How can a patient's plasmacrit be determined?

The determination of a patient's plasmacrit involves understanding the relationship between hematocrit and plasma volume. Hematocrit is the proportion of blood volume that is occupied by red blood cells (RBCs). To find the plasmacrit, which represents the fraction of plasma in the blood, you can subtract the hematocrit value from the total blood volume, which is always 1 (or 100%).

When you take the decimal hematocrit value and subtract it from 1, you are effectively calculating the remaining volume in the blood that is not occupied by RBCs, which is the plasma volume. Thus, this method provides a straightforward and accurate way to derive the physical fraction of plasma in a patient's blood sample.

This approach is commonly used in clinical settings since it gives a quick estimation of the relative volumes of plasma compared to cells in the bloodstream. Other methods, while potentially useful, do not directly address the calculation of plasmacrit as effectively as this subtraction method does, which is why this option is the most appropriate choice.