Decreased bilirubin binding by albumin in late pregnancy

Abstract

It is known that the albumin concentration of maternal plasma is reduced at term and that the binding of certain drugs and bilirubin’ in plasma is likewise reduced. This decreased plasma binding of bilirubin may be due to decreased albumin concentrations alone or a decrease in the ability of albumin to bind bilirubin. It is possible that the same factor(s) which causes decreased binding ability in cord blood may decrease the binding ability in maternal blood. We undertook this study to determine if the quantitative binding of bilirubin to albumin (binding fraction) was changed in pregnant women at term. We collected heparinized venous plasma from 19 normal pregnant subjects at term (38+ gestational weeks) and 19 normal, nonpregnant control subjects. Informed consent was obtained from all subjects in this study. Apparent total bilirubin binding capacity was measured by the peroxidase method with pooled human bilirubin-enriched plasma used as a control. Albumin was determined by the bromcresol green method. All assays have been described in detail elsewhere.’ The binding fraction was expressed as the total bilirubin binding capacity/albumin molar ratio. Statistical analysis of the data included calculations of Pearson’s product-moment correlation coefficients and multiple regression analysis. Correlation coefficients were tested for significance with the use of one-tailed t tests. The total bilirubin binding capacity in the pregnant subjects was 20.9? 2.7 (mean? SD) compared to 30.5 2 3.4 in our nonpregnant sample. The albumin concentration was 3.6 * 0.26 compared to 4.6 -C 0.29 and the binding fraction was 0.66 ‘0.11 versus 0.78 * 0.10 for the nonpregnant sample. Fig. 1 shows the relationship of total bilirubin binding capacity to binding fraction in the two groups, along with the respective linear regression equations. We found that the relationship between total bilirubin binding capacities and binding fraction was stronger in nonpregnant women (r = 0.87, p < 0.001) than in pregnant women (r = 0.76, p < O.OOl), probably because there was less spread in the total bilirubin binding capacity in the former group. The binding fraction of the pregnant group was about 15% lower than the binding fraction of the nonpregnant group, whereas