Routine red cell counts and hemoglobin determinations yield information that is valuable but very incomplete. This information concerns only the concentration of the blood. These routine methods do not furnish us absolute values for hemoglobin, for red cells, or for the oxygen-carrying capacity of the blood. With them, an anemia may be more apparent than real, as it may be dependent on or associated with a marked increase in plasma volume, while a polycythemia does not necessarily indicate an absolute increase in red cells, as it may be dependent on a decrease in plasma. In order to obtain absolute values for either red cells or hemoglobin, data relative to plasma volume or total blood volume are essential.
The volume as well as the concentration of the blood must play a rôle in pathological physiology. Absolute values are essential to the proper interpretation of certain pathological findings. Is an
KEITH NM, ROWNTREE LG, GERAGHTY JT. A METHOD FOR THE DETERMINATION OF PLASMA AND BLOOD VOLUME. Arch Intern Med (Chic). 1915;XVI(4):547-576. doi:10.1001/archinte.1915.00080040043004