Is bilirubin/albumin ratio correlated with unbound bilirubin concentration?

Background:  The American Academy of Pediatrics guidelines recommend that the total bilirubin (TB)/albumin (Alb) ratio (B/A ratio), instead of serum concentration of unbound bilirubin (UB), can be used with TB for determining treatment modality for jaundiced newborns ≥35 weeks of gestation. It is unknown, however, whether the B/A ratio is actually correlated with serum UB. Methods:  Four hundred and ninety-seven serum samples were obtained from 209 newborns ≥35 weeks of gestation, who were admitted to Kobe University Hospital. Serum UB concentration was measured using the glucose oxidase–peroxidase method. Serum TB and Alb concentrations were measured on spectrophotometry. B/A ratios were calculated and were linearly compared with serum UB. Furthermore, the accuracy of the B/A ratio was evaluated. Results:  The B/A ratio was significantly correlated with serum UB concentration. A serum UB concentration of 0.6 µg/dL was in agreement with a B/A ratio of 0.5. For comparison of the number of newborns who had serum UB concentrations ≥ or <0.6 µg/dL and B/A ratios ≥ or <0.5, we found the following characteristics: the concordance rate between serum UB concentrations and the B/A ratio was 94%, sensitivity was 51%, and specificity was 99%. Conclusions:  The B/A ratio is significantly correlated with serum UB concentration in newborns ≥35 weeks of gestation. The B/A ratio, however, is underestimated when serum UB concentrations are >0.6 µg/dL.