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 335 weeks of gestation, who were admitted to Kobe University Hospital. Serum UB concentration was measured using the glucose oxidaseperoxidase 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 mg/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 mu g/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 mu g/dL.