Knowledge Check

We have discussed two approaches to determining the hematocrit (Hct): the manual (spun) Hct and the Hct calculated from the red cell count and mean cell volume (the latter two parameters being directly measured using automated hematology analyzers). Which tends to be higher?

Spun Hct
Spun hematocrit is 1% to 3% higher than the hematocrit from automated instrument due to plasma that is trapped in the erythrocyte layer.
Automated Hct

In theory, can the hematocrit be normal with a hemoglobin (Hb) of zero?

The Hct is a function of cell size and number. It is impervious to what is inside the cell, including the amount of Hb (see next slide).

Thought experiment. Consider a spun hematocrit of blood from two different patients. The red blood cell count and the mean cell volume are identical in the two samples. The only difference is that the red blood cells in the patient whose sample is shown on the left contain normal amounts of hemoglobin, whereas those from the patient on the right lack hemoglobin altogether. Note that the hematocrit is the same for both samples.

The following is a complete blood count from a patient with cold agglutinin disease:

Cold agglutinin disease. Erythrocyte agglutination secondary to a cold agglutinin causes micro-aggregates that the instrument may measure as single cells, resulting in falsely low red blood cell (RBC) counts as well as high mean cell volume (MCV) (in this case, causing the automated counter to discard the data). Note that the hematocrit value is based on a spun hematocrit.