of one kidney, I would expect the eGFR to be in the range of 50 to 70 milliliters per minute per 1.73 meters squared. I cannot explain why it is down to 40. I suggest that you discuss this with your physician. I also recommend that you have testing of urine for blood, protein and infection as you followup your kidney testing.