Risk of kernicterus is usually assessed by measuring the total plasma/serum unconjugated bilirubin concentration. In newborns <1250 g at birth, a commonly used set of guidelines recommends exchange transfusion at bilirubin concentrations of 10 mg/dL. We have used a modified version of the peroxidase test in our nursery to measure the non-albumin bound (unbound) concentration of bilirubin as well as the total bilirubin to evaluate jaundice in this group of newborns. The data from Ritter et al and Cashore and Oh (Pediatrics 69;481-485,1982 and 69;260-266,1982), corrected for temperature effect, indicates that the level of unbound bilirubin at which significant risk of kernicterus occurs is approximately 34 nmol/L at 37° C. In this study we determined the unbound bilirubin at 10 mg/dL total bilirubin as well as the total bilirubin at which the unbound bilirubin reached 34 nmol/L to determine just how much difference there would be in the total bilirubin concentrations at which exchange transfusion is recommended by each criterion.

Unbound bilirubin concentrations were measured at the initial bilirubin concentration and after enriching the sample with bilirubin to levels above 10 mg/dL in 15 plasma samples from 12 newborns weighing <1250 g at birth. The unbound bilirubin was measured by the peroxidase test at two enzyme concentrations to correct for rate limiting dissociation of the bilirubin from albumin which in the past has led to serious underestimation of the unbound bilirubin, particularly at higher bilirubin/albumin molar ratios. The mean unbound bilirubin at 10 mg/dL total bilirubin was 9 nmol/L (SD 3, range 3 nmol/L to 16 nmol/L) while the mean total bilirubin at which the unbound bilirubin reached 34 nmol/L was 17.4 mg/dL (SD 4.2, range 13.2-28.7 mg/dL).

These data suggest that a total bilirubin of 10 mg/dL is too low a threshold for exchange transfusion in this weight group and also show a very rapid increase in unbound bilirubin as the total exceeds 10 mg/dL. Between 10 and 17.4 mg/dL of total bilirubin, the unbound bilirubin more than triples. Measuring bilirubin binding as well as total bilirubin may be worthwhile in these tiny newborns, particularly as the total bilirubin exceeds 10 mg/dL and exchange transfusion becomes a clinical consideration.