A male baby born at term to O -ve mother with Coombs +ve at delivery.Baby weighed 3 kg at bih and haemoglobinis 16 g/dL and TSB is 5 mg/dL. He passed a black stools within the first 24 hours of life and his temp was 97.7degF. After 5 days, infant's weight reduced by 60 gm, his axillary temperature reduced to 95degFand he is jaundiced upto the chest. Parents also told about yellow, seedy stool. Lab investigations show hemoglobin is now 14.4 g/dL, and his TSB is 14 mg/dL. The change in which of the following parameters is of most concern for you as a paediatrician?