REEVALUATION OF EVANS BLUE-DYE DILUTION METHOD OF PLASMA-VOLUME MEASUREMENT

To simplify the method of plasma volume measurement by Evans blue dye dilution we used, for the first time, the same venous site for injecti on of dye and collection of samples. In a series of 49 studies the dye decay between 10 and 35 min after injection was highly linear (r = 0. 991 +/- 0.01), indicating that contamination of samples is very unlike ly. We repeated the measurements after eight weeks in nine patients; t he mean difference was 16.4 +/- 19.6 ml, indicating a high degree of r eproducibility, We found that extrapolation of the dye decay curve to time zero is required for accurate estimates of plasma volume. There w as good agreement between the estimates of plasma volume obtained by e xtrapolation from only three samples taken at 10, 20 and 30 min after dye injection with the results obtained using all six samples. We also found good agreement between the estimates of plasma volume obtained by using standard curves constructed from four standard dilutions of 1 .25, 2.5, 5 and 10 mg/l and those obtained by the use of standard curv es constructed from the blank and only one standard dilution of 10 mg/ l. We therefore conclude that the Evans blue technique can be simplifi ed with minimal loss of accuracy, by using only one venous site for in jection and withdrawal, withdrawing only three samples between 10 and 30 min after injection and using a two point calibration line.