Situations where the hematocrit reading may be affected:
(1) excess chemical anticoagulant that raises the osmolality of the specimen
(2) infusions that increase plasma osmolality in excess of volume expansion
(3) removal of solute during dialysis
hematocrit in percent (Figure 1 in Stott et al) =
= (-0.0583 * (plasma osmolality)) + 58.531
hematocrit in percent (Figure 1 in Watson and Maughan) =
= (-0.0714 * (plasma osmolality)) + 63.055
If this equation is manipulated:
(hematocrit at osmolality 1) - (hematocrit at osmolality 2) =
= (0.0714 * ((osmolality 2) - (osmolality 1))
hematocrit at osmolality 2 =
= (hematocrit at osmolality 1) - (0.0714 * ((osmolality 2) - (osmolality 1)))
The most likely explanation is that at high osmolality water leaves the inside of the erythrocyte, and the erythrocyte is smaller. The volume of the erythrocyte may increase if the plasma osmolality is lower.
It is observed that the spun hematocrit may differ slightly from the hematocrit given by an automated analyzer. One explanation is the amount of trapped plasma between the erythrocytes.
