1. Determination of the plasma volume in pregnant women is a useful research tool and may become an important clinical measurement. We used three methods to determine plasma volume using Evans Blue dye: (1) the ‘usual’ method, measuring serum absorbance at a wavelength of 610 nm, (2) a two-wavelength method, and (3) precipitation of non-albumin proteins by the addition of polyethyleneglycol before measuring serum absorbance at a wavelength of 620 nm. These were each compared with the standard 125I-human serum albumin method in 20 non-pregnant subjects. Subsequently, the polyethylene glycol method was considered the standard and the three Evans Blue dye methods were compared in 20 pregnant women.

2. In non-pregnant subjects mean plasma volumes did not differ significantly according to the method used. However, the limits of agreement with 125I-human serum albumin method were closest for the polyethyleneglycol method, for both clear and turbid sera.

3. In pregnant women, mean plasma volume values did not differ according to the Evans Blue dye method used, but the limits of agreement were significantly closer with the two-wavelength method than with the ‘usual’ method (P<0.05) largely owing to the effects of turbid sera.

4. These studies demonstrate that considerable error may occur when the Evans Blue dye concentration is determined in turbid sera by the ‘usual’ method. This can be overcome by the use of the two-wavelength method or the polyethyleneglycol method. The most accurate results will be obtained if the latter method is employed routinely to determine plasma volume in pregnant women.

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