Correlation between free bilirubin and indirect bilirubin in normal newborn infants with non-hemolytic jaundice and effect of hemolysis on free bilirubin measurement by the peroxidase method.

The present study was undertaken to determine the correlation between free bilirubin and indirect bilirubin in normal newborn infants with non-hemolytic jaundice, and the possible effect of hemolysis on free bilirubin measurement by the peroxidase method. A prospective study protocol was applied at the Neonatal Unit of the Department of Pediatrics, Escola Paulista de Medicina. Forty-three newborn infants were submitted to measurement of free bilirubin and bilirubin fractions and the extent of hemolysis of the sample was determined. Data were analyzed statistically by the Student t-test. A positive and moderate correlation (r = 0.668; p < 0.01) was detected between free bilirubin and indirect bilirubin. The linear regression equation calculated by the least squares method was as follows: f(x) = 4.562 + 0.382x. The concentration of free bilirubin was inversely proportional to sample hemolysis, the difference being greater at 50 mg/dl hemolysis. Despite these results, however, the use of this correlation is delicate due to the impossibility of establishing it in individual cases. Also, since the samples may show some degree of hemolysis, this factor should be minimized by appropriate sample collection before free bilirubin measurement.