Regarding mild hydronephrosis. Sterile.

My son is 4 months old and he was admitted in hospital with fever.in hospital they have done some blood tests like CRP,CBP,CUE and after they told me that CRP level is 93.7 and in CUE PUS CELLS are >100 , PROTIENS + + +, and in remarks they mentioned bacteria present. And doctor suggested me to do USG.in ultrasound report they have mentioned that MILD HYDRONEPHROSIS present in left kidney.doctor advised to do urine culture after discharged from the hospital and we have done that report but urine culture is sterile.now again doctors suggested to do MCUG test. Please advise MCUG TEST can be done after 2 months(one of the doctor suggested) or now itself. Again we have done all tests like CUE,URINE CULTURE and CRP all are normal ,no pus cells found in urine examination