Double volume exchange tranfusion (DVET) should be performed if the total serum bilirubin(TSB) levels reach to age specific cut off for exchange transfusion or th infant shows signs of bilirubin encephalopathy irrespective of TSB levels .For < 28 wk gestation, TSB cut off is 11-14 mg/dl. For 28 to 29 wks, 30 to 31 wks and 32 to 33gestation, cut off is 12-14, 13 - 16 and 15-18 mg/dl respectively . For 34 wks of gestation, cut off is 17-19 mg /dl. Reference : page 172 Ghai Essential Pediatrics 9 th edition