Errors in estimating lung liquid volume in fetal lambs when using radiolabeled serum albumin and blue dextran

Fetal lung liquid volume is usually determined by using radio-iodinated ser um albumin (RISA) or blue dextran (BD) as volume tracers. We test-ed the re liability of both tracers at 124 (G124) and 142 days of gestation (G142; te rm = G147) when the labels were employed simultaneously. We measured the pr oportion of label bound reversibly to the lung, or apparently lost from the lung compartment, by washing out the lung with saline and 5% albumin. At G 124, volume estimates with the two labels were similar. At G142, the volume estimate with ED (36.3 +/- 8.7 ml/kg of body wt) was higher (P< 0.05) than with RISA (22.3 +/- 3.5 ml/kg). This difference resulted from reversible b inding of BD, because 5% albumin washout; released 38.5 +/- 4.0% of the ED added at the start of the experiment but a lesser amount of RISA (9.8. +/- 0.7%; P < 0.05). At G142, when RISA was used alone, its reversible binding was 1.3 +/- 0.2%. Background absorbance increased during experiments, givin g rise to an apparent increase in BD concentration. We conclude that; RISA is an effective tracer for lung liquid volume determination in the fetal la mb, whereas our findings of substantial epithelial binding of ED and large changes in background absorbance demonstrate that, under the conditions of our experiments, ED is a poor tracer dose to term.