Blood-volume determination with Evans blue dye in foals.

The rate of disappearance of Evans blue dye from the blood after an intravenous injection was studied in young foals, between 4 and 105 days of age. This was found to be age dependant, especially during the first month, the initial dye disappearance being much faster than in the adult horse. This would mean an overestimation of plasma volume, using the single sampling technique, of about 5% during the first day of life, i.e. negligible from a practical point of view considering the standard error of estimation. The effect of exercise on the venous haematocrit was studied, too, in foals less than 3 months of age. There was a significant difference between mean values before and after exercise, and this difference seemed to increase with age, indicating an increasing erythrocytestoring capacity of the spleen. This means that even in young foals, the splenic function should be considered when determining the total blood volume from the plasma volume and the venous haematocrit.